Usenix Security Symposium
Analysis of the Intel
Pentium's Ability to Support a
Secure Virtual Machine Monitor
John Scott Robin
U.S. Air Force
Cynthia E. Irvine 1
Naval Postgraduate School
A virtual machine monitor (VMM) allows multiple operating
systems to run concurrently on virtual machines (VMs) on a single
hardware platform. Each VM can be treated as an independent operating
system platform. A secure VMM would enforce an overarching security
policy on its VMs.
The potential benefits of a secure VMM for PCs include: a more secure
environment, familiar COTS operating systems and applications, and
enormous savings resulting from the elimination of the need for
separate platforms when both high assurance policy enforcement, and
COTS software are required.
This paper addresses the problem of implementing secure VMMs on the
Intel Pentium architecture. The requirements for various types of
VMMs are reviewed. We report an analysis of the virtualizability of
all of the approximately 250 instructions of the Intel Pentium
platform and address its ability to support a VMM. Current
"virtualization" techniques for the Intel Pentium architecture are
examined and several security problems are identified. An approach to
providing a virtualizable hardware base for a highly secure VMM is
A virtual machine monitor (VMM) is software for a computer
system that creates efficient, isolated programming environments that
are ``duplicates'' which provide users with the appearance of direct
access to the real machine environment. These duplicates are referred
to as virtual machines. Goldberg [12
] defines a virtual machine
(VM) as: ``a hardware-software duplicate of a real existing computer
system in which a statistically dominant subset of the virtual
processor's instructions execute on the host processor in native
mode". A VMM manages the real resources of the computer system,
exporting them to virtual machines.
A VMM offers a number of benefits not found in conventional
1.1 VMM Benefits
First, virtual machine monitors normally allow a system
manager to configure the environment in which a VM will run.
VM configurations can be different from those of the real machine.
For example, a real machine might have 32MB of memory, but a virtual
machine might have only 8 MB. This would allow a developer to test an
application on a machine with only 8 MB of memory without having to
construct a hardware version of that real machine.
Second, virtual machine monitors allow concurrent execution of
different operating systems on the same hardware. Users can run
any operating system and applications designed to run on the real
processor architecture. Thus application development for different
operating systems is easier. A developer can easily test applications
on many operating systems simultaneously while running on the same
Third, virtual machine monitors allow users to isolate untrusted
applications of unknown quality. For example, a program
downloaded from the Internet could be tested in a VM. If the program
contained a virus, the virus would be isolated to that VM. A secure
VMM will ensure that other high integrity VMs and their applications
and data are protected from corruption.
Fourth, virtual machine monitors can be used to upgrade
operating system software to a different version without losing the
ability to run the older ``legacy" operating system and its
applications. The legacy software can run in a virtual machine
exactly as it did previously on the real machine, while the new
version of the operating system runs in a separate virtual
Finally, VMMs can be used to construct system software for
scalable computers that have anywhere from 10 to 100 processors.
VMMs can facilitate the development of functional and reliable system
software for these computers.
Using a VMM, an additional software layer can be inserted between the
hardware and multiple operating systems. This VMM layer would allow
multiple copies of an operating system to run on the same scalable
computer. The VMM also allows these operating systems to share
resources with each other. This solution has most of the features of
an operating system custom-built for a scalable machine, but with
lower development costs and reduced complexity. Disco, developed for
the Stanford FLASH shared-memory multi-processor [8
] is an example of this
solution. It uses different commercial operating systems to provide
high-performance system software.
1.2 VMM Characteristics and Layers
A VMM has three characteristics [28
]. First, a VMM provides
an execution environment almost identical to the original
; any program executed on a VM should run the same as it
would on an unvirtualized machine. Exceptions to this rule result
from differences in system resource availability, timing
dependencies, and attached I/O devices. If resource availability,
e.g. physical memory, is different, the program will perform
differently. Timing dependencies may lose their validity because a
VMM may intervene and execute a different set of instructions when
certain instructions are executed by a VM. Finally, if the VM is not
configured with all of the peripheral devices required by the real
machine, application behavior will differ.
Second, a VMM must be in control of real system resources. No
program running on a VMM can access any resource not explicitly
allocated to it by the VMM. Also the VMM can regain control of
previously allocated resources.
Efficiency is the third VMM characteristic. A large percentage
of the virtual processor's instructions must be executed by the
machine's real processor, without VMM intervention. Instructions
which cannot be executed directly by the real processor are
interpreted by the VMM. Some virtual machines exhibit the recursion
property: it is possible to run a VMM inside of a VM, producing a new
level of virtual machines. The real machine is normally called Level
0. A VMM running on Level 0 is said to be Level 1, etc.
1.3 VMM Logical Modules
A VMM normally has three generic modules: dispatcher,
allocator, and interpreter. A jump to the dispatcher is placed in
every location to which the machine traps. The dispatcher then
decides which of its modules to call when a trap occurs. The second
type of module is the allocator. If a VM tries to execute a
privileged instruction that would change the resources of the VM's
environment, the VM will trap to the VMM dispatcher. The dispatcher
will handle the trap by invoking the allocator that performs the
requested resource allocation according to VMM policy. A VMM has only
one allocator module, however, it accounts for most of the complexity
of the VMM. It decides which system resources to provide to each VM,
ensuring that two different VM's do not get the same resource. The
final module type is the interpreter. For each privileged
instruction, the dispatcher will call an interpreter module to
simulate the effect of that instruction. This prevents VMs from
seeing the actual state of the real hardware. Instead they see only
their virtual machine state.
1.4 Attractions of a Secure VMM
An isolated VM constrained by an overarching security
policy enforced by the underlying secure VMM is attractive. Also, VMM
technology provides stronger isolation of virtual machines than found
in conventional multiprogramming environments [21
]. Within a constrained VM,
legacy operating systems and applications are executed unmodified and
are easily upgraded and replaced even within the context of rapidly
evolving software product lifecycles.
In the past, some virtual machine monitors, such as the SDC KVM/370
] and the DEC VAX SVS [17
], have been used to separate
mandatory security classes. A secure VMM for the Intel Pentium2
processor architecture would be very desirable
because a single machine could be used to implement critical security
policies while also running popular Win32 operating systems and
Although the x86 processor family has been used as the base for many
highly secure systems [23
], it has not been considered
as a VMM base. Recent increased interest in VMM technology suggests
that a popular hardware base for a new generation of VMMs would be
highly attractive. Before embarking on such a venture, its
feasibility must be carefully examined. This paper presents an
analysis to determine whether the Intel Pentium architecture can
support a highly secure VMM without sacrificing user
1.5 Paper Organization
The rest of this paper is organized as follows:
discusses the three different
types of VMMs and their hardware requirements. Section 3
is an analysis of the Intel Pentium architecture
with respect to the VMM hardware requirements described in Section .
asks if a VMM designed for the
Intel Pentium architecture can be secure. Finally, Section 5
presents our conclusions and future research.
2. VMM Requirements
This section discusses each type of VMM including the Type
I VMM, Type II VMM, and Hybrid VMM. It will also cover the
architectural features required for the successful implementation for
each VMM type.
2.1 Virtual Machine Monitors Types
An operating system consists of instructions to be
executed on a hardware processor. When an operating system is
virtualized, some portion, ranging from none to all, of the
instructions may be executed by underlying software. The amount of
software and hardware execution of processor instructions determines
if one has a complete software interpreter machine (CSIM), hybrid VM
(HVM), VMM, or a real machine. Each of these different types of
machines provides a normal machine environment, meaning that
processor instructions can be executed on them, viz. a VMM can host
an operating system. However, they differ in the way that the machine
environment actually executes the processor instructions. A real
machine uses only direct execution: the processor executes every
instruction of the program directly. A CSIM uses only software
interpretation: a software program emulates every processor
instruction. There has been a recent resurgence of interest in CSIM
]. A VMM requires that a
``statistically dominant subset'' of the virtual processor's
instructions be executed on the real processor [12
]. Performance will be
effected by the size of the subset.
VMMs primarily use direct execution, with occasional traps to
software. As a result, the performance of VMMs is better than CSIMs
and HVMs. An HVM is a VMM that uses software interpretation on all
privileged instructions. HVMs are possible on a larger class of
systems than VMMs. The definition of a VMM does not specify how the
VMM gains control of the machine to interpret instructions that
cannot be directly executed on the processor. As a result, there are
two different types of VMMs that can create a virtual machine
environment. These types are referred as Type I and Type II [12
]. A Type I VMM runs on a
bare machine. It is an operating system with virtualization
mechanisms. It performs the scheduling and allocation of the system's
resources. A Type II VMM runs as an application. The operating system
that controls the real hardware of the machine is called the ``host
OS." The host OS does not need or use any part of the
virtualization environment. Every OS that is run in the Type II
virtual environment is called a ``guest OS." In a Type II VMM,
the host operating system provides resource allocation and a standard
execution environment to each guest OS.
2.2 Execution of Privileged
When executing in a virtual machine, some processor
instructions can not be executed directly on the processor. These
instructions would interfere with the state of the underlying VMM or
host OS and are called sensitive instructions. The key to
implementing a VMM is to prevent the direct execution of sensitive
instructions. Some sensitive instructions in the Intel Pentium
architecture are privileged, meaning that if they are not executed at
most privileged hardware domain, they will cause a general protection
exception. Normally, a VMM is executed in privileged mode and a VM is
run in user mode; when privileged instructions are executed in a VM,
they cause a trap to the VMM. If all sensitive instructions of a
processor are privileged, the processor is considered to be
``virtualizable:" then, when executed in user mode, all
sensitive instructions will trap to the VMM. After trapping, the VMM
will execute code to emulate the proper behavior of the privileged
instruction for the virtual machine. However, if sensitive,
non-privileged instructions exist, it may be necessary for the VMM to
examine all instructions before execution to force a trap to the VMM
when a sensitive, non-privileged instruction is encountered.
The most severe performance penalty occurs when running a complete
software interpreter machine (CSIM) on the same hardware. A CSIM
emulates every instruction of the real processor. It does not meet
Goldberg's definition [12
of a virtual machine because it does not execute any of the
instructions directly on the real processor.
The following sections summarize Goldberg's analysis of processor
requirements for the types of VMMs he identified.
2.3 Type I VMM
A Type I VMM runs directly on the machine hardware. It is
an operating system or kernel that has mechanisms to support virtual
machines. It must perform scheduling and resource allocation for all
virtual machines in the system and requires drivers for hardware
To support a Type I VMM, a processor must meet three virtualization
Requirement 1 The method of executing non-privileged
instructions must be roughly equivalent in both privileged and user
mode. For example, a processor cannot use an additional bit in an
instruction word or in the address portion of an instruction when in
Requirement 2 There must be a method such as a protection
system or an address translation system to protect the real system
and any other VMs from the active VM.
Requirement 3 There must be a way to automatically signal the
VMM when a VM attempts to execute a sensitive instruction. It must
also be possible for the VMM to simulate the effect of the
Sensitive instructions include:
Requirement 3A Instructions that attempt to change or
reference the mode of the VM or the state of the machine.
Requirement 3B Instructions that read or change sensitive
registers and/or memory locations such as a clock register and
Requirement 3C Instructions that reference the storage
protection system, memory system, or address relocation system. This
class includes instructions that would allow the VM to access any
location not in its virtual memory.
Requirement 3D All I/O instructions.
2.4 Type II VMM
A Type II VMM runs as an application on a host operating
system and relies on the host OS for memory management, processor
scheduling, resource allocation, and hardware drivers. It provides
only virtualization support services. To support a Type II virtual
machine a processor must meet all of the hardware requirements for
the Type I VMM listed above. In addition, the following software
requirements must be met by the host operating system of the Type II
Weaker Requirement 3A The host OS cannot do anything to
invalidate the requirement that the method of executing
non-privileged instructions must be roughly equivalent in both
privileged and user mode.
Requirement 2 Primitives must be available in the host
OS to protect the VMM and other VMs from the active virtual
machine. Examples include a protection primitive, address translation
primitive, or a sub-process primitive.
When the virtual machine traps because it has attempted to execute a
sensitive instruction, the host OS must direct the signal to the VMM.
Therefore, the host OS needs a primitive to perform this action. The
host OS also needs a mechanism to allow a VMM to run the virtual
machine as a sub-process. The VMM must be able to simulate sensitive
A highly secure Type II VMM will require a highly secure host OS
because it will depend upon the host OS. Flaws in the host OS would
undermine the security of the Type II VMM.
2.5 Hybrid VMM
Often, if a processor does not meet the Type I or Type II
VMM requirements, it can still implement a hybrid virtual machine
monitor (HVM). A hybrid VMM has all of the advantages of normal VMMs
and avoids the performance penalties of a CSIM. It is functionally
equivalent to the real machine. However, an HVM and a VMM differ in
that an HVM interprets every privileged instruction in software,
whereas a VMM may directly execute some privileged instructions. An
HVM treats the privileged mode of hardware as a pure software
construct. In both a VMM and an HVM, all non-privileged instructions
execute directly on the processor.
An HVM has less strict hardware requirements than a VMM in tow ways.
First, the HVM does not have to directly execute non-sensitive
privileged instructions because they are all emulated in software.
Second, because of the emulation, the HVM need not provide additional
mapping of the the most privileged processor mode into another
processor privilege level. However, increased interpretative
execution usually lowers the performance of an HVM relative to a
The hardware requirements for an HVM result in the following changes
to the original Type I VMM requirements. First, Requirement 1, which
states that the method of executing non-privileged instructions must
be roughly equivalent in both privileged and user mode, is
eliminated. Second, Requirement 3A, which states that if an
instruction attempts to change or reference the mode of the VM or the
state of the machine, there must be a way to simulate the
instruction, is weakened.
3. Pentium Architecture and
identified the key architectural features of third generation
hardware pertinent to virtual machines:
- two processor modes of operation,
- a method for non-privileged programs to call privileged system
- a memory relocation or protection mechanism such as segmentation
or paging, and
- asynchronous interrupts to allow the I/O system to communicate
with the CPU.
All of these still apply to the Intel Pentium architecture. It has
four modes of operation, known as rings, or current privilege level
(CPL), 0 through 3. Ring 0, the most privileged, is occupied by
operating systems. Application programs execute in Ring 3, the least
privileged. The Pentium also has a method to control transfer of
program execution between privilege levels so that non-privileged
tasks can call privileged system routines: the call gate. The
Pentium also uses both paging and segmentation to implement its
protection mechanisms. Finally, the Pentium uses both interrupts and
exceptions to allow the I/O system to communicate with the CPU. The
architecture has 16 predefined interrupts and exceptions and 224
user-defined, or maskable, interrupts.
Despite these features, the ability of the Pentium architecture to
support virtualization is likely to be serendipitous as the processor
was not explicitly designed to support virtualization. This section
reports an analysis of the virtualizability of the Pentium against
the hardware requirements described in Section 2
Every documented instruction for the Intel Pentium 3
was analyzed for its ability to support
Any instruction in the processor's instruction set that violates rule
1, 2, 3 (3A, 3B, 3C, or 3D) will preclude the processor from running
a Type I or Type II VMM. Additionally, any instruction that violates
rule 2, 3A in its weaker form, 3B, 3C, or 3D prevents the processor
from running an HVM. By combining these two statements, one can see
that any instruction that violates rule 2, 3A in its weaker form, 3B,
3C, or 3D makes the processor non-virtualizable.
With respect to the VMM hardware requirements listed above, Intel
meets all three of the main requirements for virtualization.
Requirement 1: The method of executing non-privileged
instructions must be roughly equivalent in both privileged and user
mode. Intel meets this requirement because the method for
executing privileged and non-privileged instructions is the same. The
only difference between the two types of instructions in the Intel
architecture is that privileged instructions cause a general
protection exception if the CPL is not equal to 0.
Requirement 2: There must be a method such as a protection
system or an address translation system to protect the real system
and any other VMs from the active VM. Intel uses both
segmentation and paging to implement its protection mechanism.
Segmentation provides a mechanism to divide the linear address space
into individually protected address spaces (segments). Segments have
a descriptor privilege level (DPL) ranging from 0 to 3 that
specifies the privilege level of the segment. The DPL is used to
control access to the segment. Using DPLs, the processor can enforce
boundaries between segments to control whether one program can read
from or write into another program's segments.
Requirement 3: There must be a way to automatically signal
the VMM when a VM attempts to execute a sensitive instruction. It
must also be possible for the VMM to simulate the effect of the
instruction. The Intel architecture uses interrupts and traps to
redirect program execution and allow interrupt and exception handlers
to execute when a privileged instruction is executed by an
unprivileged task. However, the Pentium instruction set contains
sensitive, unprivileged instructions. The processor will
execute unprivileged, sensitive instructions without generating an
interrupt or exception. Thus, a VMM will never have the opportunity
to simulate the effect of the instruction.
After examining each member of the Pentium instruction set, it was
found that seventeen instructions violate Requirement 3. All
seventeen instructions violate either part B or part C of Requirement
3 and make the Intel processor non-virtualizable. To construct a
truly virtualizable Pentium chip one must focus on these
instructions. They are discussed in more detail below.
3.1 Sensitive Register
Several Intel instructions break hardware virtualization
Requirement 3B. The rule states that instructions are sensitive if
they read or change sensitive registers and/or memory locations such
as a clock register and interrupt registers.
3.1.1 SGDT, SIDT, and SLDT Instructions
The SGDT, SIDT, and SLDT instructions violate this rule in
a similar way. In protected mode, all memory accesses pass through
either the global descriptor table (GDT) or local
descriptor table (LDT). The GDT and LDT contain segment
descriptors that provide the base address, access rights, type,
length, and usage information for each segment. The interrupt
descriptor table (IDT) is similar to the GDT and LDT, but it holds
gate descriptors that provide access to interrupt and exception
handlers. The GDTR, LDTR, and IDTR all contain the linear addresses
and sizes of their respective tables.
Table 1:Important CR0 Machine Status Word
Bits Bit Flag Name Description 0 PE - Protection Enable Enable
Protected Mode when set and real mode when clear 1 MP - Monitor
Coprocessor Controls the interaction of the WAIT or FWAIT instruction
with the TS flag. 2 EM - Emulation If clear, processor has an
internal or external floating point unit 3 TS - Task Switched Allows
delayed saving of the floating point unit context on a task switch
until the unit is accessed by the new task. 4 ET - Extension Type For
386 and 468 processors, indicates whether an Intel 387 DX math
coprocessor is present (hard-coded to 1 on Pentium processors). 5 NE
- Numeric Error Enables internal or PC-style mechanism for FPU error
All three of these instructions (SGDT, SIDT, SLDT) store a special
register value into some location. The SGDT instruction stores the
contents of the GDTR in a 6-byte memory location. The SLDT
instruction stores the segment selector from the LDTR in a 16 or
32-bit general-purpose register or memory location. The SIDT
instruction stores the contents of the IDTR in a 6-byte memory
location. These instructions are normally only used by operating
systems but are not privileged in the Intel architecture. Since the
Intel processor only has one LDTR, IDTR, and GDTR, a problem arises
when multiple operating systems try to use the same registers.
Although these instructions do not protect the sensitive registers
from reading by unprivileged software, the processor allows partial
protection for these registers by only allowing tasks at CPL 0 to
load the registers. This means that if a VM tries to write to one of
these registers, a trap will be generated. The trap allows a VMM to
produce the expected result for the VM. However, if an OS in a VM
uses SGDT, SLDT, or SIDT to reference the contents of the IDTR, LDTR,
or GDTR, the register contents that are applicable to the host OS or
Type I VMM will be given. This could cause a problem if an operating
system of a virtual machine (VMOS) tries to use these values for its
own operations: it might see the state of a different VMOS executing
within a VM running on the same VMM. Therefore, a Type I VMM or Type
II VMM must provide each VM with its own virtual set of IDTR, LDTR,
and GDTR registers.
3.1.2 SMSW Instruction
The SMSW instruction stores the machine status word (bits
0 through 15 of control register 0) into a general-purpose register
or memory location. Bits 6 through 15 of CR0 are reserved bits that
are not supposed to be modified. Bits 0 through 5, however, contain
system flags that control the operating mode and state of the
processor and are described in Table 1
Although this instruction only stores the machine status word, it is
sensitive and unprivileged. Consider the following scenario: A VMOS
is running in real mode within the virtual environment created by a
VMM running in protected mode. If the VMOS checked the MSW to see if
it was in real mode, it would incorrectly see that the PE bit is set.
This means that the machine is in protected mode. If the VMOS halts
or shuts down if in protected mode, it will not be able to run
This instruction is only provided for backwards compatibility with
the Intel 286 processor [16
]. Programs written for the
Intel 386 processor and later are supposed to use the MOV instruction
to load and store control registers, which are privileged
instructions. Therefore, SMSW could be removed and only systems
requiring backward compatibility with the Intel 286 processor would
be affected. Application software written for the Intel 286 and 8086
processors should be unaffected because the SMSW instruction is a
system instruction that should not be used by application
3.1.3 PUSHF and POPF Instructions
The PUSHF and POPF instructions reverse each other's
operation. The PUSHF instruction pushes the lower 16 bits of the
EFLAGS register onto the stack and decrements the stack pointer by 2.
The POPF instruction pops a word from the top of the stack,
increments the stack pointer by 2, and stores the value in the lower
16 bits of the EFLAGS register. The PUSHFD and POPFD instructions are
the 32-bit counter-parts of the POPF and PUSHF instructions. Pushing
the EFLAGS register onto the stack allows the contents of the EFLAGS
register to be examined. Much like the lower 16 bits of the CR0
register, the EFLAGS register contains flags that control the
operating mode and state of the processor. Therefore, the
PUSHF/PUSHFD instructions prevent the Intel processor from being
virtualizable in the same way that the SMSW instruction prevents
virtualization. In virtual-8086 mode, the IOPL must equal 3 to use
the PUSHF instructions. Of the 32 flags in the EFLAGS register,
fourteen are reserved and six are arithmetic flags. Table 2
describes the bits of concern.
Table 2:Important EFLAGS Register Bits Bit
Flag Name Description 8 TF - Trap Set to enable single-step mode for
debugging. 9 IF - Interrupt Enable Controls processor response to
maskable interrupt requests. 10 DF - Direction If set, string
instructions process addresses from high to low. 12-13 IOPL - I/O
Privilege Level I/O privilege level of the currently running task. 14
NT - Nested Task Set when the current task is linked to the previous
task. 16 RF - Resume Controls processor response to debug exceptions.
17 VM - Virtual-8086 Mode Enables Virtual-8086 mode when set. 18 AC -
Alignment Check Enables alignment checking of memory references. 19
VIF - Virtual Interrupt Virtual image of the IF flag. 20 VIP -
Virtual Interrupt Pending Indicates whether or not an interrupt is
pending. 21 ID - Identification If a program can set or clear this
instruction, the CPUID instruction is supported.
The POPF instruction allows values in the EFLAGS register to be
changed. Its varies based on the processor's current operating mode.
In real-mode, or when operating at CPL 0, all non-reserved flags in
the EFLAGS register can be modified except the VM, VIP, and VIF
flags. In virtual-8086 mode, the IOPL must equal 3 to use the POPF
instructions. The IOPL allows an OS to set the privilege level needed
to perform I/O. In virtual-8086 mode, the VM, RF, IOPL, VIP, and VIF
flags are unaffected by the POPF instruction. In protected mode,
there are several conditions based on privilege levels. First, if the
CPL is greater than 0 and less than or equal to the IOPL, all flags
can be modified except IOPL, VIP, VIF, and VM. The interrupt flag is
altered when the CPL is at least as privileged as the IOPL. Finally,
if a POPF/POPFD instruction is executed without enough privilege, an
exception is not generated. However, the bits of the EFLAGS register
are not changed.
The POPF/POPFD instructions also prevent processor virtualization
because they allow modification of certain bits in the EFLAGS
register that control the operating mode and state of the
3.2 Protection System References
Many Intel instructions violate Requirement 3C:
Instructions are sensitive if they reference the storage protection
system, memory or address relocation system.
3.2.1 LAR, LSL, VERR, VERW
Four instructions violate the rule in a similar manner:
LAR, LSL, VERR, and VERW. The LAR instruction loads access rights
from a segment descriptor into a general purpose register. The LSL
instruction loads the unscrambled segment limit from the segment
descriptor into a general-purpose register. The VERR and VERW
instructions verify whether a code or data segment is readable or
writable from the current privilege level. The problem with all four
of these instructions is that they all perform the following check
during their execution: (CPL DPL) OR (RPL DPL). This conditional
checks to ensure that the current privilege level (located in bits 0
and 1 of the CS register and the SS register) and the requested
privilege level (bits 0 and 1 of any segment selector) are both
greater than the descriptor privilege level (the privilege level of a
segment). This is a problem because a VM normally does not execute at
the highest privilege (i.e., CPL 0). It is normally executed at the
user or application level (CPL 3) so that all privileged instructions
will cause traps that can be handled by the VMM. However, most
operating systems assume that they are operating at the highest
privilege level and that they can access any segment descriptor.
Therefore, if a VMOS running at CPL 3 uses any of the four
instructions listed above to examine a segment descriptor with a DPL
3, it is likely that the instruction will not execute properly.
3.2.2 POP Instruction
The reason that the POP instruction prevents
virtualization is very similar to that mentioned in the previous
paragraph. The POP instruction loads a value from the top of the
stack to a general-purpose register, memory location, or segment
register. However, the POP instruction cannot be used to load the CS
register since it contains the CPL. A value that is loaded into a
segment register must be a valid segment selector. The reason that
POP prevents virtualization is because it depends on the value of the
CPL. If the SS register is being loaded and the segment selector's
RPL and the segment descriptor's DPL are not equal to the CPL, a
general protection exception is raised. Additionally, if the DS, ES,
FS, or GS register is being loaded, the segment being pointed to is a
nonconforming code segment or data, and the RPL and CPL are greater
than the DPL, a general protection exception is raised. As in the
previous case, if a VM's CPL is 3, these privilege level checks could
cause unexpected results for a VMOS that assumes it is in CPL 0.
3.2.3 PUSH Instruction
The PUSH instruction also prevents virtualization because
it references the protection system. The PUSH instruction allows a
general-purpose register, memory location, an immediate value, or a
segment register to be pushed onto the stack. This cannot be allowed
because bits 0 and 1 of the CS and SS register contain the CPL of the
current executing task. The following scenario demonstrates why these
instructions could cause problems for virtualization. A process that
thinks it is running in CPL 0 pushes the CS register to the stack. It
then examines the contents of the CS register on the stack to check
its CPL. Upon finding that its CPL is not 0, the process may
3.2.4 CALL, JMP, INT n, and RET
The CALL instruction saves procedure linking information
to the stack and branches to the procedure given in its destination
operand. There are four types of procedure calls: near calls, far
calls to the same privilege level, far calls to a different privilege
level, and task switches. Near calls and far calls to the same
privilege level are not a problem for virtualization. Task switches
and far calls to different privilege levels are problems because they
involve the CPL, DPL, and RPL. If a far call is executed to a
different privilege level, the code segment for the procedure being
accessed has to be accessed through a call gate. A task uses a
different stack for every privilege level. Therefore, when a far call
is made to another privilege level, the processor switches to a stack
corresponding to the new privilege level of the called procedure. A
task switch operates in a manner similar to a call gate. The main
difference is that the target operand of the call instruction
specifies the segment selector of a task gate instead of a call gate.
Both call gates and task gates have many privilege level checks that
compare the CPL and RPL to DPLs. Since the VM normally operates at
user level (CPL 3), these checks will not work correctly when a VMOS
tries to access call gates or task gates at CPL 0.
The discussion above on LAR, LSL, VERR, and VERW provides a specific
example of how running a CPL 0 operating system as a CPL 3 task could
cause a problem. The JMP instruction is similar to the CALL
instruction in both the way that it executes and the reasons it
prevents virtualization. The main difference between the CALL and the
JMP instruction is that the JMP instruction transfers program control
to another location in the instruction stream and does not record
The INT instruction is also similar to the CALL instruction. The INT
n instruction performs a call to the interrupt or exception handler
specified by n. INT n does the same thing as a far call made using
the CALL instruction except that it pushes the EFLAGS register onto
the stack before pushing the return address. The INT instruction
references the protection system many times during its
The RET instruction has the opposite effect of the CALL instruction.
It transfers program control to a return address that is placed on
the stack (normally by a CALL instruction). The RET instruction can
be used for three different types of returns: near, far, and
inter-privilege-level returns. Much like the CALL instruction, the
inter-privilege-level far return examines the privilege levels and
access rights of the code and stack segments that are being returned
to determine if the operation should be allowed. The DS, ES, FS, and
GS segment registers are cleared by the RET instruction if they refer
to segments that cannot be accessed by the new privilege level.
Therefore, RET prevents virtualization because having a CPL of 3 (the
VM's privilege level) could cause the DS, ES, FS, and GS registers to
not be cleared when they should be. The IRET/IRETD instruction is
similar to the RET instruction. The main difference is it returns
control from an exception, interrupt handler, or nested task. It
prevents virtualization in the same way that the RET instruction
3.2.5 STR Instruction
Another instruction that references the protection system
is the STR instruction. The STR instruction stores the segment
selector from the task register into a general-purpose register or
memory location. The segment selector that is stored with this
instruction points to the task state segment of the currently
executing task. This instruction prevents virtualization because it
allows a task to examine its requested privilege level (RPL). Every
segment selector contains an index into the GDT or LDT, a table
indicator, and an RPL. The RPL is represented by bits 0 and 1 of the
segment selector. The RPL is an override privilege level that is
checked (along with the CPL) to determine if a task can access a
segment. The RPL is used to ensure that privileged code cannot access
a segment on behalf of an application unless the application also has
the privilege to access the segment. This is a problem because a VM
does not execute at the highest CPL or RPL (RPL = 0), but at RPL = 3.
However, most operating systems assume that they are operating at the
highest privilege level and that they can access any segment
descriptor. Therefore, if a VM running at a CPL and RPL of 3 uses STR
to store the contents of the task register and then examines the
information, it will find that it is not running at the privilege
level at which it expects to run.
3.2.6 MOVE Instruction
Two variants of the MOVE instruction prevent Intel
processor virtualization. These are the two MOV instructions that
load and store control registers. The MOV opcode that stores segment
registers allows all six of the segment registers to be stored to
either a general-purpose register or to a memory location. This is a
problem because the CS and SS registers both contain the CPL in bits
0 and 1. Thus, a task could store the CS or SS in a general-purpose
register and examine the contents of that register to find that it is
not operating at the expected privilege level. The MOV opcode that
loads segment registers does offer some protection because it does
not allow the CS register to be loaded at all. However, if the task
tries to load the SS register, several privilege checks occur that
become a problem when the VM is not operating at the privilege level
at which a VMOS is expecting-typically 0.
The analysis of Section 3
shows that the
Intel processor is not virtualizable according to Goldberg's hardware
4. Pentium-Based ``VMM''
This section will examine several security issues for a
VMM designed for the Intel Pentium architecture. We begin with a
brief review of previous secure VMMs. Second, use of Intel processors
for highly secure systems is discussed. Third, ways to provide
virtual machine monitors on unmodified Intel platforms are examined
to gain insight into the challenges faced in a virtual machine
monitor effort. Next we discuss the security impact of using
unmodified Intel platforms for VMMs. Finally, a better approach to
creating a highly secure VMM on the Intel architecture is
4.1 Are Secure VMMs Possible?
An early discussion of VMMs and security argued that the
isolation provided by a combined VMM/OS provided better software
security than a conventional multiprogramming operating system. It
was also suggested that the redundant security mechanisms found in
the VMM and the OS executing in one of its virtual machines enhanced
experiments indicated that redundant weak implementations are
insufficient to secure a system [5
KVM/370 was an early secure Type I VMM [11
]. Called a ``security
retrofit,'' two approaches to the work were examined: (1)
``hardening'' of the existing VM/370 control program (CP) to repair
identified penetration vulnerabilities and (2) a redesign of the
VM/370 CP to place all security-relevant functionality within a
formally verified security kernel based upon the reference monitor
]. (Note that
the first approach was abandoned because flaw remediation did not
provide a guarantee of the absence of yet undetected, exploitable
security flaws.) The redesigned system consisted of four
minimized security kernel and verified trusted processes executing in
Semi-trusted processes executing in real problem state. These
processes managed some global data, were audited, had access only to
Non-kernel control programs (NKCPs) that executed the
non-security relevant bulk of the VM/370 control program in real
problem state. Each NKCP executed at a single security level and had
access only to virtual addresses.
4. User VMs
executing in real problem state under the control of a NKCP, with the
same security level as the NKCP.
A security kernel is defined as hardware and software that implements
the reference monitor concept [4
]. A reference monitor
enforces authorized access relationships between the subjects and
objects within a system. It imposes three design requirements on its
mechanism must be tamperproof.
mechanism must always be invoked.
mechanism must be small enough to be to subject to analysis and tests
to ensure completeness.
The VAX Security Kernel[17
was a highly secure Type I VMM. The system's hardware, microcode, and
software were designed to meet TCSEC Class A1 assurance and security
]. The project
also maintained standard VMS and Ultrix-32 interfaces to run COTS
operating systems and applications in virtual machines.
The VAX VMM security kernel allowed multiple virtual machines to run
concurrently on a single VAX system. It could support a large number
of simultaneous users and provided isolation and controlled sharing
of sensitive data.
The VAX processor, much like the Intel Pentium processor, contained
several sensitive, unprivileged instructions. It also had four rings.
The security kernel designers modified the VAX processor microcode to
make it virtualizable. The four instructions that prevented
virtualization on the VAX processor were: CHM, REI, MOVPSL, and PROBE
]. The CHM instruction
switches to a mode of equal or increased privilege. The REI
instruction switches to a mode of equal or decreased privilege. The
MOVPSL instruction is used to read the Processor Status Longword
(similar to the machine status word in the Intel architecture). The
PROBE instruction is used to determine the accessibility of a page of
memory. These four instructions read or write one of the following
pieces of sensitive data: the current execution mode, the previous
execution mode, the modify bit of a page table entry, and the
protection bit of a page table entry.
To support compatibility with existing operating systems and
applications, some of the microcode changes included: defining a new
VM mode bit, defining a new register called VMPSL, defining a
VM-emulation exception, as well as the four instructions described
Ring compression, implemented entirely in software, was used to avoid
certain processor modifications. The protection between compressed
layers is weakened; however, this choice had little security impact
since, although the VMS operating system for the VAX used all four
rings, all three inner rings were in fact used for fully trusted
operating system software.
The VAX I/O hardware was difficult to virtualize because its I/O
mechanisms read and write various control and status registers in the
I/O space of physical memory. To overcome this difficulty, the VAX
security kernel I/O interface used a special, performance-optimized
kernel call mechanism. To use this mechanism, a virtual machine
executed a Move To Privileged Register (MTPR) instruction to a
special kernel call register. The MTPR instruction trapped the
security kernel software that performed the I/O. Untrusted device
drivers were written for each guest OS in order to run on the
The VAX security kernel applied mandatory and discretionary access
controls to virtual machines. The kernel assigned every virtual
machine an access class consisting of a secrecy class (based on the
Bell and LaPadula model [6
and an integrity class (based on the Biba model [7
]). The kernel supported access
control lists on all objects including real devices, disk and tape
volumes, and security kernel volumes. The VMM security kernel
differed from a typical secure operating system because the subjects
and objects are virtual machines and virtual disks, not files and
processes, which are implemented by each guest OS.
It is worth noting that timing channels in VMMs [33
] were addressed in the
context of the VAX VMM work [14
]. Despite the challenge of
timing channel mitigation, VMMs provide a solution to the problem of
sharing while running legacy or commercial code securely with
firewalling between the VMs managed by a highly secure VMM
The VAX security effort lead to several conclusions: (1) Every ring
of a processor can be emulated, but this is often not necessary. (2)
Emulating a start I/O instruction is simpler and cheaper than
emulating memory-mapped I/O. (3) Defining the VM as a particular
processor or family of processors makes the VM more portable than if
it were a reflection of the actual hardware. For example, if a VM is
defined to be a Pentium processor, the VM will work on a Pentium II
or Pentium III processor. (4) VM performance suffers when sensitive
instructions are forced to trap to emulation software. (5) There are
alternatives to modifying the microcode support for every privileged
instruction to meet the needs of the VMM. (6) If a VMM is a security
kernel, dependencies between the VMM and VMs must be
The Alpha architecture is designed to support virtualization [2
]. It is designed to
contain no errors that would allow protection mechanisms to be
bypassed. Even ``UNPREDICTABLE'' results or occurrences are
constrained so that security and virtualization are supported. The
processor may hold or loose information as a result of ``UNDEFINED''
operations; however, these operations can only be triggered by
privileged software. Privileged Architecture Library code (PALcode)
provides a non-microcoded interface for privileged instructions. All
privileged instructions must be implemented in PALcode and may be
processor-model specific. The Alpha architecture supports PALcode
replacement, thus allowing per-OS code to yield high performance. A
VMM on the Alpha would have PALcode for all supported operating
Unlike the Alpha which explicitly forbids state data from registers
to be spread, most processors permit leakage of information from
unpredictable results. The multiple address spaces of the Intel x86
architecture family allows such leakage [35
The observations in this section should be considered in any attempt
to design a secure Type I VMM for the Intel Pentium
4.2 Pentium Security Support
Intel 80x86 processors provide support for well understood
requirements of secure systems [32
]. These include call gates,
segmentation, several hardware privilege levels and privileged
combination of segmentation and rings are particularly supportive of
secure system design and implementation [34
]. The processor family
was the choice for past and present trusted systems: the Boeing MLS
], Gemini Trusted Network
VSLAN (B2) [26
], TIS Trusted
], and the
XTS-300 (B3) [24
4.3 Pentium Virtualization
Since the Intel Pentium architecture is not truly
virtualizable, current VMMs for the hardware base [36
] must use a bit of
``trickery" to realize a VMM. Each method must detect sensitive
but unprivileged instructions before they are executed by a VM.
4.3.1 Pure Emulation
Pure emulation allows one system architecture to be mapped
into another system architecture. By modeling a large part of the x86
instruction set in software, emulation allows x86 operating systems
and applications to run on non-x86 platforms [19
]. The disadvantage of
emulation is significant performance degradation; no instructions are
ever executed directly on the hardware. The performance degradation
in Java compilers bears witness to this observation. Advances in
compiler technology can help, but without specialized machine
support, performance will never achieve that of a Type I VMM on a
comparable hardware base. Advanced techniques, such as dynamic
translation, can improve performance. Dynamic translation allows
sequences of small, x86 architecture code to be translated into
native-CPU code ``on-the-fly." Since the native code is cached
or even optimized, it can run significantly faster. This is the
approach used by Transmeta [18
], which provides pure
emulation and is a complete software interpreter machine (CSIM) [12
]. The use of register
shadowing and soft memory may permit support of VMM technology. It is
worth pointing out that in such systems, the morphed code must be
protected from tampering or leakage of secrets. It is not clear
whether such security concerns are addressed in the current
generation of binary translation systems.
4.3.2 OS/API Emulation
Applications normally communicate with an operating system
with a set of APIs. OS/API emulation [20
] involves intercepting and
emulating the behavior of the APIs using mechanisms in the underlying
operating system. The out-of-kernel OS emulation used for certain
Mach architectures [29
might be considered a variant of this approach. This allows
applications designed for other x86 operating systems to be run. This
strategy is used in Wine which provides ``an implementation of the
Windows 3.x and Win32 API on top of X and Unix" [37
]. Wine has a program loader
that allows unmodified Windows 3.1/95/NT binary files5
to run on Intel x86-based Unix machines, such
as Linux, FreeBSD, and Solaris. It allows application binaries files
to run natively and achieves better performance than the pure
emulation technique described above. However, OS/API emulation only
works on members of the x86 OS family for which the APIs have been
emulated. Furthermore, OS/API emulation is very complex. A VMM is
less complicated and requires fewer updates with each new release of
A third technique is virtualization. Most hardware is only
designed to be driven by one device driver. The Intel Pentium CPU is
not an exception to this rule. It is designed to be configured and
used by only one operating system. Features and instructions of the
processor designed for applications are generally not a problem for
virtualization and can be executed directly by the processor. A
majority of a processor's load comes from these types of
instructions. However, as discussed above, certain sensitive
instructions are not privileged in the Intel architecture, making it
difficult for a VMM to detect when they are executed. A strategy for
virtualizing the Intel architecture would be as follows:
- Non-sensitive, unprivileged application instructions can be
executed directly on the processor with no VMM intervention.
- Sensitive, privileged instructions will be detected when they
trap after being executed in user mode. The trap should be delivered
to the VMM that will emulate the expected behavior of the instruction
- Sensitive, unprivileged instructions must be detected so that
control can be transferred to the VMM.
The hardest part of the virtualization strategy is handling the
seventeen problem instructions described in Section 3
. Lawton describes how this is accomplished for
It analyzes instructions until one of the
following conditions is encountered:
1. A problem
2. A branch
address of an instruction sequence that has already been
If 1 or 2 is encountered, a breakpoint must be set at the beginning
of the problem or branch instruction. If 3 is encountered, execution
continues normally since this code has been analyzed already and
necessary breakpoints have been installed. The complexity of this
approach may render a highly secure VMM unachievable.
Code is allowed to run natively on the processor until it reaches a
breakpoint. If the breakpoint occurred because of a problem
instruction, its behavior is emulated by the VMM. If the breakpoint
occurred because of a branch instruction, it is necessary to single
step through its execution and begin analyzing instructions again at
the branch target address. If the target address is not computed and
has already been analyzed and marked as safe, then the branch
instruction can also be marked as safe and it can run natively on the
processor on subsequent accesses. Computed branch addresses require
special attention. These instructions must be dynamically monitored
to ensure that execution does not branch to unanalyzed code. A table
might be used to keep track of the breakpoints.
Some instructions may write into memory, possibly into the address of
instructions that have already been analyzed and marked as safe. The
paging system is used to prevent this by write protecting any page of
memory in the page tables that has already been analyzed and marked
as safe. All page entries that point to the physical page with
analyzed code would have to be write protected since multiple linear
addresses can be mapped to the same physical page. When a
write-protect page fault occurs, the VMM can unprotect the page and
step through the instructions. A breakpoint can be installed before
any problematic instructions. Finally, the page should be
write-protected again. Instructions that cross page boundaries
involve tow write-protected pages. Tables are used to track
previously analyzed instructions.
Also pass-through I/O devices, timing issues, and virtualizing
descriptor loading must be addressed.
Pass-Through I/O Devices: It may be useful to allow a device
driver in the guest OS to drive hardware for a device that is not
supported by the host OS. For example, a Linux host OS will not
support a Winmodem. Pass-through devices allow a guest OS to
communicate with devices using a pass-through mechanism that handles
I/O reads and writes. Because control of the real hardware is turned
over to the VMOS, pass-through I/O devices render security
Timing: A VMM must accurately emulate system timers. Every
time slice of native code execution is bounded by an exception
generated by the system timer when the execution time slice is over.
The exception vectors to a routine defined in the VMM's IDT for a
guest OS. A mechanism is needed that measures the time between these
exceptions to emulate an accurate timer. On Intel Pentium processors,
performance monitoring could be used. The RDTSC, Read Time Stamp
Counter, instruction gives an accurate time stamp reading. The
instruction is also executable in CPL 3, allowing efficient use in
user-level VMM code.
Virtualization of Descriptor Loading: For two reasons a
Pentium-based VMM must have its own set of LDT, GDT, and IDT tables.
First, it allows the segment register mechanisms to work naturally.
Second, it allows the VMM to have its own set of exception
Since all privilege levels (0-3) in a VM are mapped into CPL 3, the
CPL is not sufficient when trying to load code that is more
privileged (i.e. numerically less) than CPL 3. CPL 3 code can load
descriptors as expected as long as the GDTR and LDTR registers point
to the guest OS's descriptor tables. When running system code in CPL
3, exceptions are generated when loading a descriptor with that has
CPL 3. This does not occur when system code is executed at CPL 0. To
solve this problem, one must trap and emulate instructions that load
the segment registers when running at CPL 3. All instructions that
examine segment registers with PL 3 must be virtualized because they
may look at the RPL field.
A private GDT and LDT for the virtualization of code at CPL 3 can
also help solve this problem. Since, the instructions that reference
the GDTR and LDTR are emulated, they can be loaded with values that
point to the private GDT and LDT. The private descriptor tables would
start out empty and generate exceptions when a segment register
loads. When this happens, a private descriptor is generated that
allows the next segment register load to execute natively. Every time
the GDTR and LDTR are reloaded, the private descriptor tables are
4.3.4 Other Virtualization Considerations
Disco is an implementation of a Type I VMM for the Flash
runs several different commercial operating systems on virtual
machines to provide high-performance system software. Some of the key
insights of the Disco implementation applicable to virtualizing the
Intel Pentium architecture are described below.
Virtual CPUs: Multiple VMMs are multiplexed onto a common
physical processor by using virtual processors. A data structure is
kept for each virtual CPU that contains register contents, TLB
contents, and other state information of the virtual CPU when it is
not running on the real CPU. The VMM is responsible for managing the
virtual CPUs and ensures that the effects of traps are handled
properly by the executing virtual processor.
Virtual Physical Memory: To virtualize physical memory, an
extra level of address translation that maintains VM
physical-to-machine address mappings is used. Virtual machines are
given physical addresses that start at address zero and continue to
the size of the VM's memory. These physical addresses could be mapped
to machine addresses used by the Intel processor using the
hardware-reloaded TLB of the Intel processor. The VMM protects and
manages the page table. When the VMOS tries to insert a
virtual-to-physical mapping in the TLB, the VMM emulates this by
translating the physical address into the corresponding VM address
and inserting this into the TLB.
Virtual I/O Devices: The VMM must intercept device accesses
from virtual machines and forward them to physical devices. Instead
of trying to use every device's real device driver, one special
device driver for each type of device is used. Each device has a
monitor call that is used to pass all command arguments to the VMM in
a single trap. Many devices such as disks and network interfaces
require direct memory access (DMA) to physical memory. Normally these
device drivers use parameters that include a DMA map. The VMM must
intercept these DMA requests and translate physical addresses into
We note that since the VMM must control devices, a VMM for the Intel
Pentium architecture must be provide device drivers for each VMOS.
Loadable drivers would be particularly convenient.
Virtual Network Interface: So that VMs can communicate with
each other, they use standard distributed protocols such as NFS.
Disco manages a virtual subnet that allows this communication. A
copy-on-write strategy for transferring data between VMs reduces the
amount of copying. Virtual devices use Ethernet addresses and do not
limit the maximum transfer unit of packets.
4.4 Unmodified Pentiums: VMM Security
To be a high-assurance secure computing system, security
policies are correctly enforced, even under hostile attack. Examples
of such systems are at least TCSEC Class B2 or an equivalent level in
the Common Criteria [1
]. The systems'
protection mechanisms must be structured and well-defined. When
dealing with highly sensitive information, labels are needed to order
information into equivalence classes. Also, for environments where
users are also categorized into equivalence classes based on
clearances or other ordering techniques, a very effective protection
mechanism is needed.
Current VMMs for the Intel architecture do not meet these
requirements although some vendors claim security as a feature [31
]. One claims that their
product can ``isolate and protect each operating environment, and the
applications and data that are running in it" [36
]. Another claim is that the
system does ``not make any assumptions concerning the software that
runs within the virtual machine. Even a rogue application or
operating system is confined..." Given such claims, it is
worthwhile to ask how well current VMMs can enforce the VM isolation
needed to support a mandatory security policy. Note that this
analysis is based on assumptions regarding how virtualization is
being accomplished. The following sections describe some potential
problems if such systems were to be used to separate mandatory
4.4.1 Resource Sharing
A problem results from resource sharing between virtual
machines. If two virtual machines have access to a floppy drive,
information can flow from one VM to the other. Files could be copied
from one VM to the floppy, thus giving the other VM access to the
4.4.2 Networking and File Sharing
A similar problem results from support of networking and
file sharing. Here two virtual machines at different security levels
could communicate information. Exploitable mechanisms include
Microsoft Networking, Samba, Novell Netware, Network File System, and
TCP/IP. For example, using TCP/IP, a VM could FTP to either a host OS
or guest Linux OS and transfer files.
4.4.3 Virtual Disks
The ability to use virtual disks is also a problem. A
virtual disk is a single file that is created in the host OS and used
to encapsulate an entire guest disk, including an operating system
and its applications. Anyone with access to this file in the host
operating system could copy all information in the virtual disk to
external media. The attacker could then install the virtual machine
monitor on his own system and open the copied virtual disk.
Another problem is that any host OS application with read access to
the file containing the virtual disk can examine the contents of
virtual disk. For example, host OS file utilities such as grep can be
used to search for specific strings in the virtual file system. Our
tests using a Linux host OS and a Windows NT guest OS showed that a
sensitive string could be located by grep in seconds on an
approximately 300 MB virtual disk.
Both problems could be remedied by restricting access to the virtual
file. Yet, to achieve this with any measure of assurance, a secure
host OS is required.
4.4.4 Program Utilities
Tools for virtual machine interoperation may cause
problems. For example, after installing VMware-Tools [36
] in a guest OS, the cursor
can move freely between the host OS desk-top and those of the VMs.
Another feature is the ability to cut and paste between virtual
machines using a feature similar to the Windows clipboard. The
potential security danger if virtual machines were running at
different mandatory security levels is obvious.
4.4.5 Host Operating System
For a Type II VMM, many security vulnerabilities emerge
due to the lack of assurance available in the underlying host
operating system. Flaws in host OS design and implementation will
render the virtual machine monitor and all virtual machines
4.4.6 Serial and Printer Ports
Implementation of serial and printer ports presents
another security problem. Before starting a virtual machine, a
configuration of the guest OS must be loaded or created. A
configuration option for parallel and serial ports is to have output
of all parallel/serial ports go to a file in the file system of host
OS. Thus on the guest OS, user attempts to print will result in
output to a host OS file. Users could easily transfer information so
that others could read the printer file in the host OS if its
permissions were not managed carefully.
4.5 Intel-Based VMM for High
We conclude that current VMMs for the Intel architecture
should not be used to enforce critical security policies.
Furthermore, it would be unwise to try to implement a high assurance
virtual machine monitor as a Type II VMM hosted on a generic
commercial operating system. Layering a highly secure VMM on top of
an operating system that does not meet reference monitor criteria
would not provide a high level of security.
Yet the Intel Pentium processor architecture has many features that
can be used to implement highly secure systems. How can these be
A better approach would be to build a Type I VMM as a microkernel.
The secure microkernel could be very small, making it easier for the
VMM to meet the reference monitor verifiability requirement. The use
of minimization, rigorous engineering, and code correspondence
contribute to ensuring that the implementation is free of intentional
as well as accidental flaws.
The Type I VMM would provide virtual environments on the machine. It
would intercept all attempts to handle low-level hardware functions
from the VMs and would control all of the devices and system features
of the CPU. The microkernel could allow each VM to choose among a
specific set of virtual devices, which may or may not map directly to
the real devices installed on the system.
There are two advantages to using a Type I VMM to separate mandatory
security levels. First, a Type I VMM can provide a high degree of
isolation between VMs. Second, existing popular commercial operating
systems for the processor and their applications can be run in this
highly secure environment without modification. A VMM eliminates the
need to port software to a special secure platform and supports the
functionality of current application suites.
The biggest disadvantage to a Type I approach is that device drivers
must be written for every device. This is a problem because of the
wide variety of peripheral types and models available. (Note that a
less secure Type II VMM avoids this problem by using existing drivers
written for the host OS.) This disadvantage can be overcome when
developing a secure solution by only supporting certain types and
manufacturers of devices. It is not out of the ordinary for highly
secure solutions to require specific types of hardware.
Before trying to implement a secure Type I VMM for the Pentium, it
might be advantageous to modify the chip. Two alternative
modifications could make virtualization easier. First, all seventeen
unprivileged, sensitive instructions of the Intel architecture could
be changed to privileged instructions. All instructions would trap
naturally and the VMM could emulate the behavior of the instruction.
However, this solution may cause problems in current operating
systems because these seventeen instructions would now trap.
An alternative is to implement a trap on op-code instruction [12
]. A new instruction is
added that allows an operating system to declare instructions that
should be treated as if they were privileged. This makes
virtualization easier without affecting current operating
Other virtualization approaches require additional code to force
sensitive, unprivileged instructions to be handled by VMM software.
As a result, two security concerns arise. First, the security kernel
may not be considered minimal because of the extra virtualization
code. Second, virtualization of the unmodified processor requires
checking every instruction before it executes. Such checking is
likely to doom to failure creation of a high assurance VMM.
5. Conclusions and Future
The feasibility of implementing a secure virtual machine
monitor on the Intel Pentium has been explored. VMM types and their
hardware requirements were reviewed. Then, a detailed study of the
virtualizability of all 250 Pentium instructions was conducted to
determine if the processor could meet the hardware requirements of
any type of VMM. The analysis showed that seventeen instructions did
not meet virtualization requirements because they were sensitive and
After defining a strategy to ``virtualize" the Pentium
architecture, an analysis was conducted to determine whether a
Pentium-based secure virtual machine monitor is able to securely
isolate classified from unclassified virtual machines could be built.
We conclude that current VMM products for the Intel architecture
should not be used as a secure virtual machine monitor.
The Intel Pentium processor family already has many features that
support the implementation of highly secure systems. Slight
modifications to the processor would significantly facilitate
development of a highly secure Type I VMM.
An effort is currently underway to examine the Intel IA64
architecture to determine how its new relate to the construction of
secure systems and virtualization. The possible use of virtualization
techniques for processors supporting fast binary translation is also
The authors wish to acknowledge the insight, guidance and suggestions
made by Steve Lipner as this research progressed. We are grateful to
Dr. Paul Karger for careful review of our manuscript, suggestions and
encouragement. We wish to thank James P. Anderson for unflagging
encouragement of our work and Timothy Levin for insightful
discussions and review of the paper. We are grateful to the
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About this document ...
Analysis of the Intel Pentium's Ability
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