hypervisor framework
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Private & Confidential Property of COSEINC
Who am I?
• Senior Security Researcher at COSEINC
• One of the developers of BluePill, a hardware-based virtualization rootkit.
• Creator of one of the most effective methods to detect virtualization rootkits.
• Experience with systems programming (kernel, device drivers) and reverse engineering for x86/x64 architectures.
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1. Review of the implementation methods for virtualization of the x86 architecture.
2. Show the complexity of using hardware supported virtualization instructions to implement virtual machines.
3. Present a framework that makes easy the task of creation of hypervisors.
4. Applications of the framework
5. Security aspects
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X86 VIRTUALIZATION The COSEINC Hypervisor Framework
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• System Virtual Machines: VM able to run multiple operating systems concurrently
• The code responsible for the virtualization is called (VMM).
• Provides isolation between the guest OS
• Physical hardware resources are shared between the multiple virtual machines
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IA-32 processor
VMM
Windows guest
Linux guest
1. Type I (native) The VMM runs directly on the host’s hardware.
Hardware resources controlled by the VMM. Examples: VMware ESX, Microsoft Hyper-V
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IA-32 processor
VMM
Windows guest
Linux guest
Operating System
• Type II – Hosted The VMM runs as an application. Hardware
resources controlled by the host OS. The COSEINC hypervisor framework creates a Type-II VMM.
Examples: VMware Workstation.
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• When the guest VM uses the same Instruction Set Architecture (ISA) of the host machine, the guest instructions can be executed in 2 ways:
– Emulation
– Direct native execution
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• The VMM must read and interpret each guest instruction
• Can be implemented using code interpretation or binary translation
• Performance penalty
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• The guest instructions are executed directly on the CPU.
• Great performance.
• Some instructions still need to be emulated.
• How to decide which instructions can be used for direct native execution?
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• Popek and Goldberg published a paper which formally defines the requirements of an ISA for the implementation of virtual machines.
• The VMMs must have 3 properties:
– 1. Equivalence
– 2. Resource control
– 3. Efficiency
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• Basically all the VMM detection methods are based on violations of the Equivalence property.
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• Violation: VMM bug exploitation.
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• Depends on the features of the host ISA.
• How to implement efficient virtual machines on the x86 architecture?
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• Innocuous instructions are instructions which doesn’t change or affect system configuration or resources.
• A efficient VMM allows the direct execution of innocuous instructions.
• Examples: – mov eax, 00204012h
– shr ebx, 03
– xor eax, eax
– cmp ebx, ecx
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• Sensitive instructions affect system resources or behavior
• The VMM must the direct execution of sensitive instructions!
• The IA-32 instruction set contains 17 sensitive instructions [2]
• Examples: – wrmsr – mov CR3, eax – out dx, eax
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• All the VMM need now is a way to intercept the execution of the sensitive instructions.
• This is easy when the sensitive instruction is privileged!
• A sensitive instruction is if it traps if the machine is executing in user mode and does not trap in system mode.
• In the x86 architecture, system mode = CPL zero (ring 0)
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INNOCUOUS INSTRUCTIONS
KERNEL code
cmp eax, ebx jnz 8c0dab00 xor edx, edx mov eax, 030h
wrmsr
cmp eax, 020Fh jnz 08000bc00 shr eax, 8
SENSITIVE INSTRUCTION
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Set CPL to RING 3 and execute the code directly on
the cpu
KERNEL code
cmp eax, ebx jnz 8c0dab00 xor edx, edx mov eax, 030h
wrmsr
cmp eax, 020Fh jnz 08000bc00 shr eax, 8
#GENERAL PROTECTION
FAULT
VMM trap handler routine
(emulation)
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• Virtualization of guest instruction would be very easy if all sensitive instructions generates a fault in ring 3.
• There are sensitive but non-privileged instructions in the x86 architecture!
• A sensitive non-privileged instruction will not generate an exception in ring 3!
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• POPFD instruction writes a DWORD value in the EFLAGS register.
• It’s a sensitive instruction because it can be used to set the IF flag.
• The IF (Interrupt Flag) controls the hardware external interrupt mechanism.
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• Problem: Executing POPFD in ring3 will not generate a fault! The CPU just ignores the IF flag modification attempt.
• How to virtualize sensitive non-privileged instructions?
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• How VMware Player VMM is able to prevent direct execution of non-privileged instructions?
• VMware Player is a Type II VMM
• The hypervisor is stored as a PE resource inside the vmware-vmx.exe executable.
• ELF executable loaded directly inside the Windows kernel memory by the vmx86.sys device driver
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vmplayer.exe
vmware-vmx.exe
USER MODE
KERNEL MODE
vmx86.sys
ntoskrnl.exe
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Vmware Hypervisor
ELF executable stored as a PE resource
• Solution: Scan all the guest code instructions and search for non-privileged instructions.
• Replace the non-privileged instructions by a privileged instruction.
• VMM handles the faults and emulates the execution of the non-privileded instruction.
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1. Review x86 virtualization implementation methods.
2. Show how to use the Intel VT® to implement virtual machines.
3. Present a framework to make easy the task of creation of hypervisors.
4. Applications of the framework
5. Security and detection discussion
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The COSEINC Hypervisor Framework
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• Virtualizable ISA
– If all sensitive instructions of some ISA are privileged, the processor is considered to be ‘virtualizable’[3]
• IA-32 is obviously not-virtualizable.
• New instruction sets created by Intel and AMD
– Intel Virtual Machine eXtensions (VMX)
– AMD Secure Virtual Machine (SVM)
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• Presentation focus on Intel VMX. AMD SVM concepts are very similar.
• New form of processor operation: the ‘VMX operation mode’
• VMX mode
– activated by the VMXON instruction.
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• VMXON fails if virtualization is locked.
• Locked by default in the BIOS for security reasons
• Ring -1.
• There’s no more need to move kernel guest code from ring 0 to ring 3. Guest kernel code can run directly in ring 0.
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• 2 types of VMX operation: – VMX root operation
– VMX non-root operation
• VMX root operation – New instructions available (VMX instructions)
– Used by the VMM (hypervisor)
• VMX non-root operation – Restricted mode of operation
– Certain instructions and events are intercepted to facilitate virtualization.
Private & Confidential Property of COSEINC
• Transitions between VMX root operation and VMX non-root operation are called ‘VMX transitions’
• Transition from the VMM to the guest: VM-ENTRY.
• Transition from the Guest VM to the VMM: VMEXIT
Private & Confidential Property of COSEINC
Hypervisor (vmx root operation)
VIRTUAL MACHINE
(vmx non-root operation)
VM-ENTRY – vmresume/vmlaunch
VM-EXIT event interception
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Creating a VMM with Intel VT® - first steps
• Detection of Intel VMX instruction support.
– CPUID
• Enable VMX (CR4)
– VMXE bit
• Check status of the Lock bit (rdmsr)
– More about in the security section
• Setup of the VMXON region
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Creating a VMM with Intel VT® - first steps
• Enable VMX instructions (VMXON)
• Create and configure the VMCS region of each guest VM.
• Launch the guest VM with VMLAUNCH instruction
• Wait for VM-exit events
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VMCS
• Virtual Machine Control Structure
• Most important vmx data structure
• One VMCS for each Virtual Machine and for each CPU core.
• It controls the behavior of VMX transitions
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VMCS
• VMM must not access the VMCS directly.
• Read and write access to the VMCS via VMREAD and VMWRITE instructions.
• Internal structure undocumented but reverse engineering it is easy.
Private & Confidential Property of COSEINC
VMXON and VMCS areas VMXON region
CPU A
VM Linux
VM Windows
VMXON region CPU B
VMCS #1A
VMCS #2A
VMCS #1B
VMCS #2B
CPU A CPU B
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VMCS logical groups
Guest-state area
Host-state area
VM-execution control fields
VM-exit control fields
VM-entry control fields
VM-exit information fields
4K-aligned physical address
6 logical areas
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Guest-state area
• Area of the VMCS where guest context information is stored.
• On #VMEXIT, guest processor state is saved in this area.
• On VMENTRY this information is loaded. • Register state:
– Control Registers – Debug Registers – RSP, RIP, RFLAGS – LDTR, GDTR, IDTR – Segment selectors – Model Specific Registers
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Guest-state area
• Non-register state
– Activity State
– Interruptibility state
– VMCS link pointer
• For future expansions
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Host-state area
• Contains information about the host (VMM)
• Processor stated is loaded from this area after each #VMEXIT
• Registers:
– RIP (Entry-point address of the hypervisor routine responsible for handling #VMEXIT events)
– RSP, RFLAGS
– MSR
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VM-execution control fields
• Controls how the VM will be executed. • The instructions that the hypervisor wants to intercept are
specified in these control fields. – Example: HLT, INVLPG, MWAIT, RDPMC, RDTSC, MOV-DR
• Exception bitmap – Bitmap which controls interception of CPU interrupts like page
faults, debug exceptions, #GP, ...
• I/O bitmap – Can be used to control interception of I/O ports
• MSR bitmap – Interception of Model Specific Registers
• Some instructions wil unconditionally result in VMEXIT
Private & Confidential Property of COSEINC
VM-entry control fields
• Controls the behavior of VM entries.
• Includes information about SMM, debug registers and some MSRs.
• Guest Event Injection:
– It’s possible to inject virtual interrupt or exception in the guest
– Types of interrupts allowed:
• External, NMI, Hardware exceptions, software interrupt.
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VM-exit fields
• `VM-exit control fields` which controls the behavior of VM exits.
• VM-exit information fields:
– Read-only fields with information about the most recent VM exit
– Exit reason
– Exit qualification
Private & Confidential Property of COSEINC
Interception
• After configuring the VMCS, the hypervisor can launch the virtual machine and wait for a VMEXIT event.
• When a instruction is intercepted in the guest, the processor will: – Save the VM-exit reason information in the VMCS
– Save guest context information
– Load the host-state area
– Transfer control to the hypervisor
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VMX ROOT-MODE RING 0
VMM
#VMEXIT event handler
VMX NON-ROOT RING 0
mov eax, 23 inc edx
xor ebx, edx sub ecx
cmp eax, 1 jnz c080df00
retn
mov cr3, ebx #VMEXIT
VMLAUNCH
#VMRESUME
Private & Confidential Property of COSEINC
1. Review x86 virtualization implementation methods.
2. Show how to use the Intel VT® to implement virtual machines.
3. Present a framework to make easy the task of creation of hypervisors.
4. Applications of the framework
5. Security and detection discussion
Private & Confidential Property of COSEINC
• Creating a VMM using these new hardware virtualization ISA is complex – More complex features always comming: EPT for
nested paging
• Very hard to find and to fix bugs • No debugger • Intel VT error codes not very useful
– Code 33 = “VM-entry failure due to invalid guest state”
– What’s exactly invalid in the guest state? – More than 40 suspects!
Private & Confidential Property of COSEINC
• The COSEINC Hypervisor Framework, referred from now as just the ‘framework’, enables you to easily create a Hosted Virtual Machine Monitor (Type II VMM) using the Windows Operating System.
• Simple and easy-to-use API exported
• Abstraction over the different hardware virtualization instruction sets (VMX-SVM)
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• 2 versions: – 32-bits Windows device driver
– 64-bits Windows device driver
• API exported methods: – Export table
– IOCTL codes for user-mode communication
• Initial version only for Windows, but porting to Mac/Linux should not be difficult.
• Release date: very soon!
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Features
• Automatic detection of the virtualization instruction sets.
• SMP support
• Evaluation of the lock bit
• Detailed error-status codes
• Plugin-like architecture
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Architecture
Framework
Ring 0 Operating System
Kernel
Ring -1
User applications Ring 3
Framework Client
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API
• The full documentation of the API will be released with the framework.
• Preliminary documentation. Subject to change.
• Function categories: – Virtual Machine management functions
• Creation and deletion of Virtual Machines.
• Executing and resuming a virtual machine.
– Interception Events functions • The framework call the registered client function callbacks.
– Root guest VM.
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Virtual Machine management
• VMSTATUS CreateVirtualMachine ( IN VMINFO *vminfo );
• This function creates a new virtual machine in the system.
• Fails if virtualization MSR is locked by the BIOS.
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VMINFO data structure
• Most important framework data structure
• Contains all the information needed to create and control a VM:
– all the GUEST context information
– GDT, LDT, Page Tables, Control Registers, ...
– Interception handler function callback address.
– Contains Event Injection information
– VMEXIT information
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GUEST_INFO
Registers
Segments
Descriptor Tables
Control
Debug
Model Specific
CONTROL_INFO
Interception
Event Injection
I/O
Interrupts
MSR
Virtual Machine
VMINFO data structure
VMEXIT info Extra info
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Interception Event management
• VMSTATUS VirtualMachineExec ( IN VMINFO *vminfo );
• This function controls the execution of the virtual machine. It can be called after the creation of the VM and to resume the execution of the VM after an intercept event.
• If the VMM must inject some event in the guest VM, the information is provided in the VMINFO data structure.
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x
VM creation and execution
Framework Client (VMM
plugin)
Framework
CreateVirtualMachine( )
Intercept event
handler
Intercept Event Message
VirtualMachineExec( )
VirtualMachineExec( )
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VM
VM Scheduler
VM message handler
Timer interrupt
VM Event Router
Virtual Machine
VM Event Manager
Hypervisor
Framework – Client communication
Root guest VM
• One of the best features of the framework:
– Automatic conversion of the host operating system into a virtual machine in runtime!
• This guest VM is called ‘root VM’
• The creation of the root VM is optional and controlled by the api.
• Root VM is shared between all loaded plugins.
Private & Confidential Property of COSEINC
1. Review x86 virtualization implementation methods.
2. Show how to use the Intel VT® to implement virtual machines.
3. Present a framework to make easy the task of creation of hypervisors.
4. Applications of the framework
5. Security and detection discussion
Private & Confidential Property of COSEINC
The COSEINC Hypervisor Framework
Private & Confidential Property of COSEINC
Applications of the framework
• Specially useful for education and research purposes
• Can abe used to create any type of small and fast VM. Not only system VMs.
• The best features are available when using the root guest VM.
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Process VM
• Whole virtualization of a process or a thread is possible with the framework.
• Normally achieved by interception of system calls.
• Additional functions will be added to the API for better memory virtualization.
• No support for EPT in the first version.
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Syscall hooking
• A great number of system monitoring and security tools are implemented using system call hooking methods.
• Old Windows OS uses INT 2eh
• Linux and newer Windows OS uses SYSENTER instructions
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Syscall mechanism - illustration
SYSENTER_EIP MSR
SYSENTER_CS MSR
mov edx, esp sysenter
mov ecx, 23h push 30h pop fs ...
nt!KiFastCallEntry
Ntdll.dll
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Windows OS syscall mechanism
Syscall hooking
• Syscall hooking methods includes:
– Patching syscall handler
– Patching of IDT table
– Patching the SYSENTER Model Specific registers
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Syscall interception
• Syscall interception using the root guest VM • No need to hook SSDT • No need to patch/modify guest kernel code • Virtualization of the SYSENTER MSR • Plugin (framework)
– VMINFO->ControlInfo->Interception->MSR
• Can also be applied to Linux guests • Virtualized IDTR for old guest operating systems
using INT xx instructions for syscall implementation.
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Instrumentation
• Instrumentation is also easy to implement using the Interruptibility controls in the VMCS.
• Performance registers are also virtualizable
• Tools:
– Optimization tools
– System statistics
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Nested virtualization
• The framework doesn’t provide support for nested virtualization
• But it is possible to add this feature via a VMM plugin.
• Also, a virtualization debugger could be implemented!
Private & Confidential Property of COSEINC
1. Review x86 virtualization implementation methods.
2. Show how to use the Intel VT® to implement virtual machines.
3. Present a framework to make easy the task of creation of hypervisors.
4. Applications of the framework
5. Security and detection discussion
Private & Confidential Property of COSEINC
64-bits
• The framework and the plugins must be digitally signed to run in 64-bit versions of Windows.
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• MSR IA32_FEATURE_CONTROL (Index 3Ah)
• Controls:
– SMX – Safer Mode eXtensions
• Disabled by default in the BIOS
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• “There is no software-visible bit whose setting indicates whether a logical processor is in VMX non-root operation. This fact may allow a VMM to prevent guest software from determining that it is running in a virtual machine.” – Intel manual 3 – 19.3
• VMX transitions are cpu-expensive operations.
• Thousand of cycles just for a simple VMEXIT.
• SyScan 2007 – Detecting BluePill
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QUESTIONS?
THANK YOU FOR YOUR TIME!
1. John Scott Robin and Cynthia E. Irvine (2000). "Analysis of the Intel Pentium's Ability to Support a Secure Virtual Machine Monitor". Proc. 9th USENIX Security Symposium.
2. Virtual Machines: Versatile Platforms for System and Processes – Jim Smith, Ravi Nair – Morgan Kaufmann - 2005
3. Intel manuals (www.intel.com)
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