hvpp

hvpp is a lightweight Intel x64/VT-x hypervisor written in C++ focused primarily on virtualization of already running operating system.

Motivation

Althoughseveralopen-sourceresearchhypervisors aimedatsimplicityalreadyexist,in my opinion this field is still somewhat unexplored and needs more open-source projects. This can especially help those who have just started exploring virtualization technologies and are looking for small/reference projects. If you're one of them, my bets are that you're really disappointed right now, because all you've got are barely dozen of (great!) projects and huge pile of Intel Manual pages.

C++ has been chosen as a language for this project because of two reasons:

• The virtualization architecture can be better expressed in OOP concepts (with such objects as VCPU, EPT).
• I didn't find other projects which would use modern C++17 features, except forbareflank.Although bareflank is compilable under Windows, Linux and UEFI, on Windows, it uses cygwin to cross-compile the hypervisor. Since the hypervisor is a self contained ELF binary, the Windows kernel is missing the debug symbols for the hypervisor, which prevents easy debugging with WinDbg.

Even though this project is primarily developed for Windows, I've decided to not use traditionalWindows Driver naming convention(akaDrvCamelCase). Instead, traditional C++snake_caseis used. The reason is that hypervisor is very "stand-alone" and doesn't depend on many OS functions. Therefore I decided to treat it as a regular C++ project.

If you want to challenge yourself in low-level programming, my advice would be to try and write a simple hypervisor. During the process you'll get invaluable knowledge and experience and you'll probably discover many new things. For instance here's a selection of some things I've learned thanks to writing this project:

• what areMTRRgood for
• NT APIs like
  • MmGetPhysicalMemoryRanges()(and\REGISTRY\MACHINE\HARDWARE\RESOURCEMAP)
  • KeIpiGenericCall- which can be used instead of undocumentedKeGenericCallDpc(but at the cost of higher IRQL)
• thatTraceLoggingAPI (built uponETW) exist
• how Meltdown vulnerability has been mitigated in Windows
• that VMWare uses not only I/O port 0x5658for backdoor, but also port 0x5659and it uses almost all GP registers for both input and output parameters
• thatbugsinotherhypervisorsexist (and found a way how to fix them)
• that VirtualBoxuses ring 1for software-based virtualization (i.e. when VT-x/AMD-V isn't available)
• how guest memoryhas been virtualized before EPT(by so called Shadow-paging, trapping page-faults andmov cr3, <source>instructions in the guest)

Also - as obvious as it might sound - I'd like to point out that if you decide to write your own VT-x hypervisor, you'llNEED Intel® 64 and IA-32 architectures software developer’s manual combined volumes: 1, 2A, 2B, 2C, 2D, 3A, 3B, 3C, 3D, and 4. So download the PDF - together withAdobe Acrobat Reader- because trust me, you don't want to read and navigate through 5000 pages with browser's built-in PDF reader.

Features

• EPT with identity mappingwith usage of 2MB pagesfor the first 512GB of physical memory (seeept.cpp). This results in faster translations of the memory. It also means splitting particular 2MB pages into 4kb pages might be desired if EPT hooking is required. This process is actually not complicated at all and this repository includes example on how to achieve that.
• Simple pass-through VM-exit handler, which can handle:
  • exceptions orNMIs
  • CPUID,(WB)INVD,INVLPG,RDTSC(P),MOV CR,MOV DR,IN/OUT,RDMSR,WRMSR,SGDT,SIDT,LGDT, LIDT,SLDT,STR,LLDT,LTR,XSETBVandINVPCIDinstructions
  • VMCALLinstruction (used for termination ofhvpp)
  • VMCLEAR,VMLAUNCH,VMRESUME,VMPTRLD,VMPTRST,VMREAD,VMWRITE,VMFUNC,VMXOFF,VMXON,INVEPT andINVVPIDinstructions raise#UD(invalid opcode exception)
• Ability to run in VMWare (tested even with KVM - I advise to turn offHyper-V Enlightenments, as it cancause conflicts). VMWare's nested virtualization makes development and debugging of hypervisors much easier.
• Simple custom memory manager (see mm.cpp). The need for custom memory manager emerges from the fact that you should think twice before calling any kernel function from VM-exit handler, because many of them can be called at IRQL <= DISPATCH_LEVEL (such asExAllocatePoolWithTag). But in VM-exit handler, interrupts are disabled and yourIRQL is effectively HIGH_LEVEL.
• Detailed code comments, which should explain what the code does and why - sometimes with direct references to Intel Manual for further reading.
• TraceLoggingAPI (which builds on ETW) - the main benefit is it can be used forreallyhigh frequency logging (10'000+ per second) and it can be used from any IRQL - which makes it a perfect candidate even for logging in VM-exit handlers.
  • hvppalready includes tracing VM-exit handler (seevmexit_stats.cpp).
• Various reimplemented classes and functions - such as bitmaps and spinlocks - to avoid calling kernel functions.
• Included simple application (hvppctrl) which should demonstrateCPUIDinstruction interception, hiding hooks in user-mode applications via EPT and communication withhvppviaVMCALL

Code workflow

Note:hvppis compiled as a static library, which is linked with thehvppdrvproject.

• Bootstrap of the driver (hvpp,driver.cpp)
  • preallocate enough memory and initialize thehvppmemory manager
  • initialize the logger
• Bootstrap of the hypervisor (hvppdrv,main.cpp)
  • createvmexit_handlerinstance
• Start the hypervisor with provided VM-exit handler:hypervisor::start(vmexit_handler& handler)
  • initialize virtual cpu (VCPU) for each logical processor
  • assign providedvmexit_handlerinstance to each VCPU
  • launch all VCPUs via IPI (inter-processor interrupt):vcpu_t::start()
    • setup VMXON region and VMCS:vcpu_t::vmx_enter()
    • vmexit_handler::setup()is called, which allows anyone to initialize the VM-exit handler and/or modify the VMCS before the launch (seevmexit_custom_handler::setup()in hvppdrv,vmexit_custom.cpp)
• Hypervisor is now running and handling VM-exits via provided VM-exit handler
• Stop the hypervisor:hypervisor::stop()
  • destroy each VCPU via IPI:vcpu_t::stop()
    • vmexit_handler::teardown()is called and switches into VMX mode (vmexit_passthrough_handler::teardown()does it byVMCALLinstruction)
    • in VMX mode,vcpu_t::vmx_leave()is called - it leaves VMX mode withVMXOFFinstruction

Compilation

Compilehvppusing Visual Studio 2017. Solution file is included. The only required dependency isWDK.

Usage

You can runhvppon Windows 7 or higher. Windows 10 is recommended though, because it supportsTraceLogging.

EnableTest-Signingboot configuration option (note that you'll need administrative privileges to use bcdeditandsccommands):

bcdedit /set testsigning on

Register driverwith Service Control Manager (yes, it's important to leave these spaces):

sc create hvpp type= kernel binPath= "C:\full\path\to\hvppdrv.sys"

Now you should restart your computer fortestsigningto take effect, otherwise you'll be unable to start the driver. But before you do, you might want to prepareDebugViewfrom SysInternals and traceview.exetool from the WDK (note thattraceviewwill work properly only on Windows 10).

After restart, launchDebugViewandTraceView.InTraceView:

• go toFile -> Create New Log Session,click onAdd Provider
  • pickManually Entered Control GUID or Hashed Name
  • paste916fcd3e-673b-4610-aaba-0b71e28acd40(arbitrarily chosen, seelib/win32/tracelog.cpp)
  • clickOK
• in the next dialog, leave theSource Of WPP Format Informationset toAuto
  • clickOK
• after that, clickNext,which will bring you toLog Session Optionsdialog
  • inLog Session Nameeditbox you can give this logging session any name you like, e.g.HvppSessionor you can leave it as it is
  • if you desire to analyze this logging session when it's stopped (e.g. withWindows Performance Analyzer) you can optionally enableLog Trace Event Data To File,which saves whole logging session into an.ETLfile
  • clickFinish

TraceViewis now set-up and ready to showtracelogsfromhvpp.You can launchhvpp now:

sc start hvpp

hvpp now performs various checks and enters VMX mode if they pass. In case of success you should see message hvpp startedin theDebugView.

Runhvppctrl:

hvppctrl.exe

• hvppctrlperformsCPUIDinstruction withEAX = 0x70707668 ('hvpp')whichhvppshould intercept and return stringhello from hvppin EAX, EBX, ECX and EDX registers (seevmexit_custom.cpp). hvppctrlshould print this string.

• hvppctrltries to "stealthily" hookntdll!ZwClosefunction using EPT. The exact process is described further below.

• hvppctrlperforms IOCTL, which should instructhvppto set one-time breakpoint whenIN/OUTinstruction manipulating with port0x64(keyboard) is executed.

Description of "stealth hooking" process

• locatesZwClosefunction inntdll.dll
  • disassembles first 16 bytes of this function and prints them
    • printed instructions should indicate that this function isNOThooked yet
  • calls this function (withNULLparameter, this function call will most likely fail with some NTSTATUS error code, which it ignores)
  • prints value ofHookCallCountand it's expected value (explained below)
• hooksntdll!ZwClosefuction usingDetours
  • disassembles first 16 bytes of this function and prints them
    • printed instructions should now indicate that the functionIShooked (byjmpbeing first instruction)
  • calls this function (withNULLparameter)
    • instead of original function, the hook function will be called - on each call, it increments variable HookCallCount
  • prints value ofHookCallCountand it's expected value - it should be 1 now, as the hooked function has been called for the first time now
• callshvppbyVMCALLinstruction andRCX = 0xc1(arbitrarily chosen),RDX = AddressOfReadPageand R8 = AddressOfExecutePage- this instructshvppto hide the hook
  • disassembles first 16 bytes of this function and prints them
    • printed instructions should now indicate that the function hook isHIDDEN(by showing original content of memory - nojmp)
  • calls this function (withNULLparameter)
    • despite what the memory returned when we read it, the hook function will be called again and theHookCallCount will be incremented again
  • prints value ofHookCallCountand it's expected value - it should be 2
• callshvppbyVMCALLinstruction andRCX = 0xc2(arbitrarily chosen) - this instructshvppto unhide the hook
  • disassembles first 16 bytes of this function and prints them
    • printed instructions should now indicate that the function hook isNOT HIDDENand it should showjmpas a first instruction again
  • calls this function (withNULLparameter)
    • because the function is still hooked, the hook function will be called andHookCallCountwill be incremented again
  • prints value ofHookCallCountand it's expected value - it should be 3
• unhooksntdll!ZwClosefunction
  • disassembles first 16 bytes of this function and prints them
    • printed instructions should now indicate that the function isNOThooked - it should show the same content as when this function wasn't hooked
  • calls this function (withNULLparameter)
    • original function will be called, therefore theHookCallCountshould not be incremented now
  • prints value ofHookCallCountand it's expected value - it should be still 3

At the same time you should see tracelog messages in theTraceView- they are generated on eachVMCALLand on each EPT Violation.

When you decide you want to turn off thehvpp,just execute:

sc stop hvpp

Remarks

• hvppis designed to virtualize already running OS - i.e. it's not capable of running multiple guests like VMWare or VirtualBox. It also lacks support for any nested VMX operations.
• hvppis designed to run only on 64bit Intel processors, which support VT-x and EPT. This makes the code more simple and less bloated.
• hvppis designed to run only on Windows - future work might focus on Linux.
• hvppcurrently doesn't exit VMX mode on sleep or hibernate (S3 and S4power states) - Intel Manual says we should do so - this is known limitation.

License

This software is open-source under the MIT license. See the LICENSE.txt file in this repository.

Detoursis licensed under MIT license (a copy of the license is included here).

udis86is licensed under the terms of the 2-clause "Simplified BSD License" (a copy of the license is included here).

Similar work

SimpleVisor:https://github.com/ionescu007/SimpleVisor

HyperPlatform:https://github.com/tandasat/HyperPlatform

HyperBone:https://github.com/DarthTon/HyperBone

Bareflank:https://github.com/Bareflank/hypervisor

ksm:https://github.com/asamy/ksm

MoRE:https://github.com/ainfosec/MoRE

hyperdbg:https://github.com/rmusser01/hyperdbg

virtdbg:https://github.com/upring/virtdbg

BluePill:https://invisiblethingslab.com/resources/bh07/nbp-0.32-public.zip

Phrack #69:http://www.phrack.org/issues/69/15.html

NOVA Microhypervisor:https://github.com/udosteinberg/NOVA

Finally, I'd especially like to suggest reading interesting writings from Satoshi Tanda (github,twitter):

• http://tandasat.github.io/HyperPlatform/
• http://tandasat.github.io/HyperPlatform/userdocument/index.html
• http://standa-note.blogspot.com/2015/08/writing-hypervisor-for-kernel-mode-code.html

And notes from LordNoteworthy (github,twitter):

• https://github.com/LordNoteworthy/cpu-internals
• https://github.com/LordNoteworthy/windows-internals

If you find this project interesting, you can buy me a coffee

BTC 3GwZMNGvLCZMi7mjL8K6iyj6qGbhkVMNMF
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