RCE Endeavors 😅

Welcome to the second and final installment of the series on WoW64 syscall hooking. This part builds on the background provided by the first part and shows what is required to build a working implementation of a syscall hook under WoW64 for x64 Windows 8.1. The code presented here should be trivially portable across different versions of Windows by simply changing syscall numbers, which again can be found on this useful table mentioned in the first part. The code provided will be a basic syscall hook that allows for filtering of one particular syscall. There is not much stopping it from being extended to support hooking an arbitrary amount of syscalls.

Getting Started

From the first part in the series, we see that each thread holds a pointer to a block of code responsible for making the switch from x86 to x64 code execution. This is the pathway that WoW64 uses to perform a syscall, and the pointer to this code resides within the thread local storage region allotted to each thread. Since the value (FS:[0xC0]) in all threads points to the same location, the easiest way to go about hooking syscalls is to replace that jump with an inline hook to our filtering code. This code can then check the syscall number, which is stored in EAX, against the desired syscall to hook. If they match, our hook gets called, otherwise execution will continue on as normal.

The process begins by getting this address. With the aid of inline assembly, this is very straightforward:

const DWORD_PTR __declspec(naked) GetWow64Address()
{
    __asm
    {
        mov eax, dword ptr fs:[0xC0]
        ret
    }
}

The return value of this function, stored in EAX, will hold the address of the block of code responsible for performing the jump to x64 bit mode. As previously mentioned, this address is the one that will be replaced with a jump to the filtering code. This is achieved with a simple overwrite of the instruction bytes at the address:

const void EnableWow64Redirect(const DWORD_PTR dwWow64Address, const LPVOID lpNewJumpLocation)
{
    unsigned char trampolineBytes[] =
    {
        0x68, 0xDD, 0xCC, 0xBB, 0xAA,       /*push 0xAABBCCDD*/
        0xC3,                               /*ret*/
        0xCC, 0xCC, 0xCC                    /*padding*/
    };
    memcpy(&trampolineBytes[1], &lpNewJumpLocation, sizeof(DWORD_PTR));
 
    WriteJump(dwWow64Address, trampolineBytes, sizeof(trampolineBytes));
}
 
const void WriteJump(const DWORD_PTR dwWow64Address, const void *pBuffer, size_t ulSize)
{
    DWORD dwOldProtect = 0;
    (void)VirtualProtect((LPVOID)dwWow64Address, PAGE_SIZE, PAGE_EXECUTE_READWRITE, &dwOldProtect);
    (void)memcpy((void *)dwWow64Address, pBuffer, ulSize);
    (void)VirtualProtect((LPVOID)dwWow64Address, PAGE_SIZE, dwOldProtect, &dwOldProtect);
}

You can see the hook being installed in a before/after form:

Above are the bytes of the original code. This carries out the inter-segment jump to x64 bit mode as is standard for all WoW64 calls.

This is the code with the hook is shown below. Here a jump to 0x13411B0 is made, which is the location of the filtering code.

This filtering code is just an implementation of what was described above, and performs a check of the syscall number against the desired target and acts accordingly

void __declspec(naked) Wow64Trampoline()
{
    __asm
    {
        cmp eax, SYSCALL_INTERCEPT
        jz NtWriteVirtualMemoryHook
        jmp lpJmpRealloc
    }
}

In the case that the syscall number is not the one to filter, execution continues as normal by the jump to lpJmpRealloc, which is just an executable region of memory that contains the bytes of the original inter-segment jump (the jmp 0033:77BE1D84 instruction above). If the syscall number does match, then our hooking function, NtWriteVirtualMemoryHook, gets invoked. The example code just prints a message denoting that the call happened then continues on to performing the syscall. There is also a pushad/popad to properly preserve the stack in the event that another syscall is made from the syscall hook.

void __declspec(naked) NtWriteVirtualMemoryHook()
{
    __asm pushad
 
    fprintf(stderr, "NtWriteVirtualMemory called.\n");
 
    __asm
    {
        popad
        jmp lpJmpRealloc
    }
}

And that is all that there is to it as far as the example code goes. It can quickly be verified by seeing if the hook gets invoked for a call to NtWriteVirtualMemory.

    //Test syscall
    int i = 0x123;
    int j = 0x678;
    SIZE_T ulBytesWritten = 0;
 
    fprintf(stderr, "i = 0x%X\n", i);
    NtWriteVirtualMemory(GetCurrentProcess(), &i, &j, sizeof(int), &ulBytesWritten);
    fprintf(stderr, "i = 0x%X\n", i);

The output below shows everything working successfully

i = 0x123
NtWriteVirtualMemory called.
i = 0x678

Get the Code

The Visual Studio 2015 RC project for this example can be found here. The source code is viewable on Github here.

Follow on Twitter for more updates.

These next two posts will cover the topic of hooking Windows system calls under WoW64, the Windows subsystem responsible for running x86 code on a 64-bit version of Windows. This post will be a brief introduction to system calls under WoW64 and how the transition between x86 and x64 occurs. It will lay the groundwork for the actual task (in part 2) of inserting a hook into this process to intercept desired system calls.

Anatomy of a System Call

On a native 64-bit version of windows, there are two ways that system calls get made: natively or through WoW64. The native case is rather straightforward. For example, take the following code snippet, compiled as x64, performing an NtWriteVirtualMemory syscall:

using pNtWriteVirtualMemory = NTSTATUS (NTAPI *)(HANDLE ProcessHandle,
    PVOID BaseAddress, PVOID Buffer, ULONG NumberOfBytesToWrite,
    PULONG NumberOfBytesWritten);
 
pNtWriteVirtualMemory NtWriteVirtualMemory = nullptr;
 
int main(int argc, char *argv[])
{
    HMODULE hModule = GetModuleHandle(L"ntdll.dll");
    NtWriteVirtualMemory = (pNtWriteVirtualMemory)GetProcAddress(hModule,
        "NtWriteVirtualMemory");
 
    int i = 0x321;
    int j = 0x123;
 
    fprintf(stderr, "j = %X\n", j);
 
    ULONG numBytesWritten = 0;
    NTSTATUS success = NtWriteVirtualMemory(GetCurrentProcess(), &j, &i,
        sizeof(int), &numBytesWritten);
 
    fprintf(stderr, "j = %X\n", j);
 
    return 0;
}

The disassembly for the call looks like the following:

00007FF6C49D193A FF 15 C0 C6 00 00    call        qword ptr [__imp_GetCurrentProcess (07FF6C49DE000h)]  
00007FF6C49D1940 48 8D 4D 64          lea         rcx,[numBytesWritten]  
00007FF6C49D1944 48 89 4C 24 20       mov         qword ptr [rsp+20h],rcx  
00007FF6C49D1949 41 B9 04 00 00 00    mov         r9d,4  
00007FF6C49D194F 4C 8D 45 24          lea         r8,[i]  
00007FF6C49D1953 48 8D 55 44          lea         rdx,[j]  
00007FF6C49D1957 48 8B C8             mov         rcx,rax  
00007FF6C49D195A FF 15 00 A8 00 00    call        qword ptr [NtWriteVirtualMemory (07FF6C49DC160h)]  

You can see the first four arguments being put in to the RCX, RDX, R8, and R9 registers, per the standard calling convention for x64 on Windows. The fifth parameter is put onto the stack. When the call is made to NtWriteVirtualMemory, the following code is executed:

00007FFC88BB1560 4C 8B D1             mov         r10,rcx  
00007FFC88BB1563 B8 39 00 00 00       mov         eax,39h  
00007FFC88BB1568 0F 05                syscall  
00007FFC88BB156A C3                   ret  

Here RCX (the first parameter) is moved into R10. Then 0x39 is moved in to EAX. Afterwards, the syscall instruction is executed, which handles the switch to kernel mode, where the actual call is carried out. The magic value of 0x39 is the syscall number corresponding to NtWriteVirtualMemory on x64 Windows 8.1. A very useful table of syscalls for x86 and x64 Windows versions can be found here. When the syscall finishes, it will return execution to the RET instruction, which subsequently returns execution back to the next instruction from the original call site.

x86 to x64 Transition

As a x86 process running under WoW64 on a 64-bit system, things change a bit. Looking at the x86 disassembly for the call initially shows nothing out of the ordinary:

00B51117 8D 45 EC             lea         eax,[numBytesWritten]  
00B5111A 50                   push        eax  
00B5111B 6A 04                push        4  
00B5111D 8D 4D F4             lea         ecx,[i]  
00B51120 51                   push        ecx  
00B51121 8D 55 F0             lea         edx,[j]  
00B51124 52                   push        edx  
00B51125 FF 15 00 30 B5 00    call        dword ptr ds:[0B53000h] ; GetCurrentProcess
00B5112B 50                   push        eax  
00B5112C FF 15 18 40 B5 00    call        dword ptr ds:[0B54018h] ; NtWriteVirtualMemory

However, looking at the call to NtWriteVirtualMemory shows the following:

77ECC810 B8 39 00 00 00       mov         eax,39h  
77ECC815 64 FF 15 C0 00 00 00 call        dword ptr fs:[0C0h]  
77ECC81C C2 14 00             ret         14h  
77ECC81F 90                   nop  

Here the syscall number is moved into EAX as in the x64 example. Then a call to a special area of memory, the FS segment, is made. This segment contains thread local data and its address is unique per thread. Despite the segment base being unique per thread, the address contained at FS:[0xC0] will always be the same. This can quickly be verified with some test code showing the addresses of the FS segments and the contents of FS:[0xC0] across different threads.

DWORD WINAPI ThreadEntry(LPVOID lpParameter)
{
    WaitForSingleObject((HANDLE)lpParameter, INFINITE);
 
    return 0;
}
 
int main(int argc, char *argv[])
{
    HANDLE hEvent = CreateEvent(nullptr, FALSE, FALSE, L"Useless event");
 
    HANDLE hThread1 = CreateThread(nullptr, 0, &ThreadEntry, hEvent, 0, nullptr);
    HANDLE hThread2 = CreateThread(nullptr, 0, &ThreadEntry, hEvent, 0, nullptr);
    HANDLE hThread3 = CreateThread(nullptr, 0, &ThreadEntry, hEvent, 0, nullptr);
 
    CONTEXT ctxThread1 = { CONTEXT_ALL };
    (void)GetThreadContext(hThread1, &ctxThread1);
 
    CONTEXT ctxThread2 = { CONTEXT_ALL };
    (void)GetThreadContext(hThread2, &ctxThread2);
 
    CONTEXT ctxThread3 = { CONTEXT_ALL };
    (void)GetThreadContext(hThread3, &ctxThread3);
 
    LDT_ENTRY ldtThread1 = { 0 };
    LDT_ENTRY ldtThread2 = { 0 };
    LDT_ENTRY ldtThread3 = { 0 };
 
    (void)GetThreadSelectorEntry(hThread1, ctxThread1.SegFs, &ldtThread1);
    (void)GetThreadSelectorEntry(hThread2, ctxThread2.SegFs, &ldtThread2);
    (void)GetThreadSelectorEntry(hThread3, ctxThread3.SegFs, &ldtThread3);
 
    NT_TIB *pTibMain = (NT_TIB *)__readfsdword(0x18);
 
    DWORD_PTR dwFSBase1 = (ldtThread1.HighWord.Bits.BaseHi << 24) |
        (ldtThread1.HighWord.Bits.BaseMid << 16) |
        ldtThread1.BaseLow;
 
    DWORD_PTR dwFSBase2 = (ldtThread2.HighWord.Bits.BaseHi << 24) |
        (ldtThread2.HighWord.Bits.BaseMid << 16) |
        ldtThread2.BaseLow;
 
    DWORD_PTR dwFSBase3 = (ldtThread3.HighWord.Bits.BaseHi << 24) |
        (ldtThread3.HighWord.Bits.BaseMid << 16) |
        ldtThread3.BaseLow;
 
    fprintf(stderr, "Thread 1 FS Segment base address: %X\n"
        "Thread 2 FS Segment base address : %X\n"
        "Thread 3 FS Segment base address : %X\n",
        dwFSBase1, dwFSBase2, dwFSBase3);
 
    DWORD_PTR dwWOW64Address1 = *(DWORD_PTR *)((unsigned char *)dwFSBase1 + 0xC0);
    DWORD_PTR dwWOW64Address2 = *(DWORD_PTR *)((unsigned char *)dwFSBase2 + 0xC0);
    DWORD_PTR dwWOW64Address3 = *(DWORD_PTR *)((unsigned char *)dwFSBase3 + 0xC0);
 
    fprintf(stderr, "Thread 1 FS:[0xC0] : %X\n"
        "Thread 2 FS:[0xC0] : %X\n"
        "Thread 3 FS:[0xC0] : %X\n",
        dwWOW64Address1, dwWOW64Address2, dwWOW64Address3);
 
    return 0;
}

The output for the following code is

Thread 1 FS Segment base address: 7FDBB000
Thread 2 FS Segment base address : 7FDB8000
Thread 3 FS Segment base address : 7FC8F000
Thread 1 FS:[0xC0] : 77E81218
Thread 2 FS:[0xC0] : 77E81218
Thread 3 FS:[0xC0] : 77E81218

which verifies the original claim.

Moving back to the original disassembly; stepping into this CALL instruction leads to the following:

77E81216 00 00                add         byte ptr [eax],al  
77E81218 EA 84 1D E8 77 33 00 jmp         0033:77E81D84  
77E8121F 00 00                add         byte ptr [eax],al  

Ignore some of the nonsense bytes — this is a result of the disassembly listing not being quite correct. The important instruction is the jump to 0x77E81D84. This is an inter-segment jump. Here the 0033 means a jump to 64-bit mode. The value prior here was 0023, which corresponds to x86. An interesting thing is that the (Visual Studio) debugger is unable to step into this address (x64 WinDbg can). Taking a look at where this address resides in memory reveals something interesting:It resides in wow64cpu.dll, which is one of the three core WoW64 DLLs that gets loaded into every process running under WoW64. That particular DLL is the one responsible for handling the transition from x86 to x64. Interestingly enough, the actual DLL is a 64-bit DLL that is loaded into a 32-bit process. The instructions, traced through to the syscall, at 0x77E81D84 are the following

wow64cpu!CpupReturnFromSimulatedCode
00000000`77e81d84 4987e6          xchg    rsp,r14
00000000`77e81d87 458b06          mov     r8d,dword ptr [r14] ds:00000000`008df5e0=77ecc78c
00000000`77e81d8a 4983c604        add     r14,4
00000000`77e81d8e 4589453c        mov     dword ptr [r13+3Ch],r8d ds:00000000`007dfdec=77eea9b0
00000000`77e81d92 45897548        mov     dword ptr [r13+48h],r14d ds:00000000`007dfdf8=008df670
00000000`77e81d96 4d8d5e04        lea     r11,[r14+4]
00000000`77e81d9a 41897d20        mov     dword ptr [r13+20h],edi ds:00000000`007dfdd0=00000000
00000000`77e81d9e 41897524        mov     dword ptr [r13+24h],esi ds:00000000`007dfdd4=00000000
00000000`77e81da2 41895d28        mov     dword ptr [r13+28h],ebx ds:00000000`007dfdd8=7f676000
00000000`77e81da6 41896d38        mov     dword ptr [r13+38h],ebp ds:00000000`007dfde8=00000000
00000000`77e81daa 9c              pushfq
00000000`77e81dab 4158            pop     r8
00000000`77e81dad 45894544        mov     dword ptr [r13+44h],r8d ds:00000000`007dfdf4=00000000
wow64cpu!TurboDispatchJumpAddressStart
00000000`77e81db1 8bc8            mov     ecx,eax
00000000`77e81db3 c1e910          shr     ecx,10h
00000000`77e81db6 41ff24cf        jmp     qword ptr [r15+rcx*8] ds:00000000`77e81b38=0000000077e822d0
wow64cpu!TurboDispatchJumpAddressEnd+0x516
00000000`77e822d0 418b5304        mov     edx,dword ptr [r11+4] ds:00000000`008df5ec=00000000
00000000`77e822d4 458b13          mov     r10d,dword ptr [r11] ds:00000000`008df5e8=008df5fc
00000000`77e822d7 eb3a            jmp     wow64cpu!TurboDispatchJumpAddressEnd+0x559 (00000000`77e82313)
wow64cpu!TurboDispatchJumpAddressEnd+0x559
00000000`77e82313 e838000000      call    wow64cpu!TurboDispatchJumpAddressEnd+0x596 (00000000`77e82350)
wow64cpu!CpupSyscallStub
00000000`77e82350 0f05            syscall
00000000`77e82352 c3              ret

The entry point to the 64-bit code resides at the symbol CpupReturnFromSimulatedCode. This code is responsible for setting up the proper parameters and stack to perform the syscall and then call it. For a more full explanation of everything involved here, see this post.

That’s all that is involved as far as performing syscalls under WoW64. This article hopefully elucidated a few things about how x86 code can perform syscalls on a x64 system. With this baseline, the next article will cover what is involved in intercepting these syscalls.

Get the Code

The Visual Studio 2015 RC project for this example can be found here. The source code is viewable on Github here.

Follow on Twitter for more updates.