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Nop Hopping: Hiding Functionality in Alignment

May 17th, 2015 2 comments

This post will cover the topic of hiding code functionality by taking advantage of compiler alignment. In order to maximize speed of data access, optimizers can try to align loops, function entries, jump destinations, etc., on a native word boundary. One example of this in actual executable code is a large series of NOP bytes after the end of a function. For example, the following was taken from an x64 library:

...
00007FFC676D7207 48 23 4C 24 38       and         rcx,qword ptr [rsp+38h]  
00007FFC676D720C 48 89 4C 24 38       mov         qword ptr [rsp+38h],rcx  
00007FFC676D7211 0F 85 91 A3 02 00    jne         00007FFC677015A8  
00007FFC676D7217 48 8B 5C 24 30       mov         rbx,qword ptr [rsp+30h]  
00007FFC676D721C 48 83 C4 20          add         rsp,20h  
00007FFC676D7220 5F                   pop         rdi  
00007FFC676D7221 C3                   ret  
00007FFC676D7222 90                   nop  
00007FFC676D7223 90                   nop  
00007FFC676D7224 90                   nop  
00007FFC676D7225 90                   nop  
00007FFC676D7226 90                   nop  
...

The NOPs are shown after the RET instruction. The size of these NOP blocks, if present, varies throughout programs. During my experimentation, I found that a majority of them (>95%) were 20 bytes or less. This leaves plenty of room for hiding functionality. One advantage of doing this is that the pages that these NOP blocks are on are already allocated, and they have executable privileges on them since they’re right next to actual executable code. This can enhance stealth since no extra allocations need to be made inside the program. Additionally, since these blocks are all over the program, it is possible to randomly select blocks to write your code in, preventing things such as signature scanning. It’s a rather overall nice technique and one that I used to use to bypass anti-cheat detection systems.

Finding the Regions

These NOP blocks are all over the place; they’re inside the main executable, and in each loaded library. This gives a very large search space. To begin, it is easiest to find and store the base address of the image and every library and its size. These will be the starting points for searching for these NOP blocks. This is done in a straightforward manner with the help of the CreateToolhelp32Snapshot API along with Module32First/Module32Next. These will return the base address of the image and its libraries as well as their sizes in memory.

const ModuleMap GetModules(const DWORD dwProcessId)
{
    ModuleMap mapModules;
 
    const HANDLE hToolhelp32 = CreateToolhelp32Snapshot(TH32CS_SNAPMODULE, dwProcessId);
    MODULEENTRY32 moduleEntry = { 0 };
    moduleEntry.dwSize = sizeof(MODULEENTRY32);
 
    const BOOL bSuccess = Module32First(hToolhelp32, &moduleEntry);
    if (!bSuccess)
    {
        fprintf(stderr, "Could not enumeate modules. Error = %X.\n",
            GetLastError());
        exit(-1);
    }
 
    do
    {
        const DWORD_PTR dwBase = (DWORD_PTR)moduleEntry.modBaseAddr;
        const DWORD_PTR dwEnd = dwBase + moduleEntry.modBaseSize;
 
        mapModules[std::wstring(moduleEntry.szModule)] = std::make_pair(dwBase, dwEnd);
 
    } while (Module32Next(hToolhelp32, &moduleEntry));
 
    CloseHandle(hToolhelp32);
 
    return mapModules;
}

Now that all modules and their sizes are stored, the next step involves enumerating them for the proper pages. These will be committed pages which have executables privileges in combination with either read/write or just read. This involves nothing more than enumerating through every modules and checking its address range with VirtualQueryEx, which will return regions of pages with the same permissions. This permission flag is masked for what is desired.

const ExecutableMap GetExecutableRegions(const HANDLE hProcess, const ModuleMap &mapModules)
{
    ExecutableMap mapExecutableRegions;
    ExecutableRegionsList lstExecutableRegions;
 
    for (auto &module : mapModules)
    {
        MEMORY_BASIC_INFORMATION memBasicInfo = { 0 };
        DWORD_PTR dwBaseAddress = module.second.first;
        const DWORD_PTR dwEndAddress = module.second.second;
 
        while (dwBaseAddress <= dwEndAddress)
        {
            const SIZE_T ulReadSize = VirtualQueryEx(hProcess, (LPCVOID)dwBaseAddress, &memBasicInfo, sizeof(MEMORY_BASIC_INFORMATION));
            if (ulReadSize > 0)
            {
                if ((memBasicInfo.State & MEM_COMMIT) &&
                    ((memBasicInfo.Protect & PAGE_EXECUTE_READWRITE) || (memBasicInfo.Protect & PAGE_EXECUTE_READ)))
                {
                    const DWORD_PTR dwRegionStart = (DWORD_PTR)memBasicInfo.AllocationBase;
                    const DWORD_PTR dwRegionEnd = dwRegionStart + (DWORD_PTR)memBasicInfo.RegionSize;
                    lstExecutableRegions.emplace_back(std::make_pair(dwRegionStart, dwRegionEnd));
                }
                dwBaseAddress += memBasicInfo.RegionSize;
            }
        }
 
        if (lstExecutableRegions.size() > 0)
        {
            mapExecutableRegions[module.first] = lstExecutableRegions;
            lstExecutableRegions.clear();
        }
    }
 
    if (mapExecutableRegions.size() == 0)
    {
        fprintf(stderr, "Could not find any executable regions.\n");
        exit(-1);
    }
 
    return mapExecutableRegions;
}

This filters the original module ranges down and only leaves ranges of pages that are committed and have the PAGE_EXECUTE_READWRITE or PAGE_EXECUTE_READ permission. These will be the ranges that are searched for NOP blocks. Now that the collection is filtered down even further, it is time to find the NOP blocks.

Finding NOP Blocks

Finding the NOP blocks is achieved in multiple steps. Since this code is a process that writes into another process, the first step involves copying over the bytes from the target process. The bytes copied over will be the executable range for the module found in the previous section. This range will then be scanned for NOP bytes, and these ranges stored. The code for this looks like the following:

const NopRangeList FindNopRanges(const HANDLE hProcess, const ExecutableMap &executableRegions, const size_t ulSize)
{
    NopRangeList nopRangeList;
 
    for (auto &executableRegion : executableRegions)
    {
        for (auto &executableAddressRange : executableRegion.second)
        {
            const DWORD_PTR dwLowerAddress = executableAddressRange.first;
            const DWORD_PTR dwHigherAddress = executableAddressRange.second;
            const DWORD_PTR dwRangeSize = dwHigherAddress - dwLowerAddress;
 
            if (dwRangeSize > ulSize)
            {
                std::unique_ptr pLocalBytes(new unsigned char[dwRangeSize]);
                SIZE_T ulBytesRead = 0;
                const bool bSuccess = BOOLIFY(ReadProcessMemory(hProcess, (LPCVOID)dwLowerAddress,
                    pLocalBytes.get(), dwRangeSize, &ulBytesRead));
                if (bSuccess && ulBytesRead == dwRangeSize)
                {
                    const DWORD_PTR dwOffset = dwLowerAddress - (DWORD_PTR)pLocalBytes.get();
 
                    NopRange nopRange = FindNops(pLocalBytes.get(), dwRangeSize, dwOffset);
                    if (nopRange.size() > 0)
                    {
                        nopRangeList.emplace_back(nopRange);
                    }
                }
                else
                {
                    fprintf(stderr, "Could not read from 0x%X. Error = %X\n",
                        executableAddressRange.first, GetLastError());
                }
            }
        }
    }
 
    return nopRangeList;
}

Here the bytes are copied into a local array with ReadProcessMemory. The offset between the address of this local array and the address that was read is calculated. This is needed because the instructions are read into this local array and the addresses are different. When these instructions are later interpreted and checked against NOP (0x90), the address of that NOP will correspond to the local array and not to the target process. Calculating the difference between these two and adding it back later will fix that problem up. At the end of this loop, nopRangeList will contain the NOP ranges for every module in the executable as shown belownop1
The topmost index will be a module and the inner index will hold an address range of NOPs within that module. For example, in the image above, nopRangeList[4][0] = [0x7FFC6701185D – 0x7FFC670118FF] is a range of NOPs in the target process found within kernel32.dll. This function also calls FindNops to do the work; the definition for FindNops is below:

const NopRange FindNops(const unsigned char * const pBytes, const size_t ulSize, const DWORD_PTR dwOffset)
{
    //Find all NOPs in the code
    const InstructionList nopList = GetNopList(pBytes, ulSize, dwOffset);
 
    //Merge continuous NOPs into an address range
    NopRange nopListMerged;
    if (nopList.size() > 1)
    {
        auto firstElem = nopList.begin();
        auto nextElem = ++firstElem;
        --firstElem;
        nopListMerged.push_back(std::make_pair(*firstElem, *firstElem));
 
        while (nextElem != nopList.end())
        {
            if (*nextElem == ((*firstElem) + 1))
            {
                auto elem = nopListMerged.back();
                const DWORD_PTR dwRangeStart = elem.first;
                const DWORD_PTR dwRangeEnd = *nextElem;
                nopListMerged.pop_back();
                nopListMerged.push_back(std::make_pair(dwRangeStart, dwRangeEnd));
            }
            else
            {
                nopListMerged.push_back(std::make_pair(*nextElem, *nextElem));
            }
 
            ++firstElem;
            ++nextElem;
        }
    }
 
    //Toss out address ranges that are too small
    NopRange nopListTrimmed;
    const int iMinNops = 20;
    for (auto &nopRange : nopListMerged)
    {
        const DWORD_PTR dwRangeStart = nopRange.first;
        const DWORD_PTR dwRangeEnd = nopRange.second;
 
        if ((dwRangeEnd - dwRangeStart) > iMinNops)
        {
            nopListTrimmed.push_back(std::make_pair(dwRangeStart, dwRangeEnd));
        }
    }
 
    return nopListTrimmed;
}

This function is responsible for finding the NOP ranges via a call to GetNopList, which returns every instruction that was a NOP in the given range. These returned NOPs will be unmerged, as shown below: nop2
Here you can see continuous addresses (0x…1000, 0x…1001, 0x…1002, …) that contain NOPs. The next loop is responsible for merging these entries into a std::pair range, containing the starting address and ending address of the range. The last loop then filters this even further to only include NOP ranges that are 20 bytes of greater.

GetNopList is implemented with the help of the BeaEngine disassembler.

const InstructionList GetInstructionList(const unsigned char * const pBytes, const size_t ulSize, const DWORD_PTR dwOffset,
    const bool bNopsOnly = false)
{
    InstructionList instructionList;
 
    DISASM disasm = { 0 };
#ifdef _M_IX86
    //Do nothing
#elif defined(_M_AMD64)
    disasm.Archi = 64;
#else
#error "Unsupported architecture"
#endif
 
    disasm.EIP = (UIntPtr)pBytes;
    int iLength = 0;
    int iLengthTotal = 0;
    do
    {
        iLength = DisasmFnc(&disasm);
        if (iLength != UNKNOWN_OPCODE)
        {
            const DWORD_PTR dwInstructionStart = (DWORD_PTR)(disasm.EIP);
            if (bNopsOnly)
            {
                if (disasm.Instruction.Opcode == NOP)
                {
                    instructionList.push_back(dwInstructionStart + dwOffset);
                }
            }
            else
            {
                instructionList.push_back(dwInstructionStart + dwOffset);
            }
 
            iLengthTotal += iLength;
            disasm.EIP += iLength;
        }
        else
        {
            ++iLengthTotal;
            ++disasm.EIP;
        }
    } while (iLengthTotal < ulSize);
 
    return instructionList;
}
 
const InstructionList GetNopList(const unsigned char * const pBytes, const size_t ulSize, const DWORD_PTR dwOffset)
{
    return GetInstructionList(pBytes, ulSize, dwOffset, true);
}

Putting Everything Together

Now the collection has been filtered down even further to only desirable NOP ranges (those >20 bytes). These will be the ones used to write in our instructions. The algorithm for doing this will be as follows:

  1. Select a module at random
  2. Select a NOP range from that module to write into at random that hasn’t been chosen already
  3. Write an instruction to the region
  4. Write an unconditional jump to the next NOP range, which will contain the next instruction and an unconditional jump.
  5. Continue doing steps 3-4 while there are instructions left to write

The code for steps 1-2 is shown below:

InstructionList SelectRegions(const HANDLE hProcess, const NopRangeList &nopRangeList, InstructionList &writeInstructions)
{
    InstructionList writtenList;
 
    auto firstElem = writeInstructions.begin();
    auto nextElem = ++firstElem;
    --firstElem;
    while(nextElem != writeInstructions.end())
    {
        bool bContinueSearching = true;
        do
        {
            const size_t ulCurrentIndexModule = std::rand() % nopRangeList.size();
            const size_t ulCurrentIndexAddressRange = std::rand() % nopRangeList[ulCurrentIndexModule].size();
 
            const DWORD_PTR dwBaseWriteAddress = nopRangeList[ulCurrentIndexModule][ulCurrentIndexAddressRange].first;
            if(std::find(writtenList.begin(), writtenList.end(), dwBaseWriteAddress) == writtenList.end())
            {
                writtenList.push_back(dwBaseWriteAddress);
                bContinueSearching = false;
            }
        } while (bContinueSearching);
 
        ++firstElem;
        ++nextElem;
    }
 
    return writtenList;
}

Here writtenList will contain the addresses in the target process to write instructions to.nop3
The rest of the algorithm involves writing in an instruction and a jump for each instruction that should be written. This is implemented in the WriteJumps function shown below:

const bool WriteJumps(const HANDLE hProcess, const InstructionList &writeInstructions, const InstructionList &selectedRegions)
{
#ifdef _M_IX86
#elif defined (_M_AMD64)
    unsigned char jmpBytes[] =
    {
        0x48, 0xB8, 0xBB, 0xBB, 0xBB, 0xBB, 0xBB, 0xBB, 0xBB, 0xBB, /*mov rax, 0xBBBBBBBBBBBBBBBB*/
        0xFF, 0xE0                                                  /*jmp rax*/
    };
#else
#error "Unsupported architecture"
#endif
 
    auto firstElem = selectedRegions.begin();
    auto nextElem = ++firstElem;
    --firstElem;
 
    int i = 0;
    while (nextElem != selectedRegions.end())
    {
        const DWORD_PTR dwInstructionSize = writeInstructions[i + 1] - writeInstructions[i];
        DWORD dwOldProtect = 0;
        bool bSuccess = BOOLIFY(VirtualProtectEx(hProcess, (LPVOID)*firstElem, dwInstructionSize, PAGE_EXECUTE_READWRITE, &dwOldProtect));
        if (bSuccess)
        {
            size_t ulBytesWritten = 0;
            bSuccess = BOOLIFY(WriteProcessMemory(hProcess, (LPVOID)*firstElem, (LPCVOID)writeInstructions[i++], dwInstructionSize,
                &ulBytesWritten));
 
            DWORD_PTR dwNextAddress = *nextElem;
            memcpy(&jmpBytes[2], &dwNextAddress, sizeof(DWORD_PTR));
 
            bSuccess = BOOLIFY(WriteProcessMemory(hProcess, (LPVOID)(*firstElem + dwInstructionSize), jmpBytes, sizeof(jmpBytes),
                &ulBytesWritten));
 
            bSuccess = BOOLIFY(VirtualProtectEx(hProcess, (LPVOID)*firstElem, dwInstructionSize, dwOldProtect, &dwOldProtect));
            if (!bSuccess)
            {
                fprintf(stderr, "Could not put permissions back on address 0x%X. Error = %X\n",
                    *firstElem, GetLastError());
                return false;
            }
 
        }
        else
        {
            fprintf(stderr, "Could not change permissions on address 0x%X. Error = %X\n",
                *firstElem, GetLastError());
            return false;
        }
 
        ++firstElem;
        ++nextElem;
    }
 
    return true;
}

For each region to be written to, this function will begin by changing the page permissions to PAGE_EXECUTE_READWRITE. Then the first WriteProcessMemory call will write the first instruction to the region. Following that, it will write an unconditional jump in the form of mov rax, <address> -> jmp rax. The page permissions will be changed back to what they were and the loop continues until there are no more instructions to write. To begin execution of these bytes, a remote thread can be created with CreateRemoteThread with the base of these instructions as the entry point.

An Example

Here is an example of what writing MessageBoxA(0, 0, 0, 0) into another process looks like. The code to be written looks like the following:

    HMODULE hModule = LoadLibrary(L"user32.dll");
    DWORD_PTR dwTargetAddress = (DWORD_PTR)GetProcAddress(hModule, "MessageBoxA");
 
#ifdef _M_IX86
#elif defined(_M_AMD64)
    DWORD dwHigh = (dwTargetAddress >> 32) & 0xFFFFFFFF;
    DWORD dwLow = (dwTargetAddress) & 0xFFFFFFFF;
 
    unsigned char pBytes[] =
    {
        0x45, 0x33, 0xC9,                               /*xor r9d, r9d*/
        0x45, 0x33, 0xC0,                               /*xor r8d, r8d*/
        0x33, 0xD2,                                     /*xor edx, edx*/
        0x33, 0xC9,                                     /*xor ecx, ecx*/
        0x68, 0x11, 0x11, 0x11, 0x11,                   /*push 0x11111111*/
        0xC7, 0x44, 0x24, 0x04, 0xDD, 0xCC, 0xBB, 0xAA, /*mov [rsp+4], 0AABBCCDD*/
        0xC3,                                           /*ret*/
        0xC3, 0xC3, 0xC3                                /*dummy*/
    };
 
    memcpy(&pBytes[11], &dwLow, sizeof(DWORD));
    memcpy(&pBytes[19], &dwHigh, sizeof(DWORD));

At the end of this code segment, pBytes will contain the bytes of a call to MessageBoxA(0, 0, 0, 0). The target process was a x64 process that I chose, it happened to be the 64-bit version of Dependency Walker (depends.exe) for this example. Here is what it looks like in action. The start address was 0x00007ffc67dcc396. nop4

The first instruction was written with a call to the next NOP range at 0x7FFC6701185D. Then at 0x7FFC6701185Dnop5The next instruction is written. This continues on until the call. nop6

nop7

nop8

nop9

nop10Eventually, when the remote thread runs, the following should appear:nop11Closing the MessageBox will return execution to normal.

Issues

The example code works on x64 only, but can be very easily ported to work on x86. This technique also doesn’t seem to work universally on all executables. For example, trying this on a 64-bit Notepad instance will crash it with the following error: “RangeChecks instrumentation code detected an out of range array access.” This is something that I am currently investigating and will hope to update soon. Edit: As a commentator pointed out (and I have confirmed), this is caused by Control Flow Guard being used for executables on Windows 8.1 and higher. The example code works without issues for x64 on Windows 7.

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.

Categories: General x86, General x86-64, Programming Tags:

Debugging Injected DLLs

May 10th, 2015 No comments

A quick post on how to debug injected DLLs through Visual Studio. This is rather straightforward, but it seems like a fair amount of people are unaware that this can be done. It might possibly because programs typically don’t have DLLs injected to them at runtime, so perhaps people think that debugging them can’t be done in a straightforward way. Fortunately, if you attach to the target process beforehand and inject a DLL, the Visual Studio debugger will detect the loaded DLL and allow for an ordinary debugging experience. The steps are rather simple:

1. Choose to attach to a process through the “Debug” menu in Visual Studio.

dbg1

2. Select the target process from the list and attach.

dbg2

3. Attach to the process and verify that breakpoints can get hit.

dbg3And that’s all there is to it. All of the useful features of the Visual Studio debugger are now available for debugging the injected DLLs.