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Before yesterdaysecret club

Preventing memory inspection on Windows

23 May 2021 at 23:00
By: jm

Have you ever wanted your dynamic analysis tool to take as long as GTA V to query memory regions while being impossible to kill and using 100% of the poor CPU core that encountered this? Well, me neither, but the technology is here and it’s quite simple!

What, Where, WTF?

As usual with my anti-debug related posts, everything starts with a little innocuous flag that Microsoft hasn’t documented. Or at least so I thought.

This time the main offender is NtMapViewOfSection, a syscall that can map a section object into the address space of a given process, mainly used for implementing shared memory and memory mapping files (The Win32 API for this would be MapViewOfFile).

NTSTATUS NtMapViewOfSection(
  HANDLE          SectionHandle,
  HANDLE          ProcessHandle,
  PVOID           *BaseAddress,
  ULONG_PTR       ZeroBits,
  SIZE_T          CommitSize,
  PLARGE_INTEGER  SectionOffset,
  PSIZE_T         ViewSize,
  SECTION_INHERIT InheritDisposition,
  ULONG           AllocationType,
  ULONG           Win32Protect);

By doing a little bit of digging around in ntoskrnl’s MiMapViewOfSection and searching in the Windows headers for known constants, we can recover the meaning behind most valid flag values.

/* Valid values for AllocationType */
MEM_RESERVE                0x00002000
SEC_PARTITION_OWNER_HANDLE 0x00040000
MEM_TOPDOWN                0x00100000
SEC_NO_CHANGE              0x00400000
SEC_FILE                   0x00800000
MEM_LARGE_PAGES            0x20000000
SEC_WRITECOMBINE           0x40000000

Initially I failed at ctrl+f and didn’t realize that 0x2000 is a known flag, so I started digging deeper. In the same function we can also discover what the flag does and its main limitations.

// --- MAIN FUNCTIONALITY ---
if (SectionOffset + ViewSize > SectionObject->SizeOfSection &&
    !(AllocationAttributes & 0x2000))
    return STATUS_INVALID_VIEW_SIZE;

// --- LIMITATIONS ---
// Image sections are not allowed
if ((AllocationAttributes & 0x2000) &&
    SectionObject->u.Flags.Image)
    return STATUS_INVALID_PARAMETER;

// Section must have been created with one of these 2 protection values
if ((AllocationAttributes & 0x2000) &&
    !(SectionObject->InitialPageProtection & (PAGE_READWRITE | PAGE_EXECUTE_READWRITE)))
    return STATUS_SECTION_PROTECTION;

// Physical memory sections are not allowed
if ((Params->AllocationAttributes & 0x20002000) &&
    SectionObject->u.Flags.PhysicalMemory)
    return STATUS_INVALID_PARAMETER;

Now, this sounds like a bog standard MEM_RESERVE and it’s possible to VirtualAlloc(MEM_RESERVE) whatever you want as well, however APIs that interact with this memory do treat it differently.

How differently you may ask? Well, after incorrectly identifying the flag as undocumented, I went ahead and attempted to create the biggest section I possibly could. Everything went well until I opened the ProcessHacker memory view. The PC was nigh unusable for at least a minute and after that process hacker remained unresponsive for a while as well. Subsequent runs didn’t seem to seize up the whole system however it still took up to 4 minutes for the NtQueryVirtualMemory call to return.

I guess you could call this a happy little accident as Bob Ross would say.

The cause

Since I’m lazy, instead of diving in and reversing, I decided to use Windows Performance Recorder. It’s a nifty tool that uses ETW tracing to give you a lot of insight into what was happening on the system. The recorded trace can then be viewed in Windows Performance Analyzer.

Trace Viewed in Windows Performance Analyzer

This doesn’t say too much, but at least we know where to look.

After spending some more time staring at the code in everyone’s favourite decompiler it became a bit more clear what’s happening. I’d bet that it’s iterating through every single page table entry for the given memory range. And because we’re dealing with terabytes of of data at a time it’s over a billion iterations. (MiQueryAddressState is a large function, and I didn’t think a short pseudocode snippet would do it justice)

This is also reinforced by the fact that from my testing the relation between view size and time taken is completely linear. To further verify this idea we can also do some quick napkin math to see if it all adds up:

instructions per second (ips) = 3.8Ghz * ~8
page table entries      (n)   = 12TB / 4096
time taken              (t)   = 3.5 minutes

instruction per page table entry = ips * t / n = ~2000

In my opinion, this number looks rather believable so, with everything added up, I’ll roll with the current idea.

Minimal Example

// file handle must be a handle to a non empty file
void* section = nullptr;
auto  status  = NtCreateSection(&section,
                                MAXIMUM_ALLOWED,
                                nullptr,
                                nullptr,
                                PAGE_EXECUTE_READWRITE,
                                SEC_COMMIT,
                                file_handle);
if (!NT_SUCCESS(status))
    return status;

// Personally largest I could get the section was 12TB, but I'm sure people with more
// memory could get it larger.
void* base = nullptr;
for (size_t i = 46;  i > 38; --i) {
    SIZE_T view_size = (1ull << i);
    status           = NtMapViewOfSection(section,
                                          NtCurrentProcess(),
                                          &base,
                                          0,
                                          0x1000,
                                          nullptr,
                                          &view_size,
                                          ViewUnmap,
                                          0x2000, // <- the flag
                                          PAGE_EXECUTE_READWRITE);

    if (NT_SUCCESS(status))
        break;
}

Do note that, ideally, you’d need to surround code with these sections because only the reserved portions of these sections cause the slowdown. Furthermore, transactions could also be a solution for needing a non-empty file without touching anything already existing or creating something visible to the user.

Conclusion

I think this is a great and powerful technique to mess with people analyzing your code. The resource usage is reasonable, all it takes to set it up is a few syscalls, and it’s unlikely to get accidentally triggered.

Process on a diet: anti-debug using job objects

20 January 2021 at 23:00
By: jm

In the second iteration of our anti-debug series for the new year, we will be taking a look at one of my favorite anti-debug techniques. In short, by setting a limit for process memory usage that is less or equal to current memory usage, we can prevent the creation of threads and modification of executable memory.

Job Object Basics

While job objects may seem like an obscure feature, the browser you are reading this article on is most likely using them (if you are a Windows user, of course). They have a ton of capabilities, including but not limited to:

  • Disabling access to user32 functionality.
  • Limiting resource usage like IO or network bandwidth and rate, memory commit and working set, and user-mode execution time.
  • Assigning a memory partition to all processes in the job.
  • Offering some isolation from the system by β€œupgrading” the job into a silo.

As far as API goes, it is pretty simple - creation does not really stand out from other object creation. The only other APIs you will really touch is NtAssignProcessToJobObject whose name is self-explanatory, and NtSetInformationJobObject through which you will set all the properties and capabilities.

NTSTATUS NtCreateJobObject(HANDLE*            JobHandle,
                           ACCESS_MASK        DesiredAccess,
                           OBJECT_ATTRIBUTES* ObjectAttributes);

NTSTATUS NtAssignProcessToJobObject(HANDLE JobHandle, HANDLE ProcessHandle);

NTSTATUS NtSetInformationJobObject(HANDLE JobHandle, JOBOBJECTINFOCLASS InfoClass,
                                   void*  Info,      ULONG              InfoLen);

The Method

With the introduction over, all one needs to create a job object, assign it to a process, and set the memory limit to something that will deny any attempt to allocate memory.

HANDLE job = nullptr;
NtCreateJobObject(&job, MAXIMUM_ALLOWED, nullptr);

NtAssignProcessToJobObject(job, NtCurrentProcess());

JOBOBJECT_EXTENDED_LIMIT_INFORMATION limits;
limits.ProcessMemoryLimit               = 0x1000;
limits.BasicLimitInformation.LimitFlags = JOB_OBJECT_LIMIT_PROCESS_MEMORY;
NtSetInformationJobObject(job, JobObjectExtendedLimitInformation,
                          &limits, sizeof(limits));

That is it. Now while it is sufficient to use only syscalls and write code where you can count the number of dynamic allocations on your fingers, you might need to look into some of the affected functions to make a more realistic program compatible with this technique, so there is more work to be done in that regard.

The implications

So what does it do to debuggers and alike?

  • Visual Studio - unable to attach.
  • WinDbg
    • Waits 30 seconds before attaching.
    • cannot set breakpoints.
  • x64dbg
    • will not be able to attach (a few months old).
    • will terminate the process of placing a breakpoint (a week or so old).
    • will fail to place a breakpoint.

Please do note that the breakpoint protection only works for pages that are not considered private. So if you compile a small test program whose total size is less than a page and have entry breakpoints or count it into the private commit before reaching anti-debug code - it will have no effect.

Conclusion

Although this method requires you to be careful with your code, I personally love it due to its simplicity and power. If you cannot see yourself using this, do not worry! You can expect the upcoming article to contain something that does not require any changes to your code.

New year, new anti-debug: Don’t Thread On Me

4 January 2021 at 23:00
By: jm

With 2020 over, I’ll be releasing a bunch of new anti-debug methods that you most likely have never seen. To start off, we’ll take a look at two new methods, both relating to thread suspension. They aren’t the most revolutionary or useful, but I’m keeping the best for last.

Bypassing process freeze

This one is a cute little thread creation flag that Microsoft added into 19H1. Ever wondered why there is a hole in thread creation flags? Well, the hole has been filled with a flag that I’ll call THREAD_CREATE_FLAGS_BYPASS_PROCESS_FREEZE (I have no idea what it’s actually called) whose value is, naturally, 0x40.

To demonstrate what it does, I’ll show how PsSuspendProcess works:

NTSTATUS PsSuspendProcess(_EPROCESS* Process)
{
  const auto currentThread = KeGetCurrentThread();
  KeEnterCriticalRegionThread(currentThread);

  NTSTATUS status = STATUS_SUCCESS;
  if ( ExAcquireRundownProtection(&Process->RundownProtect) )
  {
    auto targetThread = PsGetNextProcessThread(Process, nullptr);
    while ( targetThread )
    {
      // Our flag in action
      if ( !targetThread->Tcb.MiscFlags.BypassProcessFreeze )
        PsSuspendThread(targetThread, nullptr);

      targetThread = PsGetNextProcessThread(Process, targetThread);
    }
    ExReleaseRundownProtection(&Process->RundownProtect);
  }
  else
    status = STATUS_PROCESS_IS_TERMINATING;

  if ( Process->Flags3.EnableThreadSuspendResumeLogging )
    EtwTiLogSuspendResumeProcess(status, Process, Process, 0);

  KeLeaveCriticalRegionThread(currentThread);
  return status;
}

So as you can see, NtSuspendProcess that calls PsSuspendProcess will simply ignore the thread with this flag. Another bonus is that the thread also doesn’t get suspended by NtDebugActiveProcess! As far as I know, there is no way to query or disable the flag once a thread has been created with it, so you can’t do much against it.

As far as its usefulness goes, I’d say this is just a nice little extra against dumping and causes confusion when you click suspend in Processhacker, and the process continues to chug on as if nothing happened.

Example

For example, here is a somewhat funny code that will keep printing I am running. I am sure that seeing this while reversing would cause a lot of confusion about why the hell one would suspend his own process.

#define THREAD_CREATE_FLAGS_BYPASS_PROCESS_FREEZE 0x40

NTSTATUS printer(void*) {
    while(true) {
        std::puts("I am running\n");
        Sleep(1000);
    }
    return STATUS_SUCCESS;
}

HANDLE handle;
NtCreateThreadEx(&handle, MAXIMUM_ALLOWED, nullptr, NtCurrentProcess(),
                 &printer, nullptr, THREAD_CREATE_FLAGS_BYPASS_PROCESS_FREEZE,
                 0, 0, 0, nullptr);

NtSuspendProcess(NtCurrentProcess());

Suspend me more

Continuing the trend of NtSuspendProcess being badly behaved, we’ll again abuse how it works to detect whether our process was suspended.

The trick lies in the fact that suspend count is a signed 8-bit value. Just like for the previous one, here’s some code to give you an understanding of the inner workings:

ULONG KeSuspendThread(_ETHREAD *Thread)
{
  auto irql = KeRaiseIrql(DISPATCH_LEVEL);
  KiAcquireKobjectLockSafe(&Thread->Tcb.SuspendEvent);

  auto oldSuspendCount = Thread->Tcb.SuspendCount;
  if ( oldSuspendCount == MAXIMUM_SUSPEND_COUNT ) // 127
  {
    _InterlockedAnd(&Thread->Tcb.SuspendEvent.Header.Lock, 0xFFFFFF7F);
    KeLowerIrql(irql);
    ExRaiseStatus(STATUS_SUSPEND_COUNT_EXCEEDED);
  }

  auto prcb = KeGetCurrentPrcb();
  if ( KiSuspendThread(Thread, prcb) )
    ++Thread->Tcb.SuspendCount;

  _InterlockedAnd(&Thread->Tcb.SuspendEvent.Header.Lock, 0xFFFFFF7F);
  KiExitDispatcher(prcb, 0, 1, 0, irql);
  return oldSuspendCount;
}

If you take a look at the first code sample with PsSuspendProcess it has no error checking and doesn’t care if you can’t suspend a thread anymore. So what happens when you call NtResumeProcess? It decrements the suspend count! All we need to do is max it out, and when someone decides to suspend and resume us, they’ll actually leave the count in a state it wasn’t previously in.

Example

The simple code below is rather effective:

  • Visual Studio - prevents it from pausing the process once attached.
  • WinDbg - gets detected on attach.
  • x64dbg - pause button becomes sketchy with error messages like β€œProgram is not running” until you manually switch to the main thread.
  • ScyllaHide - older versions used NtSuspendProcess and caused it to be detected, but it was fixed once I reported it.
for(size_t i = 0; i < 128; ++i)
  NtSuspendThread(thread, nullptr);

while(true) {
  if(NtSuspendThread(thread, nullptr) != STATUS_SUSPEND_COUNT_EXCEEDED)
    std::puts("I was suspended\n");
  Sleep(1000);
}

Conclusion

If anything, I hope that this demonstrated that it’s best not to rely on NtSuspendProcess to work as well as you’d expect for tools dealing with potentially malicious or protected code. Hope you liked this post and expect more content to come out in the upcoming weeks.

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