Microsoft Windows - 'win32k.sys' TTF Font Processing win32k!fsc_RemoveDups Out-of-Bounds Pool Memory Access

Source: https://code.google.com/p/google-security-research/issues/detail?id=401&can=1

We have encountered a Windows kernel crash in the win32k!fsc_RemoveDups function while processing corrupted TTF font files, such as:

---
PAGE_FAULT_IN_NONPAGED_AREA (50)
Invalid system memory was referenced.  This cannot be protected by try-except,
it must be protected by a Probe.  Typically the address is just plain bad or it
is pointing at freed memory.
Arguments:
Arg1: ff6e7000, memory referenced.
Arg2: 00000000, value 0 = read operation, 1 = write operation.
Arg3: 91e809df, If non-zero, the instruction address which referenced the bad memory
	address.
Arg4: 00000000, (reserved)

Debugging Details:
------------------


Could not read faulting driver name

READ_ADDRESS: GetPointerFromAddress: unable to read from 8277c84c
Unable to read MiSystemVaType memory at 8275bf00
 ff6e7000 

FAULTING_IP: 
win32k!fsc_RemoveDups+85
91e809df 3918            cmp     dword ptr [eax],ebx

MM_INTERNAL_CODE:  0

DEFAULT_BUCKET_ID:  VISTA_DRIVER_FAULT

BUGCHECK_STR:  0x50

PROCESS_NAME:  csrss.exe

CURRENT_IRQL:  0

LAST_CONTROL_TRANSFER:  from 91e8015c to 91e809df

STACK_TEXT:  
969e3624 91e8015c 969e3858 fbff0e78 0000002b win32k!fsc_RemoveDups+0x85
969e36cc 91e89979 ff6de010 ff6de07c 00000001 win32k!fs_FindBitMapSize+0x2de
969e36e8 91e89b59 fbff0e78 0000002b 00000001 win32k!bGetGlyphMetrics+0x39
969e382c 91e7ec63 fbff0e78 0000002b 969e3918 win32k!lGetGlyphBitmap+0x2b
969e3850 91e7eab6 00000000 00000001 0000002b win32k!ttfdQueryFontData+0x158
969e38a0 91e7dce2 ff7af010 fbe0ccf0 00000001 win32k!ttfdSemQueryFontData+0x45
969e38e8 91e83774 ff7af010 fbe0ccf0 00000001 win32k!PDEVOBJ::QueryFontData+0x3e
969e3960 91efbc8d 969e3c3c fbe2cc94 ff713154 win32k!xInsertMetricsPlusRFONTOBJ+0x120
969e3990 91e7594d 0000000a ff7bf000 969e3cd0 win32k!RFONTOBJ::bGetGlyphMetricsPlus+0x179
969e39c8 91efb78b 969e3c1c 969e3c3c 00000008 win32k!ESTROBJ::vCharPos_H3+0xf0
969e3a0c 91e755d0 969e3cd0 0000000a 969e3c1c win32k!ESTROBJ::vInit+0x268
969e3c2c 91e75793 00000000 969e3cd0 fbe0ccf0 win32k!GreGetTextExtentExW+0x12a
969e3d0c 8264f896 0701015e 02bb0bac 0000000a win32k!NtGdiGetTextExtentExW+0x141
969e3d0c 779670f4 0701015e 02bb0bac 0000000a nt!KiSystemServicePostCall
WARNING: Frame IP not in any known module. Following frames may be wrong.
0015f434 00000000 00000000 00000000 00000000 0x779670f4
---

While we have not determined the specific root cause of the vulnerability, we have pinpointed the offending mutations to reside in the "glyf" table.

The issue reproduces on Windows 7 and 8.1. It is easiest to reproduce with Special Pools enabled for win32k.sys (leading to an immediate crash when the bug is triggered), but it is also possible to observe a crash on a default Windows installation in win32k!fsc_RemoveDups or another location in kernel space. In order to reproduce the problem with the provided samples, it might be necessary to use a custom program which displays all of the font's glyphs at various point sizes.

Attached is a proof of concept font file together with the corresponding kernel crash log.

Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/37915.zip