Windows ATMFD.DLL Write to Uninitialized Address Due to Malformed CFF Table

ID SSV:89535
Type seebug
Modified 2015-09-25T00:00:00



We have encountered a number of Windows kernel crashes in the ATMFD.DLL OpenType driver while processing corrupted OTF 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: a3a3a3db, memory referenced. Arg2: 00000001, value 0 = read operation, 1 = write operation. Arg3: 91f445c9, If non-zero, the instruction address which referenced the bad memory address. Arg4: 00000002, (reserved)

Debugging Details:

*** ERROR: Module load completed but symbols could not be loaded for ATMFD.DLL


FAULTING_IP: ATMFD+345c9 91f445c9 83483810 or dword ptr [eax+38h],10h








PROCESS_NAME: csrss.exe


ANALYSIS_VERSION: 6.3.9600.17237 (debuggers(dbg).140716-0327) x86fre

TRAP_FRAME: 879cfd8c -- (.trap 0xffffffff879cfd8c) ErrCode = 00000002 eax=a3a3a3a3 ebx=00000008 ecx=00000004 edx=fb964900 esi=fb80e380 edi=fb80e3a0 eip=91f445c9 esp=879cfe00 ebp=879cfe10 iopl=0 nv up ei ng nz ac pe cy cs=0008 ss=0010 ds=0023 es=0023 fs=0030 gs=0000 efl=00010297 ATMFD+0x345c9: 91f445c9 83483810 or dword ptr [eax+38h],10h ds:0023:a3a3a3db=???????? Resetting default scope

LAST_CONTROL_TRANSFER: from 826f4ce7 to 826902d8

879cf8dc 826f4ce7 00000003 5e64199c 00000065 nt!RtlpBreakWithStatusInstruction 879cf92c 826f57e5 00000003 c06028e8 a3a3a3db nt!KiBugCheckDebugBreak+0x1c 879cfcf0 826a3391 00000050 a3a3a3db 00000001 nt!KeBugCheck2+0x68b 879cfd74 82655c48 00000001 a3a3a3db 00000000 nt!MmAccessFault+0x104 879cfd74 91f445c9 00000001 a3a3a3db 00000000 nt!KiTrap0E+0xdc WARNING: Stack unwind information not available. Following frames may be wrong. 879cfe10 91f39eec fb964900 ff657038 91f563ec ATMFD+0x345c9 879cfe34 91f3e987 fb9bec70 91f563ec 00000f5c ATMFD+0x29eec 879d0544 91f3f6e0 fb9bec70 91f4f028 879d0790 ATMFD+0x2e987 879d0600 91f327ae fb9bec70 91f4f028 879d0790 ATMFD+0x2f6e0 879d06ec 91f32858 fb9bec70 879d0790 879d0814 ATMFD+0x227ae 879d0718 91f232b2 fb9bec70 91f4f028 879d0790 ATMFD+0x22858 879d087c 91f23689 ffffffff 879d099c fb874f58 ATMFD+0x132b2 879d08d0 91f1406d ffffffff 879d099c 00000000 ATMFD+0x13689 879d0924 91c7dcf2 ff7a5010 fbeeccf0 00000001 ATMFD+0x406d 879d096c 91c667cb ff7a5010 fbeeccf0 00000001 win32k!PDEVOBJ::QueryFontData+0x3e 879d09e0 91c91513 ffa6a130 fb81e8d0 0000004f win32k!xInsertMetricsRFONTOBJ+0x9c 879d0a14 91c935f5 00000020 879d0b2c 879d0c92 win32k!RFONTOBJ::bGetGlyphMetrics+0x131 879d0cb8 91ca6684 040101b7 00000060 00000040 win32k!GreGetCharABCWidthsW+0x147 879d0d14 82652a66 040101b7 00000040 00000040 win32k!NtGdiGetCharABCWidthsW+0xf8 879d0d14 771870f4 040101b7 00000040 00000040 nt!KiSystemServicePostCall 0012f1d4 00000000 00000000 00000000 00000000 ntdll!KiFastSystemCallRet

The crash occurs while trying to modify memory under an invalid address. More specifically, the "a3a3a3a3" bytes found in the EAX register at the time of the crash are a repetition of a random byte filled by Driver Verifier in each pool allocation before returning it to the caller. This means that the dereferenced pointer is in fact an uninitialized value from the kernel pool.

The offending code (containing the crashing instruction) is as follows:

.text:000445C6 loc_445C6: .text:000445C6 mov eax, [esi+ecx*4] .text:000445C9 or dword ptr [eax+38h], 10h .text:000445CD inc ecx .text:000445CE cmp ecx, ebx .text:000445D0 jl short loc_445C6

As shown above, there is a loop iterating EBX times over an array of addresses stored in [ESI]. At the time of the bugcheck, EBX=8, so the code expects 8 valid pointers in the array; however, only 4 pointers are properly initialized:

kd> dd fb80e380 fb80e380 fb964980 fb9649c0 fb964900 fb964940 fb80e390 a3a3a3a3 a3a3a3a3 a3a3a3a3 a3a3a3a3 fb80e3a0 a3a3a3a3 a3a3a3a3 a3a3a3a3 a3a3a3a3

The issue reproduces on Windows 7. It is easiest to reproduce with Special Pools enabled for ATMFD.DLL (leading to an immediate crash when the bug is triggered), but it should also be possible to observe a crash on a default Windows installation in ATMFD.DLL.

Attached is an archive with two proof of concept font files together with corresponding kernel crash logs.