Microsoft Windows – ‘win32k.sys’ TTF Processing RCVT TrueType Instruction Handler Out-of-Bounds Read (MS16-120)
Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=864 We have encountered a number of Windows kernel crashes in the win32k!itrp_GetCVTEntryFast function (called by the handler of the RCVT TrueType instruction) 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: fb000078, memory referenced. Arg2: 00000000, value 0 = read operation, 1 = write operation. Arg3: 8ee70ccb, If non-zero, the instruction address which referenced the bad memory address. Arg4: 00000000, (reserved) Debugging Details: ------------------ READ_ADDRESS: fb000078 Paged session pool FAULTING_IP: win32k!itrp_GetCVTEntryFast+8 8ee70ccb 8b048a mov eax,dword ptr [edx+ecx*4] MM_INTERNAL_CODE: 0 IMAGE_NAME: win32k.sys DEBUG_FLR_IMAGE_TIMESTAMP: 57349934 MODULE_NAME: win32k FAULTING_MODULE: 8ee20000 win32k DEFAULT_BUCKET_ID: WIN7_DRIVER_FAULT BUGCHECK_STR: 0x50 PROCESS_NAME: csrss.exe CURRENT_IRQL: 2 ANALYSIS_VERSION: 6.3.9600.17237 (debuggers(dbg).140716-0327) x86fre TRAP_FRAME: 897b3568 -- (.trap 0xffffffff897b3568) ErrCode = 00000000 eax=fafffcdc ebx=00000000 ecx=000000ff edx=fafffc7c esi=fafffe6e edi=00000000 eip=8ee70ccb esp=897b35dc ebp=897b3620 iopl=0 nv up ei pl nz na pe nc cs=0008 ss=0010 ds=0023 es=0023 fs=0030 gs=0000 efl=00010206 win32k!itrp_GetCVTEntryFast+0x8: 8ee70ccb 8b048a mov eax,dword ptr [edx+ecx*4] ds:0023:fb000078=???????? Resetting default scope LAST_CONTROL_TRANSFER: from 828edd87 to 82889978 STACK_TEXT: 897b30bc 828edd87 00000003 7170889f 00000065 nt!RtlpBreakWithStatusInstruction 897b310c 828ee885 00000003 00000000 00000000 nt!KiBugCheckDebugBreak+0x1c 897b34d0 8289c94d 00000050 fb000078 00000000 nt!KeBugCheck2+0x68b 897b3550 8284efa8 00000000 fb000078 00000000 nt!MmAccessFault+0x104 897b3550 8ee70ccb 00000000 fb000078 00000000 nt!KiTrap0E+0xdc 897b35d8 8ee83782 fafffe7b 8ee7bf89 00000001 win32k!itrp_GetCVTEntryFast+0x8 897b35e0 8ee7bf89 00000001 8ee81af3 00000000 win32k!itrp_RCVT+0x63 897b35e8 8ee81af3 00000000 fafffcdc faffff10 win32k!itrp_InnerExecute+0x38 897b3620 8ee7bf89 fafffcdc 8ee7f3b1 fafffd70 win32k!itrp_CALL+0x23b 897b3628 8ee7f3b1 fafffd70 fafffd38 faffff90 win32k!itrp_InnerExecute+0x38 897b36a8 8ee7cee8 fafffec8 faffff10 fafffcdc win32k!itrp_Execute+0x2b2 897b36dc 8ee85d0d fafffcdc 00000000 fa44a298 win32k!itrp_ExecutePrePgm+0x5d 897b36f8 8ee7f67c fa44a51c fafffc7c fa44a2c4 win32k!fsg_RunPreProgram+0x78 897b3758 8ee89385 00000001 897b3774 8ee892dc win32k!fs__Contour+0x1c1 897b3764 8ee892dc fa44a010 fa44a07c 897b3790 win32k!fs_ContourGridFit+0x12 897b3774 8ee89c38 fa44a010 fa44a07c 00000003 win32k!fs_NewContourGridFit+0x10 897b3790 8ee89c79 fc11ae78 00000003 897b37cc win32k!bGetGlyphOutline+0xd7 897b37b8 8ee89e72 fc11ae78 00000003 00000001 win32k!bGetGlyphMetrics+0x20 897b38fc 8ee7ef89 fc11ae78 00000003 897b39ec win32k!lGetGlyphBitmap+0x2b 897b3924 8ee7edd6 00000000 00000001 00000003 win32k!ttfdQueryFontData+0x15e 897b3974 8ee7dff2 fc396010 fc1d0cf0 00000001 win32k!ttfdSemQueryFontData+0x45 897b39bc 8ee7e169 fc396010 fc1d0cf0 00000001 win32k!PDEVOBJ::QueryFontData+0x3e 897b3a30 8ee7bc81 00000002 897b3bdc 00000000 win32k!RFONTOBJ::bInitCache+0xd4 897b3aec 8eef8655 897b3bc8 897b3b94 00000003 win32k!RFONTOBJ::bRealizeFont+0x5df 897b3b98 8eef8890 fc74ad80 00000000 00000002 win32k!RFONTOBJ::bInit+0x2f4 897b3bb0 8ee8f111 897b3bc8 00000000 00000002 win32k!RFONTOBJ::vInit+0x16 897b3bd4 8ee8f262 fc1d0cf0 897b3bf4 0678b8bd win32k!GreGetRealizationInfo+0x2a 897b3c24 8284bdc6 37010587 0459f2cc 0459f2e4 win32k!NtGdiGetRealizationInfo+0x41 897b3c24 77346bf4 37010587 0459f2cc 0459f2e4 nt!KiSystemServicePostCall 0459f2e4 00000000 00000000 00000000 00000000 ntdll!KiFastSystemCallRet --- The bugcheck is caused by an attempt to access an out-of-bounds CVT table index (255 in this case, see the ECX register), likely due to a weird behavior of the win32k!itrp_RCVT function, which allows the index to be larger than the size of the array as long as it is smaller than 256. The bug appears to only enable an out-of-bounds read primitive, since at a first glance, the corresponding WCVT instruction handler does not seem to be affected by the same problem. Still, even in its current form, the vulnerability could be used to disclose the contents of adjacent pool allocations to user-mode, potentially leaking sensitive kernel memory or facilitating a KASLR bypass. The issue reproduces on Windows 7 and 8.1. It is easiest to reproduce with Special Pools enabled for win32k.sys, but it is also possible to observe a crash on a default Windows installation. Just hovering over the proof of concept files or opening them in the default Windows Font Viewer tool should be sufficient to trigger the condition. Attached is an archive with two proof of concept font files. Proof of Concept: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/40598.zip |