Exploit-DB #37923
Description: Microsoft Windows - 'ATMFD.dll' CharString Stream Out-of-Bounds Reads (MS15-021)
Type: dos
Platform: windows
Date: 2015-08-21
Author: Google Security Research

Exploit:
Source: https://code.google.com/p/google-security-research/issues/detail?id=382&can=1

We have encountered a number of Windows kernel crashes in the ATMFD.DLL OpenType driver while processing corrupted OTF font files, such as:

---
DRIVER_PAGE_FAULT_BEYOND_END_OF_ALLOCATION (d6)
N bytes of memory was allocated and more than N bytes are being referenced.
This cannot be protected by try-except.
When possible, the guilty driver's name (Unicode string) is printed on
the bugcheck screen and saved in KiBugCheckDriver.
Arguments:
Arg1: ffb4da9f, memory referenced
Arg2: 00000000, value 0 = read operation, 1 = write operation
Arg3: 92a7a902, if non-zero, the address which referenced memory.
Arg4: 00000000, (reserved)

Debugging Details:
------------------


READ_ADDRESS:  ffb4da9f Special pool

FAULTING_IP: 
ATMFD+2a902
92a7a902 0fb600          movzx   eax,byte ptr [eax]

MM_INTERNAL_CODE:  0

IMAGE_NAME:  ATMFD.DLL

DEBUG_FLR_IMAGE_TIMESTAMP:  54e6a55a

MODULE_NAME: ATMFD

FAULTING_MODULE: 92a50000 ATMFD

DEFAULT_BUCKET_ID:  WIN7_DRIVER_FAULT

BUGCHECK_STR:  0xD6

PROCESS_NAME:  csrss.exe

CURRENT_IRQL:  2

ANALYSIS_VERSION: 6.3.9600.17237 (debuggers(dbg)[PII_REDACTED]) x86fre

TRAP_FRAME:  945bcd54 -- (.trap 0xffffffff945bcd54)
ErrCode = 00000000
eax=ffb4da9f ebx=945bd0ec ecx=ffb4da9f edx=ffb4dea8 esi=945bd2fc edi=00002932
eip=92a7a902 esp=945bcdc8 ebp=945bd4c0 iopl=0         nv up ei pl nz na po nc
cs=0008  ss=0010  ds=0023  es=0023  fs=0030  gs=0000             efl=00010202
ATMFD+0x2a902:
92a7a902 0fb600          movzx   eax,byte ptr [eax]         ds:0023:ffb4da9f=??
Resetting default scope

LAST_CONTROL_TRANSFER:  from 82723ce7 to 826bf2d8

STACK_TEXT:  
945bc8a4 82723ce7 00000003 46aca[PII_REDACTED]5 nt!RtlpBreakWithStatusInstruction
945bc8f4 827247e5 [PII_REDACTED] 00000002 nt!KiBugCheckDebugBreak+0x1c
945bccb8 826d2[PII_REDACTED]0 ffb4da9f 00000000 nt!KeBugCheck2+0x68b
945bcd3c 82684c48 00000000 ffb4da9f 00000000 nt!MmAccessFault+0x104
945bcd3c 92a7a[PII_REDACTED]0 ffb4da9f 00000000 nt!KiTrap0E+0xdc
WARNING: Stack unwind information not available. Following frames may be wrong.
945bd4c0 92a7f6e0 fab90c70 92a8f028 945bd70c ATMFD+0x2a902
945bd57c 92a727ae fab90c70 92a8f028 945bd70c ATMFD+0x2f6e0
945bd668 92a72858 fab90c70 945bd70c 945bd790 ATMFD+0x227ae
945bd694 92a632b2 fab90c70 92a8f028 945bd70c ATMFD+0x22858
945bd7f8 92a63[PII_REDACTED]b 945bd918 fb64c8b0 ATMFD+0x132b2
945bd84c 92a5406d 0000000b 945bd918 fb64c8b0 ATMFD+0x13689
945bd8a0 92badcf2 ff7a5010 fa4f4cf0 00000001 ATMFD+0x406d
945bd8e8 92bb3784 ff7a5010 fa4f4cf0 00000001 win32k!PDEVOBJ::QueryFontData+0x3e
945bd960 92c2bdcd 945bdc3c fb665704 fb64c8b0 win32k!xInsertMetricsPlusRFONTOBJ+0x120
945bd990 92ba5[PII_REDACTED]3 ff7bf020 945bdcd4 win32k!RFONTOBJ::bGetGlyphMetricsPlus+0x179
945bd9c8 92c2b8cb 945bdc1c 945bdc3c 00000008 win32k!ESTROBJ::vCharPos_H3+0xf0
945bda0c 92ba55e7 945bdcd0 00000003 945bdc1c win32k!ESTROBJ::vInit+0x268
945bdc2c 92ba57aa 00000000 945bdcd0 fa4f4cf0 win32k!GreGetTextExtentExW+0x12a
945bdd0c 82681a66 20010483 00b20b1c 00000003 win32k!NtGdiGetTextExtentExW+0x141
945bdd0c 773c70f4 20010483 00b20b1c 00000003 nt!KiSystemServicePostCall
0031f6d4 [PII_REDACTED] [PII_REDACTED] ntdll!KiFastSystemCallRet
---

The memory read instruction causing the crash is responsible for fetching the next CharString instruction from the input stream, in order to execute it as part of the PostScript state machine. This bug is similar to  issue 174 , which described the lack of the instruction pointer's bounds checking in the interpreter function, making it possible to crash the operating system or potentially disclose chunks of kernel-mode memory. While that problem was fixed in bulletin MS15-021 by introducing the missing bound checks, out-of-bounds access to the instruction stream is still possible as shown in the above crash log. The exact root cause of the vulnerability is unknown.

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 might also possible to observe a crash on a default Windows installation, depending on the specific testcase used.

Attached is an archive with two proof of concept font files together with corresponding kernel crash logs.

Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/37923.zip