NULL code pointer, changed environment, hooked functions and execution residue: pattern cooperation

After an upgrade to the new version of a productivity software package one unrelated application started to crash frequently. A crash dump was collected and the following stack trace pointed to a NULL code pointer:

0:000> r
eax=09680104 ebx=0013aefc ecx=0968a710 edx=0cdc0c0c esi=16a19058 edi=00000001
eip=00000000 esp=0013aea8 ebp=0013aeb8 iopl=0 nv up ei pl nz na po nc
cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00200202
00000000 ??              ???

0:000> k 100
ChildEBP RetAddr 
WARNING: Frame IP not in any known module. Following frames may be wrong.
0013aea4 096e28a0 0×0
0013aeb8 32e688bd dllC!Abort+0×10
0013aec8 32c82395 dllB+0×589e
0013aed8 32865718 dllB+0×18f1
0013b0c0 314de1ff dllB+0×4c6
0013b154 31293494 dllA!DllGetLCID+0×46d2d
0013b178 312af217 dllA!DllGetClassObject+0×4e896
0013f3d0 300e8721 dllA!DllGetClassObject+0×69e42
0013f578 300e7f5a application+0xcff5
0013ffc0 7c816ff7 application+0×51d5
0013fff0 00000000 kernel32!BaseProcessStart+0×23

To see if changed environment somehow affected this application the presence of any DLL hooks was checked. The following  hooked functions were found in user32.dll:

0:000> !chkimg -lo 50 -d !user32 -v
Searching for module with expression: !user32
Will apply relocation fixups to file used for comparison
Will ignore NOP/LOCK errors
Will ignore patched instructions
Image specific ignores will be applied
Comparison image path: c:\mss\USER32.dll\45F02D7C90000\USER32.dll
No range specified

Scanning section:    .text
Size: 389095
Range to scan: 7e411000-7e46ffe7
    7e4188a6-7e4188aa  5 bytes - USER32!GetWindowLongW
 [ 6a 08 68 e0 88:e9 55 77 a4 01 ]
    7e418f9c-7e418fa0  5 bytes - USER32!GetSystemMetrics (+0×6f6)
 [ 6a 10 68 00 90:e9 5f 70 b5 01 ]
    7e41945d-7e419461  5 bytes - USER32!GetWindowLongA (+0×4c1)
 [ 6a 10 68 78 9f:e9 f5 60 a2 01 ]
    7e41b6ae-7e41b6b2  5 bytes - USER32!GetClientRect (+0×17a8)
 [ 8b ff 55 8b ec:e9 4d 49 9f 01 ]
    7e41b6d4-7e41b6d8  5 bytes - USER32!GetWindowRect (+0×26)
 [ b8 74 11 00 00:e9 98 30 af 01 ]
    7e41d60d-7e41d611  5 bytes - USER32!SetWindowLongA (+0×6aa)
 [ 8b ff 55 8b ec:e9 ee 29 a5 01 ]
    7e41d62b-7e41d62f  5 bytes - USER32!SetWindowLongW (+0×1e)
 [ 6a 08 68 28 f5:e9 0e 0b b2 01 ]
    7e41fc25-7e41fc29  5 bytes - USER32!CreateWindowExW (+0×738)
 [ 8b ff 55 8b ec:e9 d6 03 b6 01 ]
    7e41ff33-7e41ff37  5 bytes - USER32!CreateWindowExA (+0×30e)
Total bytes compared: 389095(100%)
Number of errors: 52
52 errors : !user32 (7e4188a6-7e42e8d5)

The hooking DLL was found to be from that upgraded package:

0:000> u 7e4188a6
7e4188a6 e95577a401      jmp     7fe60000
7e4188ab 41              inc     ecx
7e4188ac 7ee8            jle     USER32!_GetWindowLong+0xda (7e418896)
7e4188ae 0e              push    cs
7e4188af fd              std
7e4188b0 ff              ???
7e4188b1 ff8b4d08e816    dec     dword ptr [ebx+16E8084Dh]
7e4188b7 fc              cld

0:000> u 7fe60000
7fe60000 e9bb62b080      jmp     hookA+0×62c0 (009662c0)
7fe60005 6a08            push    8
7fe60007 68e088417e      push    offset USER32!`string’+0×34 (7e4188e0)
7fe6000c e99c885bfe      jmp     USER32!GetWindowLongW+0×7 (7e4188ad)
7fe60011 0000            add     byte ptr [eax],al
7fe60013 0000            add     byte ptr [eax],al
7fe60015 0000            add     byte ptr [eax],al
7fe60017 0000            add     byte ptr [eax],al

0:000> lmv m hookA
start    end        module name
00960000 00976000   hookA     (no symbols)          
    Loaded symbol image file: hookA.dll
    Image path: C:\Program Files\CompanyA\hookA.dll
    Image name: hookA.dll
    ProductName:      ProductA

Execution residue from hookA module was also found on the problem thread raw stack and it looks like real code (not a coincidental symbolic information):

0:000> !teb
TEB at 7ffdf000
    ExceptionList:        0013f02c
    StackBase:            00140000
    StackLimit:           0010c000

    SubSystemTib:         00000000
    FiberData:            00001e00
    ArbitraryUserPointer: 00000000
    Self:                 7ffdf000
    EnvironmentPointer:   00000000
    ClientId:             00000c38 . 00000840
    RpcHandle:            00000000
    Tls Storage:          00163268
    PEB Address:          7ffdb000
    LastErrorValue:       0
    LastStatusValue:      0
    Count Owned Locks:    0
    HardErrorMode:        0

0:000> dds 0010c000 00140000
0010c000  00000000
0010c004  00000000
0010c008  00000000
00121f50  0096d7d1*** ERROR: Module load completed but symbols could not be loaded for hookA.dll

00121f54  00009924
00121f58  00121fb4
00121f5c  00000000
00121f60  00121f9c
00121f64  0096d895 hookA+0xd895
00121f68  00121f78
00121f6c  00000000
00121f70  00122008
00121f74  00000000
00121f78  00000000
00121f7c  00000000
00121f80  00000000
00121f84  00121f6c
00121f88  000001fe
00121f8c  001220fc
00121f90  0096ec98 hookA+0xec98
00121f94  00970e48 hookA+0×10e48
00121f98  00121fb4
00121f9c  7e41b6a3 USER32!_GetClientRect+0×6e
00121fa0  00122008
00121fa4  fffffa01

0:000> u 0096d895
0096d895 8bc6            mov     eax,esi
0096d897 8b4df0          mov     ecx,dword ptr [ebp-10h]
0096d89a 64890d00000000  mov     dword ptr fs:[0],ecx
0096d8a1 5f              pop     edi
0096d8a2 5e              pop     esi
0096d8a3 5b              pop     ebx
0096d8a4 8be5            mov     esp,ebp
0096d8a6 5d              pop     ebp

0:000> ub 0096d895
0096d876 8b65e8          mov     esp,dword ptr [ebp-18h]
0096d879 be0d000000      mov     esi,0Dh
0096d87e c745fcffffffff  mov     dword ptr [ebp-4],0FFFFFFFFh
0096d885 eb05            jmp     hookA+0xd88c (0096d88c)
0096d887 be72000000      mov     esi,72h
0096d88c 8d55dc          lea     edx,[ebp-24h]
0096d88f 52              push    edx
0096d890 e8fbfeffff      call    hookA+0xd790 (0096d790)

As was found the upgraded application had special DLL hooks to improve productivity and ease of use of GUI applications. Fortunately it was possible to disable that hook on a per-application basis and application crashes disappeared.

- Dmitry Vostokov @ -

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