Archive for August 4th, 2018

Crash Dump Analysis Patterns (Part 256)

Saturday, August 4th, 2018

Backwards disassembling used in memory analysis patterns such as Coincidental Symbolic Information may be ambiguous and can show Wild Code output. This may also be debugger disassembling algorithm dependent. For example, default 8-instruction backwards disassembly shows this code:

0:011> ub 00007ff8`cdc9b4bf
00007ff8`cdc9b4ab 855948 test dword ptr [rcx+48h],ebx
00007ff8`cdc9b4ae b988bf03a8 mov ecx,0A803BF88h
00007ff8`cdc9b4b3 f4 hlt
00007ff8`cdc9b4b4 0100 add dword ptr [rax],eax
00007ff8`cdc9b4b6 00488b add byte ptr [rax-75h],cl
00007ff8`cdc9b4b9 09e8 or eax,ebp
00007ff8`cdc9b4bb 117236 adc dword ptr [rdx+36h],esi
00007ff8`cdc9b4be 5f pop rdi

However, if we specify the number of instructions to disassemble except 7 and 8 we get a different result (which is more correct from the forward code execution view since we disassembled the saved return address from the stack region):

0:011> ub 00007ff8`cdc9b4bf L1
00007ff8`cdc9b4ba e81172365f call clr!JIT_MonEnter (00007ff9`2d0026d0)

0:011> ub 00007ff8`cdc9b4bf L2
00007ff8`cdc9b4b7 488b09 mov rcx,qword ptr [rcx]
00007ff8`cdc9b4ba e81172365f call clr!JIT_MonEnter (00007ff9`2d0026d0)

0:011> k L10
# Child-SP RetAddr Call Site
00 0000002a`fc23e308 00007ff9`53d06099 ntdll!NtWaitForMultipleObjects+0x14
01 0000002a`fc23e310 00007ff9`2d1a96be KERNELBASE!WaitForMultipleObjectsEx+0xf9
02 0000002a`fc23e610 00007ff9`2d1a951c clr!WaitForMultipleObjectsEx_SO_TOLERANT+0x62
03 0000002a`fc23e670 00007ff9`2d1a9315 clr!Thread::DoAppropriateWaitWorker+0x1e4
04 0000002a`fc23e770 00007ff9`2d0c2b7f clr!Thread::DoAppropriateWait+0x7d
05 0000002a`fc23e7f0 00007ff9`2d1aa491 clr!CLREventBase::WaitEx+0xc4
06 0000002a`fc23e880 00007ff9`2d1aa39e clr!AwareLock::EnterEpilogHelper+0xc2
07 0000002a`fc23e940 00007ff9`2d1c1a92 clr!AwareLock::EnterEpilog+0x62
08 0000002a`fc23e9a0 00007ff8`cdc9b4bf clr!JITutil_MonEnterWorker+0xe2
09 0000002a`fc23eb40 00007ff9`275231d3 0×00007ff8`cdc9b4bf
0a 0000002a`fc23eb80 00007ff9`27523064 mscorlib_ni+0×5031d3
0b 0000002a`fc23ec50 00007ff9`27523032 mscorlib_ni+0×503064
0c 0000002a`fc23ec80 00007ff9`2751c812 mscorlib_ni+0×503032
0d 0000002a`fc23ecd0 00007ff9`2d006bb3 mscorlib_ni+0×4fc812
0e 0000002a`fc23ed10 00007ff9`2d006a70 clr!CallDescrWorkerInternal+0×83
0f 0000002a`fc23ed50 00007ff9`2d00735d clr!CallDescrWorkerWithHandler+0×4e

We call this analysis pattern Disassembly Ambiguity. The example dump can be downloaded from here.

- Dmitry Vostokov @ + -