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The case when a function pointer or a return address becomes a Wild Pointer and EIP or RIP value lies in a valid region of memory the execution path may continue through a region called Wild Code. This might loop on itself or eventually reach non-executable or invalid pages and produce an exception. Local Buffer Overflow might lead to this behavior and also data corruption that overwrites function pointers with valid memory addresses.

My favorite example is when a function pointer points to zeroed pages with EXECUTE page attribute. What will happen next when we dereference it? All zeroes are perfect x86/x64 code:

0:001> dd 0000000`771afdf0
00000000`771afdf0  00000000 00000000 00000000 00000000
00000000`771afe00  00000000 00000000 00000000 00000000
00000000`771afe10  00000000 00000000 00000000 00000000
00000000`771afe20  00000000 00000000 00000000 00000000
00000000`771afe30  00000000 00000000 00000000 00000000
00000000`771afe40  00000000 00000000 00000000 00000000
00000000`771afe50  00000000 00000000 00000000 00000000
00000000`771afe60  00000000 00000000 00000000 00000000

0:001> u
ntdll!DbgUserBreakPoint:
00000000`771afe00 0000    add     byte ptr [rax],al
00000000`771afe02 0000    add     byte ptr [rax],al
00000000`771afe04 0000    add     byte ptr [rax],al
00000000`771afe06 0000    add     byte ptr [rax],al
00000000`771afe08 0000    add     byte ptr [rax],al
00000000`771afe0a 0000    add     byte ptr [rax],al
00000000`771afe0c 0000    add     byte ptr [rax],al
00000000`771afe0e 0000    add     byte ptr [rax],al

Now if RAX points to a valid memory page with WRITE attribute the code will modify the first byte at that address:

0:001> dq @rax
000007ff`fffdc000 00000000`00000000 00000000`035a0000
000007ff`fffdc010 00000000`0359c000 00000000`00000000
000007ff`fffdc020 00000000`00001e00 00000000`00000000
000007ff`fffdc030 000007ff`fffdc000 00000000`00000000
000007ff`fffdc040 00000000`0000142c 00000000`00001504
000007ff`fffdc050 00000000`00000000 00000000`00000000
000007ff`fffdc060 000007ff`fffd8000 00000000`00000000
000007ff`fffdc070 00000000`00000000 00000000`00000000

Therefore the code will be perfectly executed:

0:001> t
ntdll!DbgBreakPoint+0x2:
00000000`771afdf2 0000    add     byte ptr [rax],al ds:000007ff`fffdc000=00

0:001> t
ntdll!DbgBreakPoint+0x4:
00000000`771afdf4 0000    add     byte ptr [rax],al ds:000007ff`fffdc000=00

0:001> t
ntdll!DbgBreakPoint+0x6:
00000000`771afdf6 0000    add     byte ptr [rax],al ds:000007ff`fffdc000=00

0:001> t
ntdll!DbgBreakPoint+0x8:
00000000`771afdf8 0000    add     byte ptr [rax],al ds:000007ff`fffdc000=00

0:001> t
ntdll!DbgBreakPoint+0xa:
00000000`771afdfa 0000    add     byte ptr [rax],al ds:000007ff`fffdc000=00

- Dmitry Vostokov @ DumpAnalysis.org -

           

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This entry was posted on Thursday, March 27th, 2008 at 12:29 pm and is filed under Assembly Language, Crash Dump Analysis, Crash Dump Patterns. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.