Heap Corruptions in Software

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of the existing free block changes More specifically, the user-accessible portion of the heap block is overwritten by the heap manager with the FLINK and BLINK pointers, each pointing to the next and previous free block on the free list In our hypothetical example in Figure 612, B1 is inserted at the beginning of the free list corresponding to size 16 The user-accessible portion of B1 is replaced with a FLINK that points to Bx and a BLINK that points to the start of the list (itself) The existing free block Bx is also updated by the BLINK pointing to B1 Figure 613 illustrates the resulting layout after freeing block B1
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Segment FLINK=Bx BLINK=B1 B2 User accessible part
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Free Lists 0 1 2 3 127 B1 Bx
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Figure 613 Next, when the application frees block B2, the heap manager finds an adjacent free block (B1) and coalesces both blocks into one large free block As part of the coalescing process, the heap manager must remove block B1 from the free list since it no longer exists and add the new larger block to its corresponding free list The resulting large block s user-accessible part now contains FLINK and BLINK pointers that are updated according to the state of the free list So far, we have assumed that all heap blocks freed make their way to the back end allocator s free lists Although it s true that some free blocks go directly to the free lists, some of the allocations may end up going to the front end allocator s look aside list When a heap block goes into the look aside list, the primary differences can be seen in the heap block metadata:
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6 MEMORY CORRUPTION PART II HEAPS
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Memory Corruption Part II Heaps
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Heap blocks that go into the look aside list have their status bit set to busy (in comparison to free in free lists) The look aside list is a singly linked list (in comparison to the free lists doubly linked), and hence only the FLINK pointer is considered valid
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The most important aspect of freeing memory, as related to heap reuse after free, is the fact that the structure of the heap block changes once it is freed The user-accessible portion of the heap block is now used for internal bookkeeping to keep the free lists upto-date If the application overwrites any of the content (thinking the block is still busy), the FLINK and BLINK pointers become corrupt, and the structural integrity of the free list is compromised The net result is most likely a crash somewhere down the road when the heap manager tries to manipulate the free list (usually during another allocate or free call) Listing 610 shows an example of an application that allocates a block of memory and subsequently frees the block twice Listing 610
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#include <windowsh> #include <stdioh> #include <conioh> int __cdecl wmain (int argc, wchar_t* pArgs[]) { printf( Press any key to start\n ); _getch(); BYTE* pByte=(BYTE*) HeapAlloc(GetProcessHeap(), 0, 10); (*pByte)=10; HeapFree(GetProcessHeap(), 0, pByte); HeapFree(GetProcessHeap(), 0, pByte); printf( Doneexiting application\n ); return 0; }
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The source code and binary for Listing 69 can be found in the following folders: Source code: C:\AWD\6\DblFree Binary: C:\AWDBIN\WinXPx86chk\06DblFreeexe
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C:\AWDBIN\WinXPx86chk\06DblFreeexe
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To make sure that nothing out of the ordinary is happening, let s start the application under the debugger and make our way to the first heap allocation
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0:001> u wmain 06dblfree!wmain: 01001180 55 push ebp 01001181 8bec mov ebp,esp 01001183 51 push ecx 01001184 68a8100001 push offset 06dblfree!`string (010010a8) 01001189 ff1548100001 call dword ptr [06dblfree!_imp__printf (01001048)] 0100118f 83c404 add esp,4 01001192 ff1550100001 call dword ptr [06dblfree!_imp___getch (01001050)] 01001198 6a0a push 0Ah 0:001> u 06dblfree!wmain+0x1a: 0100119a 6a00 push 0 0100119c ff1508100001 call dword ptr [06dblfree!_imp__GetProcessHeap (01001008)] 010011a2 50 push eax 010011a3 ff1500100001 call dword ptr [06dblfree!_imp__HeapAlloc (01001000)] 010011a9 8945fc mov dword ptr [ebp-4],eax 010011ac 8b45fc mov eax,dword ptr [ebp-4] 010011af c6000a mov byte ptr [eax],0Ah 010011b2 8b4dfc mov ecx,dword ptr [ebp-4] 0:001> g 010011a9 eax=000830c0 ebx=7ffde000 ecx=7c9106eb edx=00080608 esi=01c7078e edi=83485b7a eip=010011a9 esp=0006ff40 ebp=0006ff44 iopl=0 nv up ei pl zr na pe nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00000246 06dblfree!wmain+0x29: 010011a9 8945fc mov dword ptr [ebp-4],eax ss:0023:0006ff40={msvcrt!__winitenv (77c61a40)}