Heap Corruptions in Software

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0:000> dd edx 00080178 000830f0 00080188 00080188 00080198 00080198 000801a8 000801a8 000801b8 000801b8 000801c8 000801c8 000801d8 000801d8 000801e8 000801e8
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This time around, you can see that the edx register contains a pointer value that is pointing to accessible, albeit incorrect, memory No longer is the array initialized to pointer values that cause an immediate access violation (baadf00d) when dereferenced As a matter of fact, stepping over the faulting instruction this time around succeeds Do we know the origins of the pointer value we just used Not at all It could be any memory location in the process The incorrect usage of the pointer value might end up causing serious problems somewhere else in the application in paths that rely on the state of that memory to be intact If we resume execution of the application, we will notice that an access violation does in fact occur, albeit much later in the execution
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0:000> g (1a875c): Access violation - code c0000005 (first chance) First chance exceptions are reported before any exception handling This exception may be expected and handled eax=0000000a ebx=00080000 ecx=00080178 edx=00000000 esi=00000002 edi=0000000f eip=7c911404 esp=0006f77c ebp=0006f99c iopl=0 nv up ei pl nz ac po nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00010212 ntdll!RtlAllocateHeap+0x6c9: 7c911404 0fb70e movzx ecx,word ptr [esi] ds:0023:00000002= 0:000> g (1a875c): Access violation - code c0000005 (!!! second chance !!!) eax=0000000a ebx=00080000 ecx=00080178 edx=00000000 esi=00000002 edi=0000000f eip=7c911404 esp=0006f77c ebp=0006f99c iopl=0 nv up ei pl nz ac po nc cs=001b ss=0023 ds=0023 es=0023 fs=003b gs=0000 efl=00000212 ntdll!RtlAllocateHeap+0x6c9: 7c911404 0fb70e movzx ecx,word ptr [esi] ds:0023:00000002= 0:000> k ChildEBP RetAddr 0007f9b0 7c80e323 ntdll!RtlAllocateHeap+0x6c9 0007fa24 7c80e00d kernel32!BasepComputeProcessPath+0xb3 0007fa64 7c80e655 kernel32!BaseComputeProcessDllPath+0xe3 0007faac 7c80e5ab kernel32!GetModuleHandleForUnicodeString+0x28 0007ff30 7c80e45c kernel32!BasepGetModuleHandleExW+0x18e
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6 MEMORY CORRUPTION PART II HEAPS
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Memory Corruption Part II Heaps
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0007ff48 0007ff54 0007ff60 0007ff70 0007ff84 0007ffc0 0007fff0
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7c80b6c0 77c39d23 77c39e78 77c39e90 01001429 7c816fd7 00000000
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kernel32!GetModuleHandleW+0x29 kernel32!GetModuleHandleA+0x2d msvcrt!__crtExitProcess+0x10 msvcrt!_cinit+0xee msvcrt!exit+0x12 06uninit!__wmainCRTStartup+0x118 kernel32!BaseProcessStart+0x23
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As you can see, the stack reporting the access violation has nothing to do with any of our own code All we really know is that when the process is about to exit, as you can see from the bottommost frame (msvcrt!__crtExitProcess+0x10), it tries to allocate memory and fails in the memory manager Typically, access violations occurring in the heap manager are good indicators that a heap corruption has occurred Backtracking the source of the corruption from this location can be an excruciatingly difficult process that should be avoided at all costs From the two previous sample runs, it should be evident that trapping a heap corruption at the point of occurrence is much more desirable than sporadic failures in code paths that we do not directly own One of the ways we can achieve this is by starting the process under the debugger and letting the heap manager use fill patterns to provide some level of protection Although the heap manager does provide this mechanism, it is not necessarily the strongest level of protection The usage of fill patterns requires that a call be made to the heap manager so that it can validate that the fill pattern is still valid Most of the time, the damage has already been done at the point of validation, and the fault caused by the heap manager still requires us to work backward and figure out what caused the fault to begin with In addition to uninitialized state, another very common scenario that results in heap corruptions is a heap overrun
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Heap Overruns and Underruns
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In the introduction to this chapter, we looked at the internal workings of the heap manager and how all heap blocks are laid out Figure 68 illustrated how a heap block is broken down and what auxiliary metadata is kept on a per-block basis for the heap manager to be capable of managing the block If a faulty piece of code overwrites any of the metadata, the integrity of the heap is compromised and the application will fault The most common form of metadata overwriting is when the owner of the heap block does not respect the boundaries of the block This phenomenon is known as a heap overrun or, reciprocally, a heap underrun Let s take a look at an example The application shown in Listing 66 simply makes a copy of the string passed in on the command line and prints out the copy
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