MEMORY CORRUPTION PART II HEAPS in Software

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6 MEMORY CORRUPTION PART II HEAPS
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The status of the previous and next heap blocks are both busy (Flags=0x7), which means that the heap manager is not capable of coalescing the memory, and the heap
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Memory Corruption Part II Heaps
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block is simply put on the free lists More specifically, the heap block will go into free list[1] because the size is 16 bytes Let s verify our theory step over the HeapFree call and use the same mechanism as previously used to see what happened to the heap block
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0:000> dt +0x000 +0x002 +0x000 +0x004 +0x005 +0x006 +0x007 _HEAP_ENTRY 0x000830c8-(0xbf*8)-(0x5*8) Size : 5 PreviousSize : 0xb SubSegmentCode : 0x000b0005 SmallTagIndex : 0x1f Flags : 0x4 UnusedBytes : 0x18 SegmentIndex : 0
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As you can see, the heap block status is indeed set to be free, and the size remains the same Since the size remains the same, it serves as an indicator that the heap manager did not coalesce the heap block with adjacent blocks Last, we verify that the block made it into the free list[1] I will leave it as an exercise for the reader to figure out what happens to the segment and heap blocks during the next call to HeapFree Here s a hint: Remember that the size of the heap block being freed is 1500 bytes and that the state of one of the adjacent blocks is set to free This concludes our overview of the internal workings of the heap manager Although it might seem like a daunting task to understand and be able to walk the various heap structures, after a little practice, it all becomes easier Before we move on to the heap corruption scenarios, one important debugger command can help us be more efficient when debugging heap corruption scenarios The extension command is called !heap and is part of the extsdll debugger extension Using this command, you can very easily display all the heap information you could possibly want Actually, all the information we just manually gathered is outputted by the !heap extension command in a split second But wait we just spent a lot of time figuring out how to analyze the heap by hand, walk the segments, and verify the heap blocks Why even bother if we have this beautiful command that does all the work for us As always, the answer lies in how the debugger arrives at the information it presents If the state of the heap is intact, the !heap extension command shows the heap state in a nice and digestible form If, however, the state of the heap has been corrupted, it is no longer sufficient to rely on the command to tell us what and how it became corrupted We need to know how to analyze the various parts of the heap to arrive at sound conclusions and possible culprits
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Heap Corruptions
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The debug session you have seen so far has involved running a process under the debugger from start to finish Another option when debugging processes is attaching the debugger to an already-running process Typically, using either approach will not dramatically change the way you debug the process The exception to the rule is when debugging heap-related issues When starting the process under the debugger, the heap manager modifies all requests to create new heaps and change the heap creation flags to enable debug-friendly heaps (unless the _NO_DEBUG_HEAP environment variable is set to 1) In comparison, attaching to an already-running process, the heaps in the process have already been created using default heap creation flags and will not have the debug-friendly flags set (unless explicitly set by the application) The heap modification flags apply across all heaps in the process, including the default process heap The biggest difference when starting a process under the debugger is that the heap blocks contain an additional fill pattern field after the user-accessible part (see Figure 68) The fill pattern is used by the heap manager to validate the integrity of the heap block during heap operations When an allocation is successful, the heap manager fills this area of the block with a specific fill pattern If an application mistakenly writes past the end of the user-accessible part, it overwrites all or portions of this fill pattern field The next time the application uses that allocation in any calls to the heap manager, the heap manager takes a close look at the fill pattern field to make sure that it hasn t changed If the fill pattern field was overwritten by the application, the heap manager immediately breaks into the debugger, giving you the opportunity to look at the heap block and try to infer why it was overwritten Writing to any area of a heap block outside the bounds of the actual user-accessible part is a serious error that can be devastating to the stability of an application
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