What Is a Heap in Software

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What Is a Heap
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previously If we are allocating 16 bytes (0x10), why is the difference in size of the free block before splitting and after only 0x5 bytes The key is to remember that the size reported must first be multiplied by the heap granularity factor of 0x8 The true size of the new free allocation is then 0x00000530 (0xa6*8), with the true size difference being 0x28 0x10 of those 0x28 bytes are our allocation size, and the remaining 0x18 bytes are all metadata associated with our heap block The next call to HeapAlloc attempts to allocate memory of size 1500 We know that free heap blocks of this size must be located in the free list[0] However, from our previous investigation, we also know that the only free heap block on the free list[0] is too small to accommodate the size we are requesting With its hands tied, the heap manager is now forced to commit more memory in the heap segment To get a better picture of the state of our heap segment, it is useful to do a manual walk of the segment The _HEAP structure contains an array of pointers to all segments currently active in the heap The array is located at the base _HEAP address plus an offset of 0x58
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0:000> dd 00080058 0:000> dt +0x000 +0x008 +0x00c +0x010 +0x014 +0x018 +0x01c +0x020 +0x024 +0x028 +0x02c +0x030 +0x034 +0x036 +0x038 0x00080000+0x58 l4 00080640 00000000 00000000 00000000 _HEAP_SEGMENT 0x00080640 Entry : _HEAP_ENTRY Signature : 0xffeeffee Flags : 0 Heap : 0x00080000 _HEAP LargestUnCommittedRange : 0xfd000 BaseAddress : 0x00080000 NumberOfPages : 0x100 FirstEntry : 0x00080680 _HEAP_ENTRY LastValidEntry : 0x00180000 _HEAP_ENTRY NumberOfUnCommittedPages : 0xfd NumberOfUnCommittedRanges : 1 UnCommittedRanges : 0x00080588 _HEAP_UNCOMMMTTED_RANGE AllocatorBackTraceIndex : 0 Reserved : 0 LastEntryInSegment : 0x00082ad0 _HEAP_ENTRY
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6 MEMORY CORRUPTION PART II HEAPS
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The _HEAP_SEGMENT data structure contains a slew of information used by the heap manager to efficiently manage all the active segments in the heap When walking a segment, the most useful piece of information is the FirstEntry field located at the base segment address plus an offset of 0x20 This field represents the first heap block in the segment If we dump out this block and get the size, we can dump out the next heap block by adding the size to the first heap block s address If we continue this process, the entire segment can be walked, and each allocation can be investigated for correctness
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Memory Corruption Part II Heaps
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0:000> dt +0x000 +0x002 +0x000 +0x004 +0x005 +0x006 +0x007 0:000> dt +0x000 +0x002 +0x000 +0x004 +0x005 +0x006 +0x007 0:000> dt +0x000 +0x002 +0x000 +0x004 +0x005 +0x006 +0x007 +0x000 +0x002 +0x000 +0x004 +0x005 +0x006 +0x007
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_HEAP_ENTRY 0x00080680 Size : 0x303 PreviousSize : 8 SubSegmentCode : 0x00080303 SmallTagIndex : 0x9a Flags : 0x7 UnusedBytes : 0x18 SegmentIndex : 0 _HEAP_ENTRY 0x00080680+(0x303*8) Size : 8 PreviousSize : 0x303 SubSegmentCode : 0x03030008 SmallTagIndex : 0x99 Flags : 0x7 UnusedBytes : 0x1e SegmentIndex : 0 _HEAP_ENTRY 0x00080680+(0x303*8)+(8*8) Size : 5 PreviousSize : 8 SubSegmentCode : 0x00080005 SmallTagIndex : 0x91 Flags : 0x7 UnusedBytes : 0x1a SegmentIndex : 0
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Size PreviousSize SubSegmentCode SmallTagIndex Flags UnusedBytes SegmentIndex
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0xa6 5 0x000500a6 0xee 0x14 0xee 0
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Let s see what the heap manager does to the segment (if anything) to try to satisfy the allocation request of size 1500 bytes Step over the HeapAlloc call and walk the segment again The heap block of interest is shown next
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+0x000 +0x002 +0x000 +0x004 +0x005 +0x006 +0x007 Size PreviousSize SubSegmentCode SmallTagIndex Flags UnusedBytes SegmentIndex : : : : : : : 0xbf 5 0x000500bf 0x10 0x7 0x1c 0
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What Is a Heap
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Before we stepped over the call to HeapAlloc, the last heap block was marked as free and with a size of 0xa6 After the call, the block status changed to busy with a size of 0xbf (0xbf*8= 0x5f8), indicating that this block is now used to hold our new allocation Since our allocation was too big to fit into the previous size of 0xa6, the heap manager committed more memory to the segment Did it commit just enough to hold our allocation Actually, it committed much more and put the remaining free memory into a new block at address 0x000830c8 The heap manager is only capable of asking for page sized allocations (4KB on x86 systems) from the virtual memory manager and returns the remainder of that allocation to the free lists The next couple of lines in our application simply free the allocations we just made What do we anticipate the heap manager to do when it executes the first HeapFree call In addition to updating the status of the heap block to free and adding it to the free lists, we expect it to try and coalesce the heap block with other surrounding free blocks Before we step over the first HeapFree call, let s take a look at the heap block associated with that call
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0:000> dt +0x000 +0x002 +0x000 +0x004 +0x005 +0x006 +0x007 0:000> dt +0x000 +0x002 +0x000 +0x004 +0x005 +0x006 +0x007 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 : 0x7 UnusedBytes : 0x18 SegmentIndex : 0 _HEAP_ENTRY 0x000830c8-(0xbf*8)-(0x5*8)-(0xb*8) Size : 0xb PreviousSize : 5 SubSegmentCode : 0x0005000b SmallTagIndex : 0 Flags : 0x7 UnusedBytes : 0x1c SegmentIndex : 0 _HEAP_ENTRY 0x000830c8-(0xbf*8) Size : 0xbf PreviousSize : 5 SubSegmentCode : 0x000500bf SmallTagIndex : 0x10 Flags : 0x7 UnusedBytes : 0x1c SegmentIndex : 0
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