MEMORY CORRUPTION PART II HEAPS in Software

Printer Quick Response Code in Software MEMORY CORRUPTION PART II HEAPS
6 MEMORY CORRUPTION PART II HEAPS
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Memory Corruption Part II Heaps
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Listing 67
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0 PsuedoTag Entries: 00000000 Virtual Alloc List: 00080050 UCR FreeList: 00080598 FreeList Usage: 00000000 00000000 00000000 00000000 FreeList[ 00 ] at 00080178: 00083188 00083188 00083180: 003a8 00378 [00] - free Unable to read nt!_HEAP_FREE_ENTRY structure at 0065004a Segment00 at 00080640: Flags: 00000000 Base: 00080000 First Entry: 00080680 Last Entry: 00180000 Total Pages: 00000100 Total UnCommit: 000000fc Largest UnCommit:000fc000 UnCommitted Ranges: (1) 00084000: 000fc000 Heap entries for Segment00 in Heap 00080000: 00000 00640 [01] 00080640: 00640 00040 [01] 00080680: 00040 01808 [01] 00081e88: 01808 00210 [01] 00082098: 00210 00228 [01] 000822c0: 00228 00090 [01] 00082350: 00090 00030 [01] 00082380: 00030 00018 [01] 00082398: 00018 00068 [01] 00082400: 00068 00230 [01] 00082630: 00230 002e0 [01] 00082910: 002e0 00320 [01] 00082c30: 00320 00320 [01] 00082f50: 00320 00030 [01] 00082f80: 00030 00030 [01] 00082fb0: 00030 00050 [01] 00083000: 00050 00048 [01] 00083048: 00048 00038 [01] 00083080: 00038 00010 [01] 00083090: 00010 00050 [01] 000830e0: 00050 00018 [01] 000830f8: 00018 00068 [01] 00083160: 00068 00020 [01] 00083180: 003a8 00378 [00] 000834f8: 00000 00000 [00] 00080000 busy (640) busy (40) busy (1800) busy (208) busy (21a) busy (84) busy (22) busy (10) busy (5b) busy (224) busy (2d8) busy (314) busy (314) busy (24) busy (24) busy (40) busy (40) busy (2a) busy (1) busy (44) busy (10) busy (5b) busy (14)
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Heap Corruptions
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The last heap entry in a segment is typically a free block In Listing 67, however, we have a couple of odd entries at the end The status of the heap blocks (0) seems to indicate that both blocks are free; however, the size of the blocks does not seem to match up Let s look at the first free block:
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00083180: 003a8 00378 [00]
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The heap block states that the size of the previous block is 003a8 and the size of the current block is 00378 Interestingly enough, the prior block is reporting its own size to be 0x20 bytes, which does not match up well Even worse, the last free block in the segment states that both the previous and current sizes are 0 If we go even further back in the heap segment, we can see that all the heap entries prior to 00083160 make sense (at least in the sense that the heap entry metadata seems intact) One of the potential theories should now start to take shape The usage of the heap block at location 00083160 seems suspect, and it s possible that the usage of that heap block caused the metadata of the following block to become corrupt Who allocated the heap block at 00083160 If we take a closer look at the block, we can see if we can recognize the content:
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0:000> dd 00083160 00083170 00083180 00083190 000831a0 000831b0 000831c0 000831d0 00083160 000d0004 00740053 0075006f 0054006f 00000068 00000000 00000000 00000000 000c0199 00690072 0064006c 00650068 00000000 00000000 00000000 00000000 00000000 0067006e 00650052 00720043 00000000 00000000 00000000 00000000
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00730069 00680053 00720070 00730061 00000000 00000000 00000000 00000000
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Parts of the block seem to resemble a string If we use the du command on the block starting at address 000830f8+0xc, we see the following:
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6 MEMORY CORRUPTION PART II HEAPS
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0:000> du 00083160+c 0008316c isStringShouldReproTheCrash
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The string definitely looks familiar It is the same string (or part of it) that we passed in on the command line Furthermore, the string seems to stretch all the way to address 000831a0, which crosses the boundary to the next reported free block at address 00083180 If we dump out the heap entry at address 00083180, we can see the following:
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0:000> dt _HEAP_ENTRY 00083180 +0x000 Size : 0x6f
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Memory Corruption Part II Heaps
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+0x002 +0x000 +0x004 +0x005 +0x006 +0x007
PreviousSize SubSegmentCode SmallTagIndex Flags UnusedBytes SegmentIndex
: : : : : :
0x75 0x0075006 0x6c l 0 0x64 d 0
The current and previous size fields correspond to part of the string that crossed the boundary of the previous block Armed with the knowledge of which string seemed to have caused the heap block overwrite, we can turn to code reviewing and figure out relatively easily that the string copy function wrote more than the maximum number of characters allowed in the destination string, causing an overwrite of the next heap block While the heap manager was unable to detect the overwrite at the exact point it occurred, it definitely detected the heap block overwrite later on in the execution, which resulted in an access violation because the heap was in an inconsistent state In the previous simplistic application, analyzing the heap at the point of the access violation yielded a very clear picture of what overwrote the heap block and subsequently, via code reviewing, who the culprit was Needless to say, it is not always possible to arrive at these conclusions merely by inspecting the contents of the heap blocks The complexity of the system can dramatically reduce your success when using this approach Furthermore, even if you do get some clues to what is overwriting the heap blocks, it might be really difficult to find the culprit by merely reviewing code Ultimately, the easiest way to figure out a heap corruption would be if we could break execution when the memory is being overwritten rather than after Fortunately, the Application Verifier tool provides a powerful facility that enables this behavior The application verifier test setting commonly used when tracking down heap corruptions is called the Heaps test setting (also referred to as pageheap) Pageheap works on the basis of surrounding the heap blocks with a protection layer that serves to isolate the heap blocks from one another If a heap block is overwritten, the protection layer detects the overwrite as close to the source as possible and breaks execution, giving the developer the ability to investigate why the overwrite occurred Pageheap runs in two different modes: normal pageheap and full pageheap The primary difference between the two modes is the strength of the protection layer Normal pageheap uses fill patterns in an attempt to detect heap block corruptions The utilization of fill patterns requires that another call be made to the heap manager post corruption so that the heap manager has the chance to validate the integrity (check fill patterns) of the heap block and report any inconsistencies Additionally, normal page heap keeps the stack trace for all allocations, making it easier to understand who allocated the memory Figure 610 illustrates what a heap block looks like when normal page heap is turned on