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6 MEMORY CORRUPTION PART II HEAPS
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Listing 61
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#include <windowsh> #include <stdioh> #include <conioh> int __cdecl wmain (int argc, wchar_t* pArgs[]) { BYTE* pAlloc1=NULL; BYTE* pAlloc2=NULL; HANDLE hProcessHeap=GetProcessHeap(); pAlloc1=(BYTE*)HeapAlloc(hProcessHeap, 0, 16); pAlloc2=(BYTE*)HeapAlloc(hProcessHeap, 0, 1500); // // Use allocated memory // HeapFree(hProcessHeap, 0, pAlloc1); HeapFree(hProcessHeap, 0, pAlloc2); }
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The source code and binary for Listing 61 can be found in the following folders: Source code: C:\AWD\6\BasicAlloc Binary: C:\AWDBIN\WinXPx86chk\06BasicAllocexe Run this application under the debugger and break on the wmain function Because we are interested in finding out more about the heap state, we must start by finding out what heaps are active in the process Each running process keeps a list of active heaps The list of heaps is stored in the PEB (process environment block), which is simply a data structure that contains a plethora of information about the process To dump out the contents of the PEB, we use the dt command, as illustrated in Listing 62 Listing 62
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0:000> dt +0x000 +0x001 +0x002 _PEB @$peb InheritedAddressSpace : 0 ReadImageFileExecOptions : 0 BeingDebugged : 0x1
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SpareBool : 0 Mutant : 0xffffffff ImageBaseAddress : 0x01000000 Ldr : 0x00191e90 _PEB_LDR_DATA ProcessParameters : 0x00020000 _RTL_USER_PROCESS_PARAMETERS SubSystemData : (null) ProcessHeap : 0x00080000 FastPebLock : 0x7c97e4c0 _RTL_CRITICAL_SECTION FastPebLockRoutine : 0x7c901005 FastPebUnlockRoutine : 0x7c9010ed EnvironmentUpdateCount : 1 KernelCallbackTable : (null) SystemReserved : [1] 0 AtlThunkSListPtr32 : 0 FreeList : (null) TlsExpansionCounter : 0 TlsBitmap : 0x7c97e480 TlsBitmapBits : [2] 1 ReadOnlySharedMemoryBase : 0x7f6f0000 ReadOnlySharedMemoryHeap : 0x7f6f0000 ReadOnlyStaticServerData : 0x7f6f0688 -> (null) AnsiCodePageData : 0x7ffb0000 OemCodePageData : 0x7ffc1000 UnicodeCaseTableData : 0x7ffd2000 NumberOfProcessors : 1 NtGlobalFlag : 0 CriticalSectionTimeout : _LARGE_INTEGER 0xffffffff`dc3cba00 HeapSegmentReserve : 0x100000 HeapSegmentCommit : 0x2000 HeapDeCommitTotalFreeThreshold : 0x10000 HeapDeCommitFreeBlockThreshold : 0x1000 NumberOfHeaps : 3 MaximumNumberOfHeaps : 0x10 ProcessHeaps : 0x7c97de80 -> 0x00080000 GdiSharedHandleTable : (null) ProcessStarterHelper : (null) GdiDCAttributeList : 0 LoaderLock : 0x7c97c0d8 OSMajorVersion : 5 OSMinorVersion : 1 OSBuildNumber : 0xa28 OSCSDVersion : 0x200 OSPlatformId : 2 ImageSubsystem : 3 ImageSubsystemMajorVersion : 4 ImageSubsystemMinorVersion : 0
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6 MEMORY CORRUPTION PART II HEAPS
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Listing 62
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ImageProcessAffinityMask : 0 GdiHandleBuffer : [34] 0 PostProcessInitRoutine : (null) TlsExpansionBitmap : 0x7c97e478 TlsExpansionBitmapBits : [32] 0 SessionId : 0 AppCompatFlags : _ULARGE_INTEGER 0x0 AppCompatFlagsUser : _ULARGE_INTEGER 0x0 pShimData : (null) AppCompatInfo : (null) CSDVersion : _UNICODE_STRING Service Pack 2 ActivationContextData : (null) ProcessAssemblyStorageMap : (null) SystemDefaultActivationContextData : 0x00080000 SystemAssemblyStorageMap : (null) MinimumStackCommit : 0
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As you can see, PEB contains quite a lot of information, and you can learn a lot by digging around in this data structure to familiarize yourself with the various components In this particular exercise, we are specifically interested in the list of process heaps located at offset 0x90 The heap list member of PEB is simply an array of pointers, where each pointer points to a data structure of type _HEAP Let s dump out the array of heap pointers and see what it contains:
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0:000> dd 7c97de80 7c97de90 7c97dea0 7c97deb0 7c97dec0 7c97ded0 7c97dee0 7c97def0
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0x7c97de80 00080000 00180000 00000000 00000000 00000000 00000000 00000000 00000000 01a801a6 00020498 7ffd2de6 00000000 ffff7e77 00000000 004e0049 004f0044
00190000 00000000 00000000 00000000 00000001 00000005 003a0044 00530057
00000000 00000000 00000000 00000000 7c9b0000 00000001 0057005c 0073005c
The dump shows that three heaps are active in our process, and the default process heap pointer is always the first one in the list Why do we have more than one heap in our process Even the simplest of applications typically contains more than one heap Most applications implicitly use components that create their own heaps A great example is the C runtime, which creates its own heap during initialization