(English) Microsoft Windows FastFAT.sys Sectors per FAT Denial of Service Vulnerability

Do You remember story about MS14-063 from last year ? It turns out there is continuation of it, but this time inside FAT12 partition.
[VIDEO] Time to stick the magic stick


Affected systems
From Windows NT to Windows 7 SP1
I personally tested it on :
* Windows XP SP3 x86
* Windows 7 SP1 x86/x64
Bug reconstruction
1. Create FAT12 partition and set WORD at offset 0x16 (Sectors per FAT) to e.g. 0x3000.
Do NOT exceed 0x3FFF value.
How to do this http://wiki.osdev.org/Loopback_Device and suggested parameters bs=512 count=32067.
Image can be empty.
Malformed value of field “Sectors per FAT” in FAT12 Boot sector, leads to attempt to read of unallocated memory region during FAT 1 mapping into cache.
My malformed partition presents in the following way:
Crash dump
Windows 7 SP1 x86

kd> !analyze -v
*                                                                             *
*                        Bugcheck Analysis                                    *
*                                                                             *
Invalid system memory was referenced.  This cannot be protected by try-except,
it must be protected by a Probe.  Typically the address is just plain bad or it
is pointing at freed memory.
Arg1: a3f00000, memory referenced.
Arg2: 00000000, value 0 = read operation, 1 = write operation.
Arg3: 82a6a05e, If non-zero, the instruction address which referenced the bad memory
Arg4: 00000000, (reserved)
Debugging Details:
READ_ADDRESS:  a3f00000
82a6a05e 8a0e            mov     cl,byte ptr [esi]
PROCESS_NAME:  explorer.exe
TRAP_FRAME:  a2447414 -- (.trap 0xffffffffa2447414)
ErrCode = 00000000
eax=000006e2 ebx=86eb8b98 ecx=a2447400 edx=00000011 esi=a3f00000 edi=0000000f
eip=82a6a05e esp=a2447488 ebp=a24474c8 iopl=0         nv up ei pl nz ac po nc
cs=0008  ss=0010  ds=0023  es=0023  fs=0030  gs=0000             efl=00010212
82a6a05e 8a0e            mov     cl,byte ptr [esi]          ds:0023:a3f00000=??
Resetting default scope
LAST_CONTROL_TRANSFER:  from 82916d5f to 828b27b8
a2446f64 82916d5f 00000003 6641e18b 00000065 nt!RtlpBreakWithStatusInstruction
a2446fb4 8291785d 00000003 00000000 00082982 nt!KiBugCheckDebugBreak+0x1c
a2447378 828c5879 00000050 a3f00000 00000000 nt!KeBugCheck2+0x68b
a24473fc 82878aa8 00000000 a3f00000 00000000 nt!MmAccessFault+0x104
a24473fc 82a6a05e 00000000 a3f00000 00000000 nt!KiTrap0E+0xdc
a24474c8 975dbb32 86e819c0 004474ec 00721e00 nt!CcMapData+0xae
a24474f4 975d9e1d a2b52fb8 85dad920 00001000 fastfat!FatReadVolumeFile+0x3a
a2447580 975da534 a2b52fb8 a2447524 00000002 fastfat!FatExamineFatEntries+0x11f
a24475e8 975ecd64 a2b52fb8 85dad920 3519a638 fastfat!FatSetupAllocationSupport+0x38a
a2447774 975ee3cf a2b52fb8 8a9b0da8 a3392fa8 fastfat!FatMountVolume+0x418
a2447794 975ee478 a2b52fb8 93528e90 3519a6ac fastfat!FatCommonFileSystemControl+0x3f
a24477e0 82b696c3 82275620 93528e90 93528e90 fastfat!FatFsdFileSystemControl+0x82
a2447804 8286ebd5 00000001 93528ff4 82275620 nt!IovCallDriver+0x258
a2447818 85499a56 270d61d5 8229bde8 93528e90 nt!IofCallDriver+0x1b
a2447878 85499c5b 8229bde8 93528e90 86f2c678 fltmgr!FltpFsControlMountVolume+0x180
a24478a8 82b696c3 8229bde8 93528e90 93528e90 fltmgr!FltpFsControl+0x5b
a24478cc 8286ebd5 00000001 8296fb88 8229bde8 nt!IovCallDriver+0x258
a24478e0 829d1dd9 82804870 8a9b0da8 82804900 nt!IofCallDriver+0x1b
a2447944 828df92e 8a9b0da8 85dac000 00000000 nt!IopMountVolume+0x1d8
a244797c 82a7ddfb 85dac008 a2447aa8 a2447a40 nt!IopCheckVpbMounted+0x64
a2447a60 82a5dd1e 8a9b0da8 851cfde8 85d66750 nt!IopParseDevice+0x7db
a2447adc 82a6e147 00000000 a2447b30 00000040 nt!ObpLookupObjectName+0x4fa
a2447b38 82a64c25 02effc44 851cfde8 828b1d01 nt!ObOpenObjectByName+0x165
a2447bb4 82a884a4 02effca0 00100081 02effc44 nt!IopCreateFile+0x673
a2447c00 828758c6 02effca0 00100081 02effc44 nt!NtCreateFile+0x34
a2447c00 776170f4 02effca0 00100081 02effc44 nt!KiSystemServicePostCall
02effc00 77615614 7573a9d9 02effca0 00100081 ntdll!KiFastSystemCallRet
02effc04 7573a9d9 02effca0 00100081 02effc44 ntdll!ZwCreateFile+0xc
02effca8 772ce99f 00004000 00100081 00000007 KERNELBASE!CreateFileW+0x35e
02effcd4 761a2fc5 03cc0038 00000001 00000007 kernel32!CreateFileWImplementation+0x69
02effd38 761a2bc9 03cc0038 00000001 00000000 SHELL32!CLocalInterruptSource::v_CreateEvent+0x41
02effd5c 761aac0f 0368f598 00000000 02effd94 SHELL32!CFSInterruptSource::GetEvent+0x7b
02effd9c 761aa92a 02effdc0 00000000 006e7a90 SHELL32!CChangeNotify::_GetInterruptEvents+0x93
02effdc8 7611561b 00000000 00000000 00000067 SHELL32!CChangeNotify::_MessagePump+0x67
02effde0 75fd43c0 006e7a90 00000000 00000000 SHELL32!CChangeNotify::s_ThreadProc+0x4f
02effe68 772cee1c 0201ed74 02effeb4 776337eb SHLWAPI!WrapperThreadProc+0x1b5
02effe74 776337eb 0201ed74 758640f9 00000000 kernel32!BaseThreadInitThunk+0xe
02effeb4 776337be 75fd42ed 0201ed74 00000000 ntdll!__RtlUserThreadStart+0x70
02effecc 00000000 75fd42ed 0201ed74 00000000 ntdll!_RtlUserThreadStart+0x1b
82a6a05e 8a0e            mov     cl,byte ptr [esi]
SYMBOL_NAME:  nt!CcMapData+ae
FOLLOWUP_NAME:  MachineOwner
IMAGE_NAME:  ntkrpamp.exe
FAILURE_BUCKET_ID:  0x50_VRF_nt!CcMapData+ae
BUCKET_ID:  0x50_VRF_nt!CcMapData+ae
Followup: MachineOwner

Vulnerable code:

Line 72:
CcMapData (
    __in PFILE_OBJECT FileObject,
    __in PLARGE_INTEGER FileOffset,
    __in ULONG Length,
    __in ULONG Flags,
    __out PVOID *Bcb,
    __deref_out_bcount(Length) PVOID *Buffer
Line 155:
    ULONG PageCount = ADDRESS_AND_SIZE_TO_SPAN_PAGES((ULongToPtr(FileOffset->LowPart)), Length);
    PETHREAD Thread = PsGetCurrentThread();
    BOOLEAN ReturnStatus;
    DebugTrace(+1, me, "CcMapData\n", 0 );
    MmSavePageFaultReadAhead( Thread, &SavedState );
    ReturnStatus = CcMapDataCommon( FileObject,
                                    Buffer );
Line 193
            //  Loop to touch each page
            BaseAddress = *Buffer;
            while (PageCount != 0) {
                MmSetPageFaultReadAhead( Thread, PageCount - 1 );
                ch = *((volatile UCHAR *)(BaseAddress));   // <----------   CRASH!!!!
                BaseAddress = (PCHAR) BaseAddress + PAGE_SIZE;
                PageCount -= 1;

PageCount is calculated based on controlled Length value, which in this case (read of FATx) consists of:
Sectors per FAT * Bytes per sector

kd> . frame /c 5
kd> dd ebp+10 L1 //get Length value
f53e1510  00721e00
kd> ?00721e00 / 0x200(= Bytes per sector)
Evaluate expression: 14607 = 0000390f (= Sectors per FAT)

FileOffset equals:

kd> dt _LARGE_INTEGER poi(ebp+c)
   +0x000 LowPart          : 0x200
   +0x004 HighPart         : 0
   +0x000 u                : __unnamed
   +0x000 QuadPart         : 512

PageCount for these values equals:

Evaluate expression: 1826 = 00000722

Each While-Loop iteration, BaseAddress pointer is increased by PAGE_SIZE, so in this scenario memory region which will be “cover” by this loop is:

kd> ?(0x722 * 0x1000) / 0x400
PageCount     PAGE_SIZE     1KB
Evaluate expression: 7304 = 00001c88
kd> ?00001c88 / 0x400
Evaluate expression: 7 = 00000007   ~   7MB

Let’s check how many bytes we are really able to cache:

Line 241:
CcMapDataCommon (
    IN PFILE_OBJECT FileObject,
    IN ULONG Length,
    IN ULONG Flags,
    OUT PVOID *Bcb,
    OUT PVOID *Buffer
Line 350:
    //  Get pointer to SharedCacheMap.
    SharedCacheMap = FileObject->SectionObjectPointer->SharedCacheMap;
    //  Call local routine to Map or Access the file data.  If we cannot map
    //  the data because of a Wait condition, return FALSE.
    if (FlagOn(Flags, MAP_WAIT)) {
        *Buffer = CcGetVirtualAddress( SharedCacheMap,
                                       (PVACB *)&TempBcb,
                                       &ReceivedLength );
        ASSERT( ReceivedLength >= Length ); // maybe convert it to IF ?

checking ReceiveLength in CcGetVirtualAddress:

Line 388:
ULONG VacbOffset = FileOffset.LowPart & (VACB_MAPPING_GRANULARITY - 1);
Line  449:
*ReceivedLength = VACB_MAPPING_GRANULARITY - VacbOffset;


Line 31:
#define VACB_MAPPING_GRANULARITY         (0x40000)

Fragments of code suggest us that ReceiveLength should at least be equal Lenght and maximum value of ReceiveLength can be 0x40000 bytes which is 256KB.
MSDN CcMapData

CcMapData cannot map data across view boundaries in the cache manager. The cache manager manages files in the system in 256 KB-aligned views. (The cache manager’s view size is specified by the system-defined constant VACB_MAPPING_GRANULARITY, which is set to 256 KB in ntifs.h.) Mapped regions cannot span more than one 256 KB view. Therefore, the largest region that can be mapped is 256 KB, beginning at a 256 KB-aligned offset in the file.

Code and documentation clearly points that mapped region of file data can’t exceed 256KB.
In our case kernel will act like there is ~7MB of available space what leads to crash after iteration over available 256KB’s.
Why bug exists?
Lack of verification Length value, leads to iteration in this loop:

Line 198
            while (PageCount != 0) {
                MmSetPageFaultReadAhead( Thread, PageCount - 1 );
                ch = *((volatile UCHAR *)(BaseAddress));   // <----------   CRASH!!!!
                BaseAddress = (PCHAR) BaseAddress + PAGE_SIZE;
                PageCount -= 1;

over available mapped/allocated region.

Not always triggerable

During my research I observed that not each value of Sectors per FAT bigger than 0x0200 will cause crash. E.g. Sectors per FAT set to 0x4000 causes exception in Fastfat!FatCommonRead:
Stack trace when Sectors per FAT == 40 00

Windows XP SP3
ChildEBP RetAddr  Args to Child
bad46fc0 b7e0d505 86a69e50 bad46fec 804e24f1 Fastfat!FatExceptionFilter+0x5
bad46fcc 804e24f1 bad46ff4 00000000 bad46ff4 Fastfat!FatFsdRead+0x12b
bad46ff4 804db49a bad470d8 bad47518 bad47128 nt!_except_handler3+0x61
bad47018 804db46b bad470d8 bad47518 bad47128 nt!ExecuteHandler2+0x26
bad470c8 804dc6a1 bad470d8 bad47128 c000000d nt!ExecuteHandler+0x24
bad473fc b7e12fc4 c000000d 86a69e50 8667b888 nt!ExRaiseStatus+0xb5
bad474b8 b7e0d69a 86a69e50 8667b888 86a501e8 Fastfat!FatCommonRead+0x66b
bad47528 804e37f7 8686ba98 8667b888 00000000 Fastfat!FatFsdRead+0x13d
bad47538 804f95d8 00000000 86b01ad0 86b01ae0 nt!IopfCallDriver+0x31
bad4754c 804f95ff 8686ba98 86b01b08 86b01ae8 nt!IopPageReadInternal+0xf4
bad4756c 804f9264 86a501e8 86b01b08 86b01ae8 nt!IoPageRead+0x1b
bad475e0 804eba6a 0dead8c0 c7fc0000 c031ff00 nt!MiDispatchFault+0x274
bad47630 804e1718 00000000 c7fc0000 00000000 nt!MmAccessFault+0x5bc
bad47630 8056d716 00000000 c7fc0000 00000000 nt!KiTrap0E+0xcc
bad47708 b7e15b25 86a501e8 bad47734 00001000 nt!CcMapData+0xef
bad4773c b7e1e8b6 86a84e28 8698b3a8 00000000 Fastfat!FatReadDirectoryFile+0x92
bad47770 b7e137b7 86a84e28 8686bb90 bad47880 Fastfat!FatLocateVolumeLabel+0x7f
bad4790c b7e10a93 86a84e28 86b6a718 86bd3af0 Fastfat!FatMountVolume+0x49b
bad4792c b7e10a38 86a84e28 8690eaf0 8690ec78 Fastfat!FatCommonFileSystemControl+0x49
bad47978 804e37f7 86a2d928 8690eaf0 8690ec9c Fastfat!FatFsdFileSystemControl+0x85
b7e0d485 53              push    ebx
kd> .exr poi(poi(ebp+c))
ExceptionAddress: b7e12fc4 (Fastfat!FatCommonRead+0x0000066b)
   ExceptionCode: c000000d  // STATUS_INVALID_PARAMETER
   ExceptionFlags: 00000001
NumberParameters: 0

I did not dig deeper, so I cannot point place in code where validation on probably FileObject->Length is made and cause this exception. But it looks like there is already some kind of Length value validation but NOT sufficiently accurate.

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