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sys: FspFastIoDeviceControl: fix exploit reported by Tay Kiat Loong

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The WinFsp "transact" protocol is used by user mode file systems to interface
with the FSD. This protocol works via the DeviceIoControl API and uses the
FSP_IOCTL_TRANSACT control code. The FSP_IOCTL_TRANSACT code is marked as
METHOD_BUFFERED.

When the DeviceIoControl call is forwarded as an IRP, the METHOD_BUFFERED flag
instructs the kernel to copy user mode buffers to kernel mode buffers (and
vice-versa). However when the DeviceIoControl call is forwarded via the FastIO
mechanism the METHOD_BUFFERED flag is ignored. This means that when WinFsp
added support for DeviceIoControl FastIO, the FSD started accessing user mode
buffers directly.

This means that a malicious file system could attempt exploits like changing
or freeing a buffer while the FSD is reading it. Tay Kiat Loong developed a
POC exploit which demonstrated this vulnerability.

This commit fixes the problem by patching FspFastIoDeviceControl to add the
missing METHOD_BUFFERED handling.
This commit is contained in:
Bill Zissimopoulos
2026-02-17 15:21:35 +02:00
parent aed8bb745f
commit ae40f0edb1
1 changed files with 31 additions and 12 deletions
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43
src/sys/devctl.c
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@@ -72,15 +72,24 @@ BOOLEAN FspFastIoDeviceControl(
if (!Result)
FSP_RETURN();
#if 0
PDEVICE_OBJECT FsctlDeviceObject = DeviceObject;
if (!FspDeviceReference(FsctlDeviceObject))
PVOID SystemBuffer = 0;
if (0 != InputBufferLength || 0 != OutputBufferLength)
{
IoStatus->Status = STATUS_CANCELLED;
IoStatus->Information = 0;
FSP_RETURN();
SystemBuffer = FspAllocNonPaged(
InputBufferLength > OutputBufferLength ? InputBufferLength : OutputBufferLength);
if (0 == SystemBuffer)
FSP_RETURN(Result = FALSE);
if (0 != InputBuffer)
try
{
RtlCopyMemory(SystemBuffer, InputBuffer, InputBufferLength);
}
except (EXCEPTION_EXECUTE_HANDLER)
{
FspFree(SystemBuffer);
FSP_RETURN(Result = FALSE);
}
}
#endif
ASSERT(0 == IoGetTopLevelIrp());
IoSetTopLevelIrp((PIRP)FSRTL_FAST_IO_TOP_LEVEL_IRP);
@@ -88,18 +97,28 @@ BOOLEAN FspFastIoDeviceControl(
IoStatus->Status = FspVolumeFastTransact(
FileObject->FsContext2,
IoControlCode,
InputBuffer,
SystemBuffer,
InputBufferLength,
OutputBuffer,
0 != OutputBufferLength ? SystemBuffer : 0,
OutputBufferLength,
IoStatus,
(PIRP)FSRTL_FAST_IO_TOP_LEVEL_IRP);
IoSetTopLevelIrp(0);
#if 0
FspDeviceDereference(FsctlDeviceObject);
#endif
if (0 != SystemBuffer)
{
if (0 != OutputBuffer)
try
{
RtlCopyMemory(OutputBuffer, SystemBuffer, OutputBufferLength);
}
except (EXCEPTION_EXECUTE_HANDLER)
{
IoStatus->Status = GetExceptionCode();
}
FspFree(SystemBuffer);
}
FSP_LEAVE_BOOL(
"%s, FileObject=%p",