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Code Overwrite and System Code Write Protection


A driver with a bug that causes corruption or misinterpretation of its own data structures can reference memory the driver doesn’t own when it interprets corrupted data as a memory pointer value. The target of the pointer can be anything in the virtual address space, including data belonging to other drivers, invalid memory, or the code of other drivers or the kernel. As with buffer overruns, by the time that corruption is detected and the system crashes, it’s usually impossible to identify the driver that caused the corruption. Enabling special pool increases the chance of catching wild-pointer bugs, but it does not catch code corruption.

When you run Notmyfault and select the Code Overwrite option, the Myfault driver corrupts the entry point to the NtReadFile kernel function. One of two things will happen at this point: if your system has 255 MB or less of physical memory, you’ll get a crash for which an analysis points at Myfault.sys. The stop code description that a verbose analysis displays tells you that Myfault attempted to write to read-only memory:

An attempt was made to write to readonly memory. The guilty driver is on the
stack trace (and is typically the current instruction pointer).
When possible, the guilty driver’s name (Unicode string) is printed on
the bugcheck screen and saved in KiBugCheckDriver.
Arg1: 804bb7fd, Virtual address for the attempted write.
Arg2: 004bb121, PTE contents.
Arg3: b804db60, (reserved)
Arg4: 0000000b, (reserved)

However, if you have more than 255 MB of memory, you’ll get a different type of crash because the attempt to corrupt the memory isn’t caught. Because NtReadFile is a commonly executed system service that is used by the Windows subsystem to read keyboard and mouse input, the system will almost immediately crash as a thread attempts to execute the corrupted code and generates an illegal instruction fault. The analysis of crashes generated with this bug is always wrong, but it might vary, with Win32k.sys and Ntoskrnl.exe commonly being the analyzer’s best guess as to what’s responsible. The bugcheck description for these crashes is:

This is a very common bugcheck. Usually the exception address pinpoints
the driver/function that caused the problem. Always note this address
as well as the link date of the driver/image that contains this address.
Arg1: c0000005, The exception code that was not handled
Arg2: 80461885, The address that the exception occurred at
Arg3: 00000000, Parameter 0 of the exception
Arg4: 00000000, Parameter 1 of the exception

The reason for the different behaviors on different configurations relates to a mechanism called system code write protection. If system code write protection is enabled, the memory manager maps Ntoskrnl.exe, the HAL, and boot drivers using standard physical pages (4 KB on x86 and x64, and 8 KB on IA64). Because the granularity of protection in an image is the standard page size, the memory manager can write-protect code pages so that an attempt to modify them generates an access fault (as seen in the first crash). However, when system code write protection is disabled on systems with more than 255 MB of RAM, the memory manager uses large pages (4 MB on x86, and 16 MB on IA64 and x86-64) to map Ntoskrnl.exe and the HAL.

If system code write protection is off and crash analysis reports unlikely causes for a crash or you suspect code corruption, you should enable it. Verifying at least one driver with the Driver Verifier is the easiest way to enable it. You can also enable it manually by adding two registry values under HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management. First, specify the amount of RAM at which the memory manager uses large pages instead of standard pages to map Ntoskrnl.exe as an effectively infinite value. You do this by creating a DWORD value called LargePageMinimum and setting it to 0xFFFFFFFF. Then add another DWORD value named EnforceWriteProtection and set it to 1. You must reboot for the changes to take effect.

When the debugger has access to the image files included in a crash dump, the analysis internally executes the !chkimg debugger command to verify that a copy of an image in a crash dump matches the on-disk image and reports any differences. Note that chkimg will always report discrepancies in Ntoskrnl.exe if you’ve enabled the Driver Verifier.

Source of Information : Microsoft Press Windows Internals 5th Edition


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