Register a callback in Kernel from a manually mapped driver.
Registering a callback in the kernel without taking further precautions will either not work or lead to a nice BSOD. This small driver, designed to be manually mapped, is able to register a callback in the kernel without triggering any crash.
By finding a codecave in a legit module in the kernel (here we take APCI driver, which is not protected by PatchGuard), we can write a JMP to our routines so that the start address of the registered function for the callback is in a valid module.
On top of that, we need to find a way to circumvent MmVerifyCallbackFunction, which is a routine designed to check for the presence of unwanted callbacks.
If we take a look at this function, we see that it's just a call to MmVerifyCallbackFunctionCheckFlags, with arg = 0x20
Then we have a look at MmVerifyCallbackFunctionCheckFlags
The 0x68 here corresponds to the offset of attribute Flags for struct _LDR_DATA_TABLE_ENTRY, as defined in internals.h.
So all we need to do, is find the corresponding data table entry and change the flag so the function will not crash the OS.
Unfortunately, the MiLookupDataTableEntry function is not exported by ntoskrnl, so we need to make a signature of this function and find it ourselves in memory.
Note that using this is easily detectable as we patch the .text section of a module, and discrepancies can be checked versus the file on disk