ocxl: Fix page fault handler in case of fault on dying process
If a process exits without doing proper cleanup, there's a window where an opencapi device can try to access the memory of the dying process and may trigger a page fault. That's an expected scenario and the ocxl driver holds a reference on the mm_struct of the process until the opencapi device is notified of the process exiting. However, if mm_users is already at 0, i.e. the address space of the process has already been destroyed, the driver shouldn't try resolving the page fault, as it will fail, but it can also try accessing already freed data. It is fixed by only calling the bottom half of the page fault handler if mm_users is greater than 0 and get a reference on mm_users instead of mm_count. Otherwise, we can safely return a translation fault to the device, as its associated memory context is being removed. The opencapi device will be properly cleaned up shortly after when closing the file descriptors. Fixes: 5ef3166e ("ocxl: Driver code for 'generic' opencapi devices") Cc: stable@vger.kernel.org # v4.16+ Signed-off-by: Frederic Barrat <fbarrat@linux.ibm.com> Reviewed-By: Alastair D'Silva <alastair@d-silva.org> Acked-by: Andrew Donnellan <andrew.donnellan@au1.ibm.com> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
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