Description of problem: The PASID is being freed too early. It needs to stay around until after device drivers that might be using it have had a chance to clear it out of the hardware. The relevant refcounts are: mmget() /mmput() refcount the mm's address space mmgrab()/mmdrop() refcount the mm itself The PASID is currently tied to the life of the mm's address space and freed in __mmput(). This makes logical sense because the PASID can't be used once the address space is gone. But, this misses an important point: even after the address space is gone, the PASID will still be programmed into a device. Device drivers might, for instance, still need to flush operations that are outstanding and need to use that PASID. They do this at file->release() time. Device drivers call the IOMMU driver to hold a reference on the mm itself and drop it at file->release() time. But, the IOMMU driver holds a reference on the mm itself, not the address space. The address space (and the PASID) is long gone by the time the driver tries to clean up. This is effectively a use-after-free bug on the PASID.