Hi, Dave, On Thu, Apr 28, 2022 at 08:09:04AM -0700, Dave Hansen wrote: > On 4/25/22 21:20, Fenghua Yu wrote: > >>From 84aa68f6174439d863c40cdc2db0e1b89d620dd0 Mon Sep 17 00:00:00 2001 > > From: Fenghua Yu <fenghua...@intel.com> > > Date: Fri, 15 Apr 2022 00:51:33 -0700 > > Subject: [PATCH] iommu/sva: Fix PASID use-after-free issue > > > > A PASID might be still used on ARM after it is freed in __mmput(). > > Is it really just ARM?
Actually it should happen on X86 as well. I will remove "on ARM" in the changelog. > > > process: > > open()->sva_bind()->ioasid_alloc() = N; // Get PASID N for the mm > > exit(); > > exit_mm()->__mmput()->mm_pasid_drop()->mm->pasid = -1; // PASID -1 > > exit_files()->release(dev)->sva_unbind()->use mm->pasid; // Failure > > > > To avoid the use-after-free issue, free the PASID after no device uses it, > > i.e. after all devices are unbound from the mm. > > > > sva_bind()/sva_unbind() call mmgrab()/mmdrop() to track mm->mm_count. > > __mmdrop() is called only after mm->mm_count is zero. So freeing the PASID > > in __mmdrop() guarantees the PASID is safely freed only after no device > > is bound to the mm. > > Does this changelog work for everyone? > > == > > tl;dr: 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. > Do you want me to change the changlog to add both this paragraph and the following paragraph? > -- > > As a reminder: > > 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 ->release() time. > > Device drivers hold a reference on the mm itself and drop it at > ->release() time. But, the device driver holds a reference 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. > > To fix this, move the PASID free operation from __mmput() to __mmdrop(). > This ensures that the device drivers' existing mmgrab() keeps the PASID > allocated until they drop their mm reference. > Thank you very much! -Fenghua _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu
