[PATCH v2] iommu/sva: Fix PASID use-after-free issue

[Fenghua Yu]

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.

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 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.

To fix this, move the PASID free operation from __mmput() to __mmdrop().
This ensures that the IOMMU driver's existing mmgrab() keeps the PASID
allocated until it drops its mm reference.

Fixes: 701fac40384f ("iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit")

Reported-by: Zhangfei Gao <zhangfei.gao@xxxxxxxxxxx>
Tested-by: Zhangfei Gao <zhangfei.gao@xxxxxxxxxxx>
Suggested-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
Suggested-by: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
Reviewed-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
---

v2:
- Dave Hansen rewrites the change log.
- Add Tested-by: Zhangfei Gao <zhangfei.gao@xxxxxxxxxxx>
- Add Reviewed-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>

The original patch was posted and discussed in:
https://lore.kernel.org/lkml/YmdzFFx7fN586jcf@xxxxxxxxxxxxxxxxx/

 kernel/fork.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 9796897560ab..35a3beff140b 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -792,6 +792,7 @@ void __mmdrop(struct mm_struct *mm)
 	mmu_notifier_subscriptions_destroy(mm);
 	check_mm(mm);
 	put_user_ns(mm->user_ns);
+	mm_pasid_drop(mm);
 	free_mm(mm);
 }
 EXPORT_SYMBOL_GPL(__mmdrop);
@@ -1190,7 +1191,6 @@ static inline void __mmput(struct mm_struct *mm)
 	}
 	if (mm->binfmt)
 		module_put(mm->binfmt->module);
-	mm_pasid_drop(mm);
 	mmdrop(mm);
 }
 
-- 
2.32.0