[tip: core/urgent] mm: Fix PASID use-after-free issue

[tip-bot2 for Fenghua Yu]

The following commit has been merged into the core/urgent branch of tip:

Commit-ID:     2667ed10d9f01e250ba806276740782c89d77fda
Gitweb:        https://git.kernel.org/tip/2667ed10d9f01e250ba806276740782c89d77fda
Author:        Fenghua Yu <fenghua.yu@xxxxxxxxx>
AuthorDate:    Thu, 28 Apr 2022 11:00:41 -07:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Sun, 01 May 2022 10:17:17 +02:00

mm: Fix PASID use-after-free issue

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.

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>
Suggested-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
Suggested-by: Jacob Pan <jacob.jun.pan@xxxxxxxxxxxxxxx>
Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Tested-by: Zhangfei Gao <zhangfei.gao@xxxxxxxxxxx>
Reviewed-by: Jean-Philippe Brucker <jean-philippe@xxxxxxxxxx>
Link: https://lore.kernel.org/r/20220428180041.806809-1-fenghua.yu@xxxxxxxxx
---
 kernel/fork.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 9796897..35a3bef 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);
 }