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

[Fenghua Yu]

A PASID might be still used on ARM after it is freed in __mmput().

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.

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>
---
 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