iommu/iova: Fix race between FQ timeout and teardown

It turns out to be possible for hotplugging out a device to reach the
stage of tearing down the device's group and default domain before the
domain's flush queue has drained naturally. At this point, it is then
possible for the timeout to expire just before the del_timer() call
in free_iova_flush_queue(), such that we then proceed to free the FQ
resources while fq_flush_timeout() is still accessing them on another
CPU. Crashes due to this have been observed in the wild while removing
NVMe devices.

Close the race window by using del_timer_sync() to safely wait for any
active timeout handler to finish before we start to free things. We
already avoid any locking in free_iova_flush_queue() since the FQ is
supposed to be inactive anyway, so the potential deadlock scenario does
not apply.

Fixes: 9a005a8 ("iommu/iova: Add flush timer")
Reviewed-by: John Garry <[email protected]>
Signed-off-by: Xiongfeng Wang <[email protected]>
[ rm: rewrite commit message ]
Signed-off-by: Robin Murphy <[email protected]>
Link: https://lore.kernel.org/r/0a365e5b07f14b7344677ad6a9a734966a8422ce.1639753638.git.robin.murphy@arm.com
Signed-off-by: Joerg Roedel <[email protected]>

Author: fenghusthu

drivers/iommu/iova.c

@@ -83,8 +83,7 @@ static void free_iova_flush_queue(struct iova_domain *iovad)
 	if (!has_iova_flush_queue(iovad))
 		return;
 
-	if (timer_pending(&iovad->fq_timer))
-		del_timer(&iovad->fq_timer);
+	del_timer_sync(&iovad->fq_timer);
 
 	fq_destroy_all_entries(iovad);