linux-kernel - [PATCH 4.4 71/72] dm cache: fix a crash due to incorrect work item cancelling

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Message-Id: <20200310123619.207004558@linuxfoundation.org>
Date:   Tue, 10 Mar 2020 13:39:24 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Mikulas Patocka <mpatocka@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>
Subject: [PATCH 4.4 71/72] dm cache: fix a crash due to incorrect work item cancelling

From: Mikulas Patocka <mpatocka@...hat.com>

commit 7cdf6a0aae1cccf5167f3f04ecddcf648b78e289 upstream.

The crash can be reproduced by running the lvm2 testsuite test
lvconvert-thin-external-cache.sh for several minutes, e.g.:
  while :; do make check T=shell/lvconvert-thin-external-cache.sh; done

The crash happens in this call chain:
do_waker -> policy_tick -> smq_tick -> end_hotspot_period -> clear_bitset
-> memset -> __memset -- which accesses an invalid pointer in the vmalloc
area.

The work entry on the workqueue is executed even after the bitmap was
freed. The problem is that cancel_delayed_work doesn't wait for the
running work item to finish, so the work item can continue running and
re-submitting itself even after cache_postsuspend. In order to make sure
that the work item won't be running, we must use cancel_delayed_work_sync.

Also, change flush_workqueue to drain_workqueue, so that if some work item
submits itself or another work item, we are properly waiting for both of
them.

Fixes: c6b4fcbad044 ("dm: add cache target")
Cc: stable@...r.kernel.org # v3.9
Signed-off-by: Mikulas Patocka <mpatocka@...hat.com>
Signed-off-by: Mike Snitzer <snitzer@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 drivers/md/dm-cache-target.c |    4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

--- a/drivers/md/dm-cache-target.c
+++ b/drivers/md/dm-cache-target.c
@@ -2193,8 +2193,8 @@ static void wait_for_migrations(struct c
 
 static void stop_worker(struct cache *cache)
 {
-	cancel_delayed_work(&cache->waker);
-	flush_workqueue(cache->wq);
+	cancel_delayed_work_sync(&cache->waker);
+	drain_workqueue(cache->wq);
 }
 
 static void requeue_deferred_cells(struct cache *cache)