[PATCH] fs/mbcache: cancel shrink work before destroying the cache

[Hyungjung Joo]

From: HyungJung Joo <jhj140711@xxxxxxxxx>

mb_cache_destroy() calls shrinker_free() and then frees all cache
entries and the cache itself, but it does not cancel the pending
c_shrink_work work item first.

If mb_cache_entry_create() schedules c_shrink_work via schedule_work()
and the work item is still pending or running when mb_cache_destroy()
runs, mb_cache_shrink_worker() will access the cache after its memory
has been freed, causing a use-after-free.

This is only reachable by a privileged user (root or CAP_SYS_ADMIN)
who can trigger the last put of a mounted ext2/ext4/ocfs2 filesystem.

Cancel the work item with cancel_work_sync() before calling
shrinker_free(), ensuring the worker has finished and will not be
rescheduled before the cache is torn down.

Signed-off-by: Hyungjung Joo <jhj140711@xxxxxxxxx>
---
 fs/mbcache.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/mbcache.c b/fs/mbcache.c
index 480d02d6ebf0..2a6319b4072c 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -406,6 +406,7 @@ void mb_cache_destroy(struct mb_cache *cache)
 {
 	struct mb_cache_entry *entry, *next;
 
+	cancel_work_sync(&cache->c_shrink_work);
 	shrinker_free(cache->c_shrink);
 
 	/*
-- 
2.34.1