[PATCH 3.19 137/177] dm snapshot: fix a possible invalid memory access on unload

Greg Kroah-Hartman
2015-03-16T14:17:22Z

From: Mikulas Patocka <mpatocka@redhat.com>

commit 22aa66a3ee5b61e0f4a0bfeabcaa567861109ec3 upstream.

When the snapshot target is unloaded, snapshot_dtr() waits until
pending_exceptions_count drops to zero. Then, it destroys the snapshot.
Therefore, the function that decrements pending_exceptions_count
should not touch the snapshot structure after the decrement.

pending_complete() calls free_pending_exception(), which decrements
pending_exceptions_count, and then it performs up_write(&s->lock) and it
calls retry_origin_bios() which dereferences s->origin. These two
memory accesses to the fields of the snapshot may touch the dm_snapshot
struture after it is freed.

This patch moves the call to free_pending_exception() to the end of
pending_complete(), so that the snapshot will not be destroyed while
pending_complete() is in progress.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

--- a/drivers/md/dm-snap.c
+++ b/drivers/md/dm-snap.c
@@ -1432,8 +1432,6 @@ out:
full_bio->bi_private = pe->full_bio_private;
atomic_inc(&full_bio->bi_remaining);
}
- free_pending_exception(pe);
-
increment_pending_exceptions_done_count();

up_write(&s->lock);
@@ -1450,6 +1448,8 @@ out:
}

retry_origin_bios(s, origin_bios);
+
+ free_pending_exception(pe);
}

static void commit_callback(void *context, int success)