[PATCH 4.9 02/27] audit: Fix use after free in audit_remove_watch_rule()

[Greg Kroah-Hartman]

4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jan Kara <jack@xxxxxxx>

commit d76036ab47eafa6ce52b69482e91ca3ba337d6d6 upstream.

audit_remove_watch_rule() drops watch's reference to parent but then
continues to work with it. That is not safe as parent can get freed once
we drop our reference. The following is a trivial reproducer:

mount -o loop image /mnt
touch /mnt/file
auditctl -w /mnt/file -p wax
umount /mnt
auditctl -D
<crash in fsnotify_destroy_mark()>

Grab our own reference in audit_remove_watch_rule() earlier to make sure
mark does not get freed under us.

Reported-by: Tony Jones <tonyj@xxxxxxx>
Signed-off-by: Jan Kara <jack@xxxxxxx>
Tested-by: Tony Jones <tonyj@xxxxxxx>
Signed-off-by: Paul Moore <paul@xxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/audit_watch.c |   12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

--- a/kernel/audit_watch.c
+++ b/kernel/audit_watch.c
@@ -457,13 +457,15 @@ void audit_remove_watch_rule(struct audi
 	list_del(&krule->rlist);
 
 	if (list_empty(&watch->rules)) {
+		/*
+		 * audit_remove_watch() drops our reference to 'parent' which
+		 * can get freed. Grab our own reference to be safe.
+		 */
+		audit_get_parent(parent);
 		audit_remove_watch(watch);
-
-		if (list_empty(&parent->watches)) {
-			audit_get_parent(parent);
+		if (list_empty(&parent->watches))
 			fsnotify_destroy_mark(&parent->mark, audit_watch_group);
-			audit_put_parent(parent);
-		}
+		audit_put_parent(parent);
 	}
 }