[PATCH 3.16 113/294] audit: Fix use after free in audit_remove_watch_rule()

[Ben Hutchings]

3.16.50-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Jan Kara <j...@suse.cz>

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 <to...@suse.de>
Signed-off-by: Jan Kara <j...@suse.cz>
Tested-by: Tony Jones <to...@suse.de>
Signed-off-by: Paul Moore <p...@paul-moore.com>
Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
---
 kernel/audit_watch.c | 12 +++++++-----
 1 file changed, 7 insertions(+), 5 deletions(-)

--- a/kernel/audit_watch.c
+++ b/kernel/audit_watch.c
@@ -455,13 +455,15 @@ 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);
        }
 }