inotify: race use after free/double free in inotify inode marks

There is a race in the inotify add/rm watch code.  A task can find and
remove a mark which doesn't have all of it's references.  This can
result in a use after free/double free situation.

Task A					Task B
------------				-----------
inotify_new_watch()
 allocate a mark (refcnt == 1)
 add it to the idr
					inotify_rm_watch()
					 inotify_remove_from_idr()
					  fsnotify_put_mark()
					      refcnt hits 0, free
 take reference because we are on idr
 [at this point it is a use after free]
 [time goes on]
 refcnt may hit 0 again, double free

The fix is to take the reference BEFORE the object can be found in the
idr.

Signed-off-by: Eric Paris <eparis@redhat.com>
Cc: <stable@kernel.org>

fs/notify/inotify/inotify_user.c

@@ -546,21 +546,24 @@ retry:
 	if (unlikely(!idr_pre_get(&group->inotify_data.idr, GFP_KERNEL)))
 		goto out_err;
 
+	/* we are putting the mark on the idr, take a reference */
+	fsnotify_get_mark(&tmp_ientry->fsn_entry);
+
 	spin_lock(&group->inotify_data.idr_lock);
 	ret = idr_get_new_above(&group->inotify_data.idr, &tmp_ientry->fsn_entry,
 				group->inotify_data.last_wd+1,
 				&tmp_ientry->wd);
 	spin_unlock(&group->inotify_data.idr_lock);
 	if (ret) {
+		/* we didn't get on the idr, drop the idr reference */
+		fsnotify_put_mark(&tmp_ientry->fsn_entry);
+
 		/* idr was out of memory allocate and try again */
 		if (ret == -EAGAIN)
 			goto retry;
 		goto out_err;
 	}
 
-	/* we put the mark on the idr, take a reference */
-	fsnotify_get_mark(&tmp_ientry->fsn_entry);
-
 	/* we are on the idr, now get on the inode */
 	ret = fsnotify_add_mark(&tmp_ientry->fsn_entry, group, inode);
 	if (ret) {