fanotify: correct broken ref counting in case adding a mark failed

If adding a mount or inode mark failed fanotify_free_mark() is called explicitly.
But at this time the mark has already been put into the destroy list of the
fsnotify_mark kernel thread. If the thread is too slow it will try to decrease
the reference of a mark, that has already been freed by fanotify_free_mark().
(If its fast enough it will only decrease the marks ref counter from 2 to 1 - note
that the counter has been increased to 2 in add_mark() - which has practically no
effect.)

This patch fixes the ref counting by not calling free_mark() explicitly, but
decreasing the ref counter and rely on the fsnotify_mark thread to cleanup in
case adding the mark has failed.
Signed-off-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Signed-off-by: Eric Paris <eparis@redhat.com>

fs/notify/fanotify/fanotify_user.c

@@ -594,11 +594,10 @@ static int fanotify_add_vfsmount_mark(struct fsnotify_group *group,
 {
 	struct fsnotify_mark *fsn_mark;
 	__u32 added;
+	int ret = 0;
 
 	fsn_mark = fsnotify_find_vfsmount_mark(group, mnt);
 	if (!fsn_mark) {
-		int ret;
-
 		if (atomic_read(&group->num_marks) > group->fanotify_data.max_marks)
 			return -ENOSPC;
 
@@ -608,17 +607,16 @@ static int fanotify_add_vfsmount_mark(struct fsnotify_group *group,
 
 		fsnotify_init_mark(fsn_mark, fanotify_free_mark);
 		ret = fsnotify_add_mark(fsn_mark, group, NULL, mnt, 0);
-		if (ret) {
-			fanotify_free_mark(fsn_mark);
-			return ret;
-		}
+		if (ret)
+			goto err;
 	}
 	added = fanotify_mark_add_to_mask(fsn_mark, mask, flags);
-	fsnotify_put_mark(fsn_mark);
+
 	if (added & ~mnt->mnt_fsnotify_mask)
 		fsnotify_recalc_vfsmount_mask(mnt);
-
-	return 0;
+err:
+	fsnotify_put_mark(fsn_mark);
+	return ret;
 }
 
 static int fanotify_add_inode_mark(struct fsnotify_group *group,
@@ -627,6 +625,7 @@ static int fanotify_add_inode_mark(struct fsnotify_group *group,
 {
 	struct fsnotify_mark *fsn_mark;
 	__u32 added;
+	int ret = 0;
 
 	pr_debug("%s: group=%p inode=%p\n", __func__, group, inode);
 
@@ -642,8 +641,6 @@ static int fanotify_add_inode_mark(struct fsnotify_group *group,
 
 	fsn_mark = fsnotify_find_inode_mark(group, inode);
 	if (!fsn_mark) {
-		int ret;
-
 		if (atomic_read(&group->num_marks) > group->fanotify_data.max_marks)
 			return -ENOSPC;
 
@@ -653,16 +650,16 @@ static int fanotify_add_inode_mark(struct fsnotify_group *group,
 
 		fsnotify_init_mark(fsn_mark, fanotify_free_mark);
 		ret = fsnotify_add_mark(fsn_mark, group, inode, NULL, 0);
-		if (ret) {
-			fanotify_free_mark(fsn_mark);
-			return ret;
-		}
+		if (ret)
+			goto err;
 	}
 	added = fanotify_mark_add_to_mask(fsn_mark, mask, flags);
-	fsnotify_put_mark(fsn_mark);
+
 	if (added & ~inode->i_fsnotify_mask)
 		fsnotify_recalc_inode_mask(inode);
-	return 0;
+err:
+	fsnotify_put_mark(fsn_mark);
+	return ret;
 }
 
 /* fanotify syscalls */