fanotify: fix handling of events on child sub-directory

commit b469e7e47c upstream.

When an event is reported on a sub-directory and the parent inode has
a mark mask with FS_EVENT_ON_CHILD|FS_ISDIR, the event will be sent to
fsnotify() even if the event type is not in the parent mark mask
(e.g. FS_OPEN).

Further more, if that event happened on a mount or a filesystem with
a mount/sb mark that does have that event type in their mask, the "on
child" event will be reported on the mount/sb mark.  That is not
desired, because user will get a duplicate event for the same action.

Note that the event reported on the victim inode is never merged with
the event reported on the parent inode, because of the check in
should_merge(): old_fsn->inode == new_fsn->inode.

Fix this by looking for a match of an actual event type (i.e. not just
FS_ISDIR) in parent's inode mark mask and by not reporting an "on child"
event to group if event type is only found on mount/sb marks.

[backport hint: The bug seems to have always been in fanotify, but this
                patch will only apply cleanly to v4.19.y]

Cc: <stable@vger.kernel.org> # v4.19
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Jan Kara <jack@suse.cz>
[amir: backport to v4.9]
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

fs/notify/fsnotify.c

@@ -158,9 +158,9 @@ int __fsnotify_parent(const struct path *path, struct dentry *dentry, __u32 mask
 	parent = dget_parent(dentry);
 	p_inode = parent->d_inode;
 
-	if (unlikely(!fsnotify_inode_watches_children(p_inode)))
+	if (unlikely(!fsnotify_inode_watches_children(p_inode))) {
 		__fsnotify_update_child_dentry_flags(p_inode);
-	else if (p_inode->i_fsnotify_mask & mask) {
+	} else if (p_inode->i_fsnotify_mask & mask & ~FS_EVENT_ON_CHILD) {
 		struct name_snapshot name;
 
 		/* we are notifying a parent so come up with the new mask which
@@ -264,6 +264,10 @@ int fsnotify(struct inode *to_tell, __u32 mask, const void *data, int data_is,
 	else
 		mnt = NULL;
 
+	/* An event "on child" is not intended for a mount mark */
+	if (mask & FS_EVENT_ON_CHILD)
+		mnt = NULL;
+
 	/*
 	 * Optimization: srcu_read_lock() has a memory barrier which can
 	 * be expensive.  It protects walking the *_fsnotify_marks lists.