fanotify: fix false positive on permission events

commit 7746e3b upstream.

fsnotify_get_mark_safe() may return false for a mark on an unrelated group,
which results in bypassing the permission check.

Fix by skipping over detached marks that are not in the current group.

CC: stable@vger.kernel.org
Fixes: abc7757 ("fsnotify: Provide framework for dropping SRCU lock in ->handle_event")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Link: https://patch.msgid.link/20260410144950.156160-1-mszeredi@redhat.com
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Miklos Szeredi

fs/notify/fsnotify.c

@@ -398,7 +398,7 @@ static struct fsnotify_mark *fsnotify_first_mark(struct fsnotify_mark_connector
 	return hlist_entry_safe(node, struct fsnotify_mark, obj_list);
 }
 
-static struct fsnotify_mark *fsnotify_next_mark(struct fsnotify_mark *mark)
+struct fsnotify_mark *fsnotify_next_mark(struct fsnotify_mark *mark)
 {
 	struct hlist_node *node = NULL;
 

fs/notify/mark.c

@@ -380,9 +380,6 @@ EXPORT_SYMBOL_GPL(fsnotify_put_mark);
  */
 static bool fsnotify_get_mark_safe(struct fsnotify_mark *mark)
 {
-	if (!mark)
-		return true;
-
 	if (refcount_inc_not_zero(&mark->refcnt)) {
 		spin_lock(&mark->lock);
 		if (mark->flags & FSNOTIFY_MARK_FLAG_ATTACHED) {
@@ -423,15 +420,22 @@ bool fsnotify_prepare_user_wait(struct fsnotify_iter_info *iter_info)
 	int type;
 
 	fsnotify_foreach_iter_type(type) {
+		struct fsnotify_mark *mark = iter_info->marks[type];
+
 		/* This can fail if mark is being removed */
-		if (!fsnotify_get_mark_safe(iter_info->marks[type])) {
-			__release(&fsnotify_mark_srcu);
-			goto fail;
+		while (mark && !fsnotify_get_mark_safe(mark)) {
+			if (mark->group == iter_info->current_group) {
+				__release(&fsnotify_mark_srcu);
+				goto fail;
+			}
+			/* This is a mark in an unrelated group, skip */
+			mark = fsnotify_next_mark(mark);
+			iter_info->marks[type] = mark;
 		}
 	}
 
 	/*
-	 * Now that both marks are pinned by refcount in the inode / vfsmount
+	 * Now that all marks are pinned by refcount in the inode / vfsmount / etc
 	 * lists, we can drop SRCU lock, and safely resume the list iteration
 	 * once userspace returns.
 	 */

include/linux/fsnotify_backend.h

@@ -817,6 +817,7 @@ static inline void fsnotify_clear_sb_marks_by_group(struct fsnotify_group *group
 }
 extern void fsnotify_get_mark(struct fsnotify_mark *mark);
 extern void fsnotify_put_mark(struct fsnotify_mark *mark);
+struct fsnotify_mark *fsnotify_next_mark(struct fsnotify_mark *mark);
 extern void fsnotify_finish_user_wait(struct fsnotify_iter_info *iter_info);
 extern bool fsnotify_prepare_user_wait(struct fsnotify_iter_info *iter_info);