fsnotify: fix oops in fsnotify_clear_marks_by_group_flags()

fsnotify_clear_marks_by_group_flags() can race with
fsnotify_destroy_marks() so when fsnotify_destroy_mark_locked() drops
mark_mutex, a mark from the list iterated by
fsnotify_clear_marks_by_group_flags() can be freed and we dereference free
memory in the loop there.

Fix the problem by keeping mark_mutex held in
fsnotify_destroy_mark_locked().  The reason why we drop that mutex is that
we need to call a ->freeing_mark() callback which may acquire mark_mutex
again.  To avoid this and similar lock inversion issues, we move the call
to ->freeing_mark() callback to the kthread destroying the mark.

Signed-off-by: Jan Kara <jack@suse.cz>
Reported-by: Ashish Sangwan <a.sangwan@samsung.com>
Suggested-by: Lino Sanfilippo <LinoSanfilippo@gmx.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/notify/mark.c b/fs/notify/mark.c
index 92e48c7..3e594ce 100644 (file)
--- a/fs/notify/mark.c
+++ b/fs/notify/mark.c
@@ -152,31 +152,15 @@ void fsnotify_destroy_mark_locked(struct fsnotify_mark *mark,
                BUG();
 
        list_del_init(&mark->g_list);
-
        spin_unlock(&mark->lock);
 
        if (inode && (mark->flags & FSNOTIFY_MARK_FLAG_OBJECT_PINNED))
                iput(inode);
-       /* release lock temporarily */
-       mutex_unlock(&group->mark_mutex);
 
        spin_lock(&destroy_lock);
        list_add(&mark->g_list, &destroy_list);
        spin_unlock(&destroy_lock);
        wake_up(&destroy_waitq);
-       /*
-        * We don't necessarily have a ref on mark from caller so the above destroy
-        * may have actually freed it, unless this group provides a 'freeing_mark'
-        * function which must be holding a reference.
-        */
-
-       /*
-        * Some groups like to know that marks are being freed.  This is a
-        * callback to the group function to let it know that this mark
-        * is being freed.
-        */
-       if (group->ops->freeing_mark)
-               group->ops->freeing_mark(mark, group);
 
        /*
         * __fsnotify_update_child_dentry_flags(inode);
@@ -191,8 +175,6 @@ void fsnotify_destroy_mark_locked(struct fsnotify_mark *mark,
         */
 
        atomic_dec(&group->num_marks);
-
-       mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);
 }
 
 void fsnotify_destroy_mark(struct fsnotify_mark *mark,
@@ -205,7 +187,10 @@ void fsnotify_destroy_mark(struct fsnotify_mark *mark,
 
 /*
  * Destroy all marks in the given list. The marks must be already detached from
- * the original inode / vfsmount.
+ * the original inode / vfsmount. Note that we can race with
+ * fsnotify_clear_marks_by_group_flags(). However we hold a reference to each
+ * mark so they won't get freed from under us and nobody else touches our
+ * free_list list_head.
  */
 void fsnotify_destroy_marks(struct list_head *to_free)
 {
@@ -406,7 +391,7 @@ struct fsnotify_mark *fsnotify_find_mark(struct hlist_head *head,
 }
 
 /*
- * clear any marks in a group in which mark->flags & flags is true
+ * Clear any marks in a group in which mark->flags & flags is true.
  */
 void fsnotify_clear_marks_by_group_flags(struct fsnotify_group *group,
                                         unsigned int flags)
@@ -460,6 +445,7 @@ static int fsnotify_mark_destroy(void *ignored)
 {
        struct fsnotify_mark *mark, *next;
        struct list_head private_destroy_list;
+       struct fsnotify_group *group;
 
        for (;;) {
                spin_lock(&destroy_lock);
@@ -471,6 +457,14 @@ static int fsnotify_mark_destroy(void *ignored)
 
                list_for_each_entry_safe(mark, next, &private_destroy_list, g_list) {
                        list_del_init(&mark->g_list);
+                       group = mark->group;
+                       /*
+                        * Some groups like to know that marks are being freed.
+                        * This is a callback to the group function to let it
+                        * know that this mark is being freed.
+                        */
+                       if (group && group->ops->freeing_mark)
+                               group->ops->freeing_mark(mark, group);
                        fsnotify_put_mark(mark);
                }