Patch "fsnotify: fix oops in fsnotify_clear_marks_by_group_flags()" has been added to the 4.1-stable tree

This is a note to let you know that I've just added the patch titled

fsnotify: fix oops in fsnotify_clear_marks_by_group_flags()

to the 4.1-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
fsnotify-fix-oops-in-fsnotify_clear_marks_by_group_flags.patch
and it can be found in the queue-4.1 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Jan Kara <[email protected]>
Date: Thu, 6 Aug 2015 15:46:42 -0700
Subject: fsnotify: fix oops in fsnotify_clear_marks_by_group_flags()

From: Jan Kara <[email protected]>

commit 8f2f3eb59dff4ec538de55f2e0592fec85966aab upstream.

fsnotify_clear_marks_by_group_flags() can race with fsnotify_destroy_marks() so that 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 thus the next entry pointer we have cached may become stale and we dereference free memory.

Fix the problem by first moving marks to free to a special private list and then always free the first entry in the special list. This method is safe even when entries from the list can disappear once we drop the lock.

---
 fs/notify/mark.c |   30 +++++++++++++++++++++++++-----
 1 file changed, 25 insertions(+), 5 deletions(-)

@@ -412,16 +412,36 @@ void fsnotify_clear_marks_by_group_flags
 					 unsigned int flags)
 {
 	struct fsnotify_mark *lmark, *mark;
+	LIST_HEAD(to_free);
 
+	/*
+	 * We have to be really careful here. Anytime we drop mark_mutex, e.g.
+	 * fsnotify_clear_marks_by_inode() can come and free marks. Even in our
+	 * to_free list so we have to use mark_mutex even when accessing that
+	 * list. And freeing mark requires us to drop mark_mutex. So we can
+	 * reliably free only the first mark in the list. That's why we first
+	 * move marks to free to to_free list in one go and then free marks in
+	 * to_free list one by one.
+	 */
 	mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);
 	list_for_each_entry_safe(mark, lmark, &group->marks_list, g_list) {
-		if (mark->flags & flags) {
-			fsnotify_get_mark(mark);
-			fsnotify_destroy_mark_locked(mark, group);
-			fsnotify_put_mark(mark);
-		}
+		if (mark->flags & flags)
+			list_move(&mark->g_list, &to_free);
 	}
 	mutex_unlock(&group->mark_mutex);
+
+	while (1) {
+		mutex_lock_nested(&group->mark_mutex, SINGLE_DEPTH_NESTING);
+		if (list_empty(&to_free)) {
+			mutex_unlock(&group->mark_mutex);
+			break;
+		}
+		mark = list_first_entry(&to_free, struct fsnotify_mark, g_list);
+		fsnotify_get_mark(mark);
+		fsnotify_destroy_mark_locked(mark, group);
+		mutex_unlock(&group->mark_mutex);
+		fsnotify_put_mark(mark);
+	}
 }
 
 /*

Patches currently in stable-queue which might be from [email protected] are

queue-4.1/fsnotify-fix-oops-in-fsnotify_clear_marks_by_group_flags.patch