[PATCH 3.16.y-ckt 062/130] fsnotify: fix oops in fsnotify_clear_marks_by_group_flags()

Luis Henriques luis.henriques at canonical.com
Fri Sep 4 13:07:30 UTC 2015


3.16.7-ckt17 -stable review patch.  If anyone has any objections, please let me know.

------------------

From: Jan Kara <jack at suse.com>

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.

Signed-off-by: Jan Kara <jack at suse.com>
Reported-by: Ashish Sangwan <a.sangwan at samsung.com>
Reviewed-by: Ashish Sangwan <a.sangwan at samsung.com>
Cc: Lino Sanfilippo <LinoSanfilippo at gmx.de>
Signed-off-by: Andrew Morton <akpm at linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 fs/notify/mark.c | 30 +++++++++++++++++++++++++-----
 1 file changed, 25 insertions(+), 5 deletions(-)

diff --git a/fs/notify/mark.c b/fs/notify/mark.c
index d90deaa08e78..e7b1ca99d82d 100644
--- a/fs/notify/mark.c
+++ b/fs/notify/mark.c
@@ -293,16 +293,36 @@ void fsnotify_clear_marks_by_group_flags(struct fsnotify_group *group,
 					 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);
+	}
 }
 
 /*




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