[3.19.y-ckt stable] Patch "fsnotify: fix oops in fsnotify_clear_marks_by_group_flags()" has been added to staging queue

Kamal Mostafa kamal at canonical.com
Mon Sep 21 22:26:11 UTC 2015


This is a note to let you know that I have just added a patch titled

    fsnotify: fix oops in fsnotify_clear_marks_by_group_flags()

to the linux-3.19.y-queue branch of the 3.19.y-ckt extended stable tree 
which can be found at:

    http://kernel.ubuntu.com/git/ubuntu/linux.git/log/?h=linux-3.19.y-queue

This patch is scheduled to be released in version 3.19.8-ckt7.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.19.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From 3cd962613d4f134049055621558f67a0f5882c30 Mon Sep 17 00:00:00 2001
From: Jan Kara <jack at suse.com>
Date: Thu, 6 Aug 2015 15:46:42 -0700
Subject: fsnotify: fix oops in fsnotify_clear_marks_by_group_flags()

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: Kamal Mostafa <kamal 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 92e48c7..39ddcaf 100644
--- a/fs/notify/mark.c
+++ b/fs/notify/mark.c
@@ -412,16 +412,36 @@ 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);
+	}
 }

 /*
--
1.9.1





More information about the kernel-team mailing list