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From:   Siddh Raman Pant <code@siddh.me>
To:     David Howells <dhowells@redhat.com>,
        Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
        Eric Dumazet <edumazet@google.com>,
        "Fabio M. De Francesco" <fmdefrancesco@gmail.com>
Cc:     linux-security-modules <linux-security-module@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-kernel-mentees 
        <linux-kernel-mentees@lists.linuxfoundation.org>,
        syzbot+c70d87ac1d001f29a058@syzkaller.appspotmail.com
Message-ID: <20220728155121.12145-1-code@siddh.me>
Subject: [PATCH v3] kernel/watch_queue: Make pipe NULL while clearing watch_queue
Date:   Thu, 28 Jul 2022 21:21:21 +0530
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If not done, a reference to a freed pipe remains in the watch_queue,
as this function is called before freeing a pipe in free_pipe_info()
(see line 834 of fs/pipe.c).

This causes a UAF when post_one_notification() tries to access the pipe
on a key update, which is reported by syzbot.

We also need to use READ_ONCE() in post_one_notification() to prevent the
compiler from optimising and loading a non-NULL value from wqueue->pipe.

Bug report: https://syzkaller.appspot.com/bug?id=3D1870dd7791ba05f2ea7f47f7=
cbdde701173973fc
Reported-and-tested-by: syzbot+c70d87ac1d001f29a058@syzkaller.appspotmail.c=
om

Signed-off-by: Siddh Raman Pant <code@siddh.me>
---
Changes in v3:
- Restore the original unlock order, and clear before unlock.
- Use READ_ONCE() in post path.

This was explained by David Howells <dhowells@redhat.com> in
reply to v1. Not added Suggested-by since he didn't reply yet.

Changes in v2:
- Removed the superfluous ifdef guard.

 kernel/watch_queue.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index bb9962b33f95..617425e34252 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -99,7 +99,7 @@ static bool post_one_notification(struct watch_queue *wqu=
eue,
 =09=09=09=09  struct watch_notification *n)
 {
 =09void *p;
-=09struct pipe_inode_info *pipe =3D wqueue->pipe;
+=09struct pipe_inode_info *pipe =3D READ_ONCE(wqueue->pipe);
 =09struct pipe_buffer *buf;
 =09struct page *page;
 =09unsigned int head, tail, mask, note, offset, len;
@@ -637,6 +637,12 @@ void watch_queue_clear(struct watch_queue *wqueue)
 =09=09spin_lock_bh(&wqueue->lock);
 =09}
=20
+=09/* Clearing the watch queue, so we should clean the associated pipe. */
+=09if (wqueue->pipe) {
+=09=09wqueue->pipe->watch_queue =3D NULL;
+=09=09wqueue->pipe =3D NULL;
+=09}
+
 =09spin_unlock_bh(&wqueue->lock);
 =09rcu_read_unlock();
 }
--=20
2.35.1