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From:   Siddh Raman Pant <code@siddh.me>
To:     Eric Biggers <ebiggers@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        David Howells <dhowells@redhat.com>,
        Randy Dunlap <rdunlap@infradead.org>,
        Mauro Carvalho Chehab <mchehab@kernel.org>,
        Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
        Eric Dumazet <edumazet@google.com>
Cc:     linux-kernel <linux-kernel@vger.kernel.org>,
        linux-kernel-mentees 
        <linux-kernel-mentees@lists.linuxfoundation.org>
Message-ID: <cf470fb02971377e32564224f17897c55a374857.1659618705.git.code@siddh.me>
Subject: [PATCH 1/3 v4] kernel/watch_queue: Remove dangling pipe reference while clearing watch_queue
Date:   Thu,  4 Aug 2022 19:00:22 +0530
In-Reply-To: <cover.1659618705.git.code@siddh.me>
References: <cover.1659618705.git.code@siddh.me>
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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).

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.

Signed-off-by: Siddh Raman Pant <code@siddh.me>
---
Changes in v4:
- Brought the lines towards the start rather than the end.
- Removed incorrect NULLing of wqueue->pipe->watch_queue.
The latter was pointed out by Eric Biggers <ebiggers@kernel.org>
in reply to v3.

Changes in v3:
- Restored the original unlock order, and clear before unlock.
- Used READ_ONCE() in post path.
This was explained by David Howells <dhowells@redhat.com> in
reply to v1.

Changes in v2:
- Removed the superfluous ifdef guard.

 kernel/watch_queue.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index a6f9bdd956c3..8999c4e3076d 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;
@@ -606,6 +606,9 @@ void watch_queue_clear(struct watch_queue *wqueue)
 =09/* Prevent new notifications from being stored. */
 =09wqueue->defunct =3D true;
=20
+=09/* This pipe will get freed by caller, and we are anyways clearing. */
+=09wqueue->pipe =3D NULL;
+
 =09while (!hlist_empty(&wqueue->watches)) {
 =09=09watch =3D hlist_entry(wqueue->watches.first, struct watch, queue_nod=
e);
 =09=09hlist_del_init_rcu(&watch->queue_node);
--=20
2.35.1