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From:   Siddh Raman Pant <code@siddh.me>
To:     Mauro Carvalho Chehab <mchehab@kernel.org>,
        Randy Dunlap <rdunlap@infradead.org>,
        David Howells <dhowells@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        "Fabio M. De Francesco" <fmdefrancesco@gmail.com>,
        Eric Dumazet <edumazet@google.com>,
        Christophe JAILLET <christophe.jaillet@wanadoo.fr>,
        Eric Biggers <ebiggers@kernel.org>
Cc:     keyrings <keyrings@vger.kernel.org>,
        linux-security-module <linux-security-module@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-kernel <linux-kernel@vger.kernel.org>
Message-ID: <20230111161934.336743-1-code@siddh.me>
Subject: [PATCH v4] kernel/watch_queue: NULL the dangling *pipe, and use it for clear check
Date:   Wed, 11 Jan 2023 21:49:34 +0530
MIME-Version: 1.0
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Precedence: bulk

NULL the dangling pipe reference while clearing watch_queue.

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).

The sole use of wqueue->defunct is for checking if the watch queue has
been cleared, but wqueue->pipe is also NULLed while clearing.

Thus, wqueue->defunct is superfluous, as wqueue->pipe can be checked
for NULL. Hence, the former can be removed.

Signed-off-by: Siddh Raman Pant <code@siddh.me>
---
Changes in v4:
- Drop preceeding kerneldoc-changes patch and change appropriately.

Changes in v3 (8 Jan 2023):
- Minor rephrase of comment before NULLing in watch_queue_clear().

Changes in v2 (6 Aug 2022):
- Merged the NULLing and removing defunct patches.
- Removed READ_ONCE barrier in lock_wqueue().
- Better commit messages.

 include/linux/watch_queue.h |  3 +--
 kernel/watch_queue.c        | 12 ++++++------
 2 files changed, 7 insertions(+), 8 deletions(-)

diff --git a/include/linux/watch_queue.h b/include/linux/watch_queue.h
index fc6bba20273b..45cd42f55d49 100644
--- a/include/linux/watch_queue.h
+++ b/include/linux/watch_queue.h
@@ -38,7 +38,7 @@ struct watch_filter {
 struct watch_queue {
 =09struct rcu_head=09=09rcu;
 =09struct watch_filter __rcu *filter;
-=09struct pipe_inode_info=09*pipe;=09=09/* The pipe we're using as a buffe=
r */
+=09struct pipe_inode_info=09*pipe;=09=09/* Pipe we use as a buffer, NULL i=
f queue closed */
 =09struct hlist_head=09watches;=09/* Contributory watches */
 =09struct page=09=09**notes;=09/* Preallocated notifications */
 =09unsigned long=09=09*notes_bitmap;=09/* Allocation bitmap for notes */
@@ -46,7 +46,6 @@ struct watch_queue {
 =09spinlock_t=09=09lock;
 =09unsigned int=09=09nr_notes;=09/* Number of notes */
 =09unsigned int=09=09nr_pages;=09/* Number of pages in notes[] */
-=09bool=09=09=09defunct;=09/* T when queues closed */
 };
=20
 /*
diff --git a/kernel/watch_queue.c b/kernel/watch_queue.c
index a6f9bdd956c3..6ead921c15c0 100644
--- a/kernel/watch_queue.c
+++ b/kernel/watch_queue.c
@@ -43,7 +43,7 @@ MODULE_LICENSE("GPL");
 static inline bool lock_wqueue(struct watch_queue *wqueue)
 {
 =09spin_lock_bh(&wqueue->lock);
-=09if (unlikely(wqueue->defunct)) {
+=09if (unlikely(!wqueue->pipe)) {
 =09=09spin_unlock_bh(&wqueue->lock);
 =09=09return false;
 =09}
@@ -105,9 +105,6 @@ static bool post_one_notification(struct watch_queue *w=
queue,
 =09unsigned int head, tail, mask, note, offset, len;
 =09bool done =3D false;
=20
-=09if (!pipe)
-=09=09return false;
-
 =09spin_lock_irq(&pipe->rd_wait.lock);
=20
 =09mask =3D pipe->ring_size - 1;
@@ -603,8 +600,11 @@ void watch_queue_clear(struct watch_queue *wqueue)
 =09rcu_read_lock();
 =09spin_lock_bh(&wqueue->lock);
=20
-=09/* Prevent new notifications from being stored. */
-=09wqueue->defunct =3D true;
+=09/*
+=09 * This pipe can be freed by callers like free_pipe_info().
+=09 * Removing this reference also prevents new notifications.
+=09 */
+=09wqueue->pipe =3D NULL;
=20
 =09while (!hlist_empty(&wqueue->watches)) {
 =09=09watch =3D hlist_entry(wqueue->watches.first, struct watch, queue_nod=
e);
--=20
2.39.0