linux-kernel - [PATCH AUTOSEL 5.4 4/4] poll_wait: add mb() to fix theoretical race between waitqueue

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-Id: <20250113183633.1784590-4-sashal@kernel.org>
Date: Mon, 13 Jan 2025 13:36:33 -0500
From: Sasha Levin <sashal@...nel.org>
To: linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Cc: Oleg Nesterov <oleg@...hat.com>,
	Christian Brauner <brauner@...nel.org>,
	Sasha Levin <sashal@...nel.org>,
	kees@...nel.org,
	arnd@...db.de,
	ak@...ux.intel.com
Subject: [PATCH AUTOSEL 5.4 4/4] poll_wait: add mb() to fix theoretical race between waitqueue_active() and .poll()

From: Oleg Nesterov <oleg@...hat.com>

[ Upstream commit cacd9ae4bf801ff4125d8961bb9a3ba955e51680 ]

As the comment above waitqueue_active() explains, it can only be used
if both waker and waiter have mb()'s that pair with each other. However
__pollwait() is broken in this respect.

This is not pipe-specific, but let's look at pipe_poll() for example:

	poll_wait(...); // -> __pollwait() -> add_wait_queue()

	LOAD(pipe->head);
	LOAD(pipe->head);

In theory these LOAD()'s can leak into the critical section inside
add_wait_queue() and can happen before list_add(entry, wq_head), in this
case pipe_poll() can race with wakeup_pipe_readers/writers which do

	smp_mb();
	if (waitqueue_active(wq_head))
		wake_up_interruptible(wq_head);

There are more __pollwait()-like functions (grep init_poll_funcptr), and
it seems that at least ep_ptable_queue_proc() has the same problem, so the
patch adds smp_mb() into poll_wait().

Link: https://lore.kernel.org/all/20250102163320.GA17691@redhat.com/
Signed-off-by: Oleg Nesterov <oleg@...hat.com>
Link: https://lore.kernel.org/r/20250107162717.GA18922@redhat.com
Signed-off-by: Christian Brauner <brauner@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 include/linux/poll.h | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/include/linux/poll.h b/include/linux/poll.h
index 7e0fdcf905d2..a4af5e14dffe 100644
--- a/include/linux/poll.h
+++ b/include/linux/poll.h
@@ -43,8 +43,16 @@ typedef struct poll_table_struct {
 
 static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
 {
-	if (p && p->_qproc && wait_address)
+	if (p && p->_qproc && wait_address) {
 		p->_qproc(filp, wait_address, p);
+		/*
+		 * This memory barrier is paired in the wq_has_sleeper().
+		 * See the comment above prepare_to_wait(), we need to
+		 * ensure that subsequent tests in this thread can't be
+		 * reordered with __add_wait_queue() in _qproc() paths.
+		 */
+		smp_mb();
+	}
 }
 
 /*
-- 
2.39.5