net: kcm: Fix race condition in kcm_unattach()

syzbot found a race condition when kcm_unattach(psock)
and kcm_release(kcm) are executed at the same time.

kcm_unattach() is missing a check of the flag
kcm->tx_stopped before calling queue_work().

If the kcm has a reserved psock, kcm_unattach() might get executed
between cancel_work_sync() and unreserve_psock() in kcm_release(),
requeuing kcm->tx_work right before kcm gets freed in kcm_done().

Remove kcm->tx_stopped and replace it by the less
error-prone disable_work_sync().

Fixes: ab7ac4e ("kcm: Kernel Connection Multiplexor module")
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=e62c9db591c30e174662
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=d199b52665b6c3069b94
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=be6b1fdfeae512726b4e
Signed-off-by: Sven Stegemann <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

Author: Nevsor

include/net/kcm.h

@@ -71,7 +71,6 @@ struct kcm_sock {
 	struct list_head wait_psock_list;
 	struct sk_buff *seq_skb;
 	struct mutex tx_mutex;
-	u32 tx_stopped : 1;
 
 	/* Don't use bit fields here, these are set under different locks */
 	bool tx_wait;

net/kcm/kcmsock.c

@@ -430,7 +430,7 @@ static void psock_write_space(struct sock *sk)
 
 	/* Check if the socket is reserved so someone is waiting for sending. */
 	kcm = psock->tx_kcm;
-	if (kcm && !unlikely(kcm->tx_stopped))
+	if (kcm)
 		queue_work(kcm_wq, &kcm->tx_work);
 
 	spin_unlock_bh(&mux->lock);
@@ -1693,12 +1693,6 @@ static int kcm_release(struct socket *sock)
 	 */
 	__skb_queue_purge(&sk->sk_write_queue);
 
-	/* Set tx_stopped. This is checked when psock is bound to a kcm and we
-	 * get a writespace callback. This prevents further work being queued
-	 * from the callback (unbinding the psock occurs after canceling work.
-	 */
-	kcm->tx_stopped = 1;
-
 	release_sock(sk);
 
 	spin_lock_bh(&mux->lock);
@@ -1714,7 +1708,7 @@ static int kcm_release(struct socket *sock)
 	/* Cancel work. After this point there should be no outside references
 	 * to the kcm socket.
 	 */
-	cancel_work_sync(&kcm->tx_work);
+	disable_work_sync(&kcm->tx_work);
 
 	lock_sock(sk);
 	psock = kcm->tx_psock;