Re: [PATCH net-next] net: kcm: Fix race condition in kcm_unattach()

[Hillf Danton <hdanton@sina.com>]

#syz test upstream master

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().

Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
Reported-by: syzbot+e62c9db591c30e174662@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=e62c9db591c30e174662
Reported-by: syzbot+d199b52665b6c3069b94@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d199b52665b6c3069b94
Signed-off-by: Sven Stegemann <sven@stegemann.de>
---
 include/net/kcm.h | 1 -
 net/kcm/kcmsock.c | 9 ++-------
 2 files changed, 2 insertions(+), 8 deletions(-)

diff --git a/include/net/kcm.h b/include/net/kcm.h
index 441e993be634..d9c35e71ecea 100644
--- a/include/net/kcm.h
+++ b/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;
diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
index a4971e6fa943..2f66b5279f2a 100644
--- a/net/kcm/kcmsock.c
+++ b/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);
@@ -1714,6 +1708,7 @@ static int kcm_release(struct socket *sock)
 	/* Cancel work. After this point there should be no outside references
 	 * to the kcm socket.
 	 */
+	disable_work(&kcm->tx_work);
 	cancel_work_sync(&kcm->tx_work);
 
 	lock_sock(sk);
-- 
2.50.1