Re: net/sctp: recursive locking in sctp_do_peeloff

[Marcelo Ricardo Leitner]

On Tue, Mar 14, 2017 at 09:52:15PM -0700, Cong Wang wrote:
> On Fri, Mar 10, 2017 at 12:04 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> > On Fri, Mar 10, 2017 at 8:46 PM, Marcelo Ricardo Leitner
> > <marcelo.leitner@xxxxxxxxx> wrote:
> >> On Fri, Mar 10, 2017 at 4:11 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> >>> Hello,
> >>>
> >>> I've got the following recursive locking report while running
> >>> syzkaller fuzzer on net-next/9c28286b1b4b9bce6e35dd4c8a1265f03802a89a:
> >>>
> >>> [ INFO: possible recursive locking detected ]
> >>> 4.10.0+ #14 Not tainted
> >>> ---------------------------------------------
> >>> syz-executor3/5560 is trying to acquire lock:
> >>>  (sk_lock-AF_INET6){+.+.+.}, at: [<ffffffff8401ebcd>] lock_sock
> >>> include/net/sock.h:1460 [inline]
> >>>  (sk_lock-AF_INET6){+.+.+.}, at: [<ffffffff8401ebcd>]
> >>> sctp_close+0xcd/0x9d0 net/sctp/socket.c:1497
> >>>
> >>> but task is already holding lock:
> >>>  (sk_lock-AF_INET6){+.+.+.}, at: [<ffffffff84038110>] lock_sock
> >>> include/net/sock.h:1460 [inline]
> >>>  (sk_lock-AF_INET6){+.+.+.}, at: [<ffffffff84038110>]
> >>> sctp_getsockopt+0x450/0x67e0 net/sctp/socket.c:6611
> >>>
> >>> other info that might help us debug this:
> >>>  Possible unsafe locking scenario:
> >>>
> >>>        CPU0
> >>>        ----
> >>>   lock(sk_lock-AF_INET6);
> >>>   lock(sk_lock-AF_INET6);
> >>>
> >>>  *** DEADLOCK ***
> >>>
> >>>  May be due to missing lock nesting notation
> >>
> >> Pretty much the case, I suppose. The lock held by sctp_getsockopt() is
> >> on one socket, while the other lock that sctp_close() is getting later
> >> is on the newly created (which failed) socket during peeloff
> >> operation.
> >
> >
> > Does this mean that never-ever lock 2 sockets at a time except for
> > this case? If so, it probably suggests that this case should not do it
> > either.
> >
> 
> Yeah, actually for the error path we don't even need to lock sock
> since it is newly allocated and no one else could see it yet.
> 

Agreed.

> Instead of checking for the status of the sock, I believe the following
> one-line fix should do the trick too. Can you give it a try?
> 
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index 0f378ea..4de62d4 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -1494,7 +1494,7 @@ static void sctp_close(struct sock *sk, long timeout)
> 
>         pr_debug("%s: sk:%p, timeout:%ld\n", __func__, sk, timeout);
> 
> -       lock_sock(sk);
> +       lock_sock_nested(sk, SINGLE_DEPTH_NESTING);
>         sk->sk_shutdown = SHUTDOWN_MASK;
>         sk->sk_state = SCTP_SS_CLOSING;

I refrained on doing this just because it will change the lock signature
for the first level too, as sctp_close() can be called directly, and
might avoid some other lockdep detections.

Then you probably also need:
diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index 465a9c8464f9..02506b4406d2 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -1543,7 +1543,7 @@ static void sctp_close(struct sock *sk, long timeout)
 	 * held and that should be grabbed before socket lock.
 	 */
 	spin_lock_bh(&net->sctp.addr_wq_lock);
-	bh_lock_sock(sk);
+	bh_lock_sock_nested(sk);
 
 	/* Hold the sock, since sk_common_release() will put sock_put()
 	 * and we have just a little more cleanup.

because sctp_close will re-lock the socket a little later (for backlog
processing).

  Marcelo