Re: net/sctp: recursive locking in sctp_do_peeloff

[Cong Wang]

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.

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;