Re: possible deadlock in sk_diag_fill

[Andrei Vagin]

On Tue, May 15, 2018 at 07:19:39AM +0200, Dmitry Vyukov wrote:
> On Mon, May 14, 2018 at 8:00 PM, Andrei Vagin <avagin@xxxxxxxxxxxxx> wrote:
> >> >> Hello,
> >> >>
> >> >> syzbot found the following crash on:
> >> >>
> >> >> HEAD commit:    c1c07416cdd4 Merge tag 'kbuild-fixes-v4.17' of git://git.k..
> >> >> git tree:       upstream
> >> >> console output: https://syzkaller.appspot.com/x/log.txt?x=12164c97800000
> >> >> kernel config:  https://syzkaller.appspot.com/x/.config?x=5a1dc06635c10d27
> >> >> dashboard link: https://syzkaller.appspot.com/bug?extid=c1872be62e587eae9669
> >> >> compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
> >> >> userspace arch: i386
> >> >>
> >> >> Unfortunately, I don't have any reproducer for this crash yet.
> >> >>
> >> >> IMPORTANT: if you fix the bug, please add the following tag to the commit:
> >> >> Reported-by: syzbot+c1872be62e587eae9669@xxxxxxxxxxxxxxxxxxxxxxxxx
> >> >>
> >> >>
> >> >> ======================================================
> >> >> WARNING: possible circular locking dependency detected
> >> >> 4.17.0-rc3+ #59 Not tainted
> >> >> ------------------------------------------------------
> >> >> syz-executor1/25282 is trying to acquire lock:
> >> >> 000000004fddf743 (&(&u->lock)->rlock/1){+.+.}, at: sk_diag_dump_icons
> >> >> net/unix/diag.c:82 [inline]
> >> >> 000000004fddf743 (&(&u->lock)->rlock/1){+.+.}, at:
> >> >> sk_diag_fill.isra.5+0xa43/0x10d0 net/unix/diag.c:144
> >> >>
> >> >> but task is already holding lock:
> >> >> 00000000b6895645 (rlock-AF_UNIX){+.+.}, at: spin_lock
> >> >> include/linux/spinlock.h:310 [inline]
> >> >> 00000000b6895645 (rlock-AF_UNIX){+.+.}, at: sk_diag_dump_icons
> >> >> net/unix/diag.c:64 [inline]
> >> >> 00000000b6895645 (rlock-AF_UNIX){+.+.}, at: sk_diag_fill.isra.5+0x94e/0x10d0
> >> >> net/unix/diag.c:144
> >> >>
> >> >> which lock already depends on the new lock.
> >> >
> >> > In the code, we have a comment which explains why it is safe to take this lock
> >> >
> >> > /*
> >> >  * The state lock is outer for the same sk's
> >> >  * queue lock. With the other's queue locked it's
> >> >  * OK to lock the state.
> >> >  */
> >> > unix_state_lock_nested(req);
> >> >
> >> > It is a question how to explain this to lockdep.
> >>
> >> Do I understand it correctly that (&u->lock)->rlock associated with
> >> AF_UNIX is locked under rlock-AF_UNIX, and then rlock-AF_UNIX is
> >> locked under (&u->lock)->rlock associated with AF_NETLINK? If so, I
> >> think we need to split (&u->lock)->rlock by family too, so that we
> >> have u->lock-AF_UNIX and u->lock-AF_NETLINK.
> >
> > I think here is another problem. lockdep woried about
> > sk->sk_receive_queue vs unix_sk(s)->lock.
> >
> > sk_diag_dump_icons() takes sk->sk_receive_queue and then
> > unix_sk(s)->lock.
> >
> > unix_dgram_sendmsg takes unix_sk(sk)->lock and then sk->sk_receive_queue.
> >
> > sk_diag_dump_icons() takes locks for two different sockets, but
> > unix_dgram_sendmsg() takes locks for one socket.
> >
> > sk_diag_dump_icons
> >         if (sk->sk_state == TCP_LISTEN) {
> >                 spin_lock(&sk->sk_receive_queue.lock);
> >                 skb_queue_walk(&sk->sk_receive_queue, skb) {
> >                         unix_state_lock_nested(req);
> >                                 spin_lock_nested(&unix_sk(s)->lock,
> >
> >
> > unix_dgram_sendmsg
> >         unix_state_lock(other)
> >                 spin_lock(&unix_sk(s)->lock)
> >         skb_queue_tail(&other->sk_receive_queue, skb);
> >                 spin_lock_irqsave(&list->lock, flags);
> 
> 
> Do you mean the following?
> There is socket 1 with state lock (S1) and queue lock (Q2), and socket
> 2 with state lock (S2) and queue lock (Q2). unix_dgram_sendmsg lock
> S1->Q1. And sk_diag_dump_icons locks Q1->S2.
> If yes, then this looks pretty much as deadlock. Consider that 2
> unix_dgram_sendmsg in 2 different threads lock S1 and S2 respectively.
> Now 2  sk_diag_dump_icons in 2 different threads lock Q1 and Q2
> respectively. Now sk_diag_dump_icons want to lock S's, and
> unix_dgram_sendmsg want to lock Q's. Nobody can proceed.

Q1 and S1 belongs to a listen socket, so they can't be taken from
unix_dgram_sendmsg().