netdev - Re: [PATCH net] kcm: avoid potential race in kcm_tx

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Date:   Wed, 12 Oct 2022 07:38:57 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org,
        eric.dumazet@...il.com, syzbot <syzkaller@...glegroups.com>,
        Tom Herbert <tom@...bertland.com>
Subject: Re: [PATCH net] kcm: avoid potential race in kcm_tx_work

On Wed, Oct 12, 2022 at 7:00 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
> On Wed, 12 Oct 2022 at 15:34, 'Eric Dumazet' via syzkaller
> <syzkaller@...glegroups.com> wrote:
> >
> > syzbot found that kcm_tx_work() could crash [1] in:
> >
> >         /* Primarily for SOCK_SEQPACKET sockets */
> >         if (likely(sk->sk_socket) &&
> >             test_bit(SOCK_NOSPACE, &sk->sk_socket->flags)) {
> > <<*>>   clear_bit(SOCK_NOSPACE, &sk->sk_socket->flags);
> >                 sk->sk_write_space(sk);
> >         }
> >
> > I think the reason is that another thread might concurrently
> > run in kcm_release() and call sock_orphan(sk) while sk is not
> > locked. kcm_tx_work() find sk->sk_socket being NULL.
>
> Does it make sense to add some lockdep annotations to sock_orphan()
> and maybe some other similar functions to catch such cases earlier?

I thought about that, but this seems net-next material.

>
>
> > [1]
> > BUG: KASAN: null-ptr-deref in instrument_atomic_write include/linux/instrumented.h:86 [inline]
> > BUG: KASAN: null-ptr-deref in clear_bit include/asm-generic/bitops/instrumented-atomic.h:41 [inline]
> > BUG: KASAN: null-ptr-deref in kcm_tx_work+0xff/0x160 net/kcm/kcmsock.c:742
> > Write of size 8 at addr 0000000000000008 by task kworker/u4:3/53
> >
> > CPU: 0 PID: 53 Comm: kworker/u4:3 Not tainted 5.19.0-rc3-next-20220621-syzkaller #0
> > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
> > Workqueue: kkcmd kcm_tx_work
> > Call Trace:
> > <TASK>
> > __dump_stack lib/dump_stack.c:88 [inline]
> > dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:106
> > kasan_report+0xbe/0x1f0 mm/kasan/report.c:495
> > check_region_inline mm/kasan/generic.c:183 [inline]
> > kasan_check_range+0x13d/0x180 mm/kasan/generic.c:189
> > instrument_atomic_write include/linux/instrumented.h:86 [inline]
> > clear_bit include/asm-generic/bitops/instrumented-atomic.h:41 [inline]
> > kcm_tx_work+0xff/0x160 net/kcm/kcmsock.c:742
> > process_one_work+0x996/0x1610 kernel/workqueue.c:2289
> > worker_thread+0x665/0x1080 kernel/workqueue.c:2436
> > kthread+0x2e9/0x3a0 kernel/kthread.c:376
> > ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:302
> > </TASK>
> >
> > Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
> > Reported-by: syzbot <syzkaller@...glegroups.com>
> > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> > Cc: Tom Herbert <tom@...bertland.com>
> > ---
> >  net/kcm/kcmsock.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
> > index 1215c863e1c410fa9ba5b9c3706152decfb3ebac..27725464ec08fe2b5f2e86202636cbc895568098 100644
> > --- a/net/kcm/kcmsock.c
> > +++ b/net/kcm/kcmsock.c
> > @@ -1838,10 +1838,10 @@ static int kcm_release(struct socket *sock)
> >         kcm = kcm_sk(sk);
> >         mux = kcm->mux;
> >
> > +       lock_sock(sk);
> >         sock_orphan(sk);
> >         kfree_skb(kcm->seq_skb);
> >
> > -       lock_sock(sk);
> >         /* Purge queue under lock to avoid race condition with tx_work trying
> >          * to act when queue is nonempty. If tx_work runs after this point
> >          * it will just return.
> > --
> > 2.38.0.rc1.362.ged0d419d3c-goog
> >
> > --
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