netdev - Re: [PATCH net v4] l2tp: Serialize access to sk_user_data with sk_callback

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Message-ID: <CANn89i+iRoHnJ=+MFB5N3c36t5AeeDpd7aHqheBdgKjhNH17qA@mail.gmail.com>
Date:   Fri, 18 Nov 2022 02:28:03 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jakub Sitnicki <jakub@...udflare.com>
Cc:     patchwork-bot+netdevbpf@...nel.org, netdev@...r.kernel.org,
        davem@...emloft.net, kuba@...nel.org, pabeni@...hat.com,
        tparkin@...alix.com, g1042620637@...il.com
Subject: Re: [PATCH net v4] l2tp: Serialize access to sk_user_data with sk_callback_lock

On Thu, Nov 17, 2022 at 1:57 AM Jakub Sitnicki <jakub@...udflare.com> wrote:
>
> On Thu, Nov 17, 2022 at 01:40 AM -08, Eric Dumazet wrote:
> > On Thu, Nov 17, 2022 at 1:07 AM Eric Dumazet <edumazet@...gle.com> wrote:
> >>
> >> On Wed, Nov 16, 2022 at 5:30 AM <patchwork-bot+netdevbpf@...nel.org> wrote:
> >> >
> >> > Hello:
> >> >
> >> > This patch was applied to netdev/net.git (master)
> >> > by David S. Miller <davem@...emloft.net>:
> >> >
> >> > On Mon, 14 Nov 2022 20:16:19 +0100 you wrote:
> >> > > sk->sk_user_data has multiple users, which are not compatible with each
> >> > > other. Writers must synchronize by grabbing the sk->sk_callback_lock.
> >> > >
> >> > > l2tp currently fails to grab the lock when modifying the underlying tunnel
> >> > > socket fields. Fix it by adding appropriate locking.
> >> > >
> >> > > We err on the side of safety and grab the sk_callback_lock also inside the
> >> > > sk_destruct callback overridden by l2tp, even though there should be no
> >> > > refs allowing access to the sock at the time when sk_destruct gets called.
> >> > >
> >> > > [...]
> >> >
> >> > Here is the summary with links:
> >> >   - [net,v4] l2tp: Serialize access to sk_user_data with sk_callback_lock
> >> >     https://git.kernel.org/netdev/net/c/b68777d54fac
> >> >
> >> >
> >>
> >> I guess this patch has not been tested with LOCKDEP, right ?
> >>
> >> sk_callback_lock always needs _bh safety.
> >>
> >> I will send something like:
> >>
> >> diff --git a/net/l2tp/l2tp_core.c b/net/l2tp/l2tp_core.c
> >> index 754fdda8a5f52e4e8e2c0f47331c3b22765033d0..a3b06a3cf68248f5ec7ae8be2a9711d0f482ac36
> >> 100644
> >> --- a/net/l2tp/l2tp_core.c
> >> +++ b/net/l2tp/l2tp_core.c
> >> @@ -1474,7 +1474,7 @@ int l2tp_tunnel_register(struct l2tp_tunnel
> >> *tunnel, struct net *net,
> >>         }
> >>
> >>         sk = sock->sk;
> >> -       write_lock(&sk->sk_callback_lock);
> >> +       write_lock_bh(&sk->sk_callback_lock);
> >
> > Unfortunately this might still not work, because
> > setup_udp_tunnel_sock->udp_encap_enable() probably could sleep in
> > static_branch_inc() ?
> >
> > I will release the syzbot report, and let you folks work on a fix, thanks.
>
> Ah, the problem is with pppol2tp_connect racing with itself. Thanks for
> the syzbot report. I will take a look. I live for debugging deadlocks
> :-)

Hi Jakub, any updates on this issue ? (Other syzbot reports with the
same root cause are piling up)

Thanks