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

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Date:   Thu, 17 Nov 2022 01:07:50 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     patchwork-bot+netdevbpf@...nel.org
Cc:     Jakub Sitnicki <jakub@...udflare.com>, 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 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);

        ret = l2tp_validate_socket(sk, net, tunnel->encap);
        if (ret < 0)
@@ -1522,7 +1522,7 @@ int l2tp_tunnel_register(struct l2tp_tunnel
*tunnel, struct net *net,
        if (tunnel->fd >= 0)
                sockfd_put(sock);

-       write_unlock(&sk->sk_callback_lock);
+       write_unlock_bh(&sk->sk_callback_lock);
        return 0;

 err_sock:
@@ -1531,7 +1531,7 @@ int l2tp_tunnel_register(struct l2tp_tunnel
*tunnel, struct net *net,
        else
                sockfd_put(sock);

-       write_unlock(&sk->sk_callback_lock);
+       write_unlock_bh(&sk->sk_callback_lock);
 err:
        return ret;
 }