netdev - Re: [PATCH net-next] tcp: drop skb dst in tcp_rcv

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Date:   Fri, 29 Apr 2022 22:20:17 -0400
From:   Neal Cardwell <ncardwell@...gle.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next] tcp: drop skb dst in tcp_rcv_established()

On Fri, Apr 29, 2022 at 9:15 PM Eric Dumazet <eric.dumazet@...il.com> wrote:
>
> From: Eric Dumazet <edumazet@...gle.com>
>
> In commit f84af32cbca7 ("net: ip_queue_rcv_skb() helper")
> I dropped the skb dst in tcp_data_queue().
>
> This only dealt with so-called TCP input slow path.
>
> When fast path is taken, tcp_rcv_established() calls
> tcp_queue_rcv() while skb still has a dst.
>
> This was mostly fine, because most dsts at this point
> are not refcounted (thanks to early demux)
>
> However, TCP packets sent over loopback have refcounted dst.
>
> Then commit 68822bdf76f1 ("net: generalize skb freeing
> deferral to per-cpu lists") came and had the effect
> of delaying skb freeing for an arbitrary time.
>
> If during this time the involved netns is dismantled, cleanup_net()
> frees the struct net with embedded net->ipv6.ip6_dst_ops.
>
> Then when eventually dst_destroy_rcu() is called,
> if (dst->ops->destroy) ... triggers an use-after-free.
>
> It is not clear if ip6_route_net_exit() lacks a rcu_barrier()
> as syzbot reported similar issues before the blamed commit.
>
> ( https://groups.google.com/g/syzkaller-bugs/c/CofzW4eeA9A/m/009WjumTAAAJ )
>
> Fixes: 68822bdf76f1 ("net: generalize skb freeing deferral to per-cpu lists")
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  net/ipv4/tcp_input.c | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index cc3de8dc57970c97316ad1591cac0ca5f1a24c47..97cfcd85f84e6f873c3e60c388e6c27628451a7d 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -5928,6 +5928,7 @@ void tcp_rcv_established(struct sock *sk, struct sk_buff *skb)
>                         NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPHPHITS);
>
>                         /* Bulk data transfer: receiver */
> +                       skb_dst_drop(skb);
>                         __skb_pull(skb, tcp_header_len);
>                         eaten = tcp_queue_rcv(sk, skb, &fragstolen);
>
> --

Nice catch. Thanks, Eric!

Acked-by: Neal Cardwell <ncardwell@...gle.com>

neal