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

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Date:   Fri, 29 Apr 2022 22:47:23 -0400
From:   Soheil Hassas Yeganeh <soheil@...gle.com>
To:     Neal Cardwell <ncardwell@...gle.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>
Subject: Re: [PATCH net-next] tcp: drop skb dst in tcp_rcv_established()

On Fri, Apr 29, 2022 at 10:20 PM Neal Cardwell <ncardwell@...gle.com> wrote:
>
> 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

Acked-by: Soheil Hassas Yeganeh <soheil@...gle.com>

Thank you for the fix!