netdev - Re: [PATCH net] udp: preserve skb->dst if required for IP options processing

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Date:   Mon, 17 Jul 2017 14:26:10 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Paolo Abeni <pabeni@...hat.com>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: [PATCH net] udp: preserve skb->dst if required for IP options
 processing

On Mon, 2017-07-17 at 22:59 +0200, Paolo Abeni wrote:
> Eric noticed that in udp_recvmsg() we still need to access
> skb->dst while processing the IP options.
> Since commit 0a463c78d25b ("udp: avoid a cache miss on dequeue")
> skb->dst is no more available at recvmsg() time and bad things
> will happen if we enter the relevant code path.
> 
> This commit address the issue, avoid clearing skb->dst if
> any IP options are present into the relevant skb.
> Since the IP CB is contained in the first skb cacheline, we can
> test it to decide to leverage the consume_stateless_skb()
> optimization, without measurable additional cost in the faster
> path.
> 
> Fixes: b65ac44674dd ("udp: try to avoid 2 cache miss on dequeue")

Strange.... I thought bug was coming from
commit 0a463c78d25b ("udp: avoid a cache miss on dequeue")

Please also add original reporter, from syzkaller team.

Reported-by: Andrey Konovalov <andreyknvl@...gle.com>

> Reported-by: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Paolo Abeni <pabeni@...hat.com>
> ---
>  net/ipv4/udp.c | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
> index 25294d4..b057653 100644
> --- a/net/ipv4/udp.c
> +++ b/net/ipv4/udp.c
> @@ -1388,6 +1388,11 @@ void skb_consume_udp(struct sock *sk, struct sk_buff *skb, int len)
>  		unlock_sock_fast(sk, slow);
>  	}
>  
> +	/* we cleared the head states previously only if the skb lacks any IP
> +	 * options, see __udp_queue_rcv_skb().
> +	 */
> +	if (unlikely(IPCB(skb)->opt.optlen > 0))
> +		skb_release_head_state(skb);
>  	consume_stateless_skb(skb);
>  }
>  EXPORT_SYMBOL_GPL(skb_consume_udp);
> @@ -1779,8 +1784,12 @@ static int __udp_queue_rcv_skb(struct sock *sk, struct sk_buff *skb)
>  		sk_mark_napi_id_once(sk, skb);
>  	}
>  
> -	/* clear all pending head states while they are hot in the cache */
> -	skb_release_head_state(skb);
> +	/* At recvmsg() time we need skb->dst to process IP options-related
> +	 * cmsg, elsewhere can we clear all pending head states while they are
> +	 * hot in the cache
> +	 */
> +	if (likely(IPCB(skb)->opt.optlen == 0))
> +		skb_release_head_state(skb);
>  
>  	rc = __udp_enqueue_schedule_skb(sk, skb);
>  	if (rc < 0) {