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

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Message-ID: <1500327358.5566.30.camel@edumazet-glaptop3.roam.corp.google.com>
Date:   Mon, 17 Jul 2017 14:35:58 -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 14:30 -0700, Eric Dumazet wrote:
> 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.
> 
> Also the problem was not skb->dst at all, but fact that 
> skb->head can not be freed if IP options need later to be fetched.
> 
> ( __ip_options_echo() -> 
>    sptr = skb_network_header(skb);
> 
> ... -> use after free


Oh well, I read again the whole thing, and it seems you are right.

Acked-by: Eric Dumazet <edumazet@...gle.com>