netdev - Re: [PATCH net] udp: on peeking bad csum, drop packets even if not at head

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Message-ID: <CAF=yD-+oht8gPcNdSU1zuj5TrOhq0cuwzzcwiCn676YzKNsn=Q@mail.gmail.com>
Date:   Mon, 21 Aug 2017 20:12:04 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net] udp: on peeking bad csum, drop packets even if not at head

On Mon, Aug 21, 2017 at 6:40 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Mon, 2017-08-21 at 17:39 -0400, Willem de Bruijn wrote:
>> From: Willem de Bruijn <willemb@...gle.com>
>>
>> When peeking, if a bad csum is discovered, the skb is unlinked from
>> the queue with __sk_queue_drop_skb and the peek operation restarted.
>>
>> __sk_queue_drop_skb only drops packets that match the queue head. With
>> sk_peek_off, the skb need not be at head, causing the call to fail and
>> the same skb to be found again on restart.
>>
>> Walk the queue to find the correct skb. Limit the walk to sk_peek_off,
>> to bound cycle cost to at most twice the original skb_queue_walk in
>> __skb_try_recv_from_queue.
>>
>> The operation may race with updates to sk_peek_off. As the operation
>> is retried, it will eventually succeed.
>>
>> Signed-off-by: Willem de Bruijn <willemb@...gle.com>
>
> You forgot the Fixes: tag, that such a bug fix deserves.

Indeed, sorry. I'm looking into that now. It should be the patch that
introduced peeking at offset, but need to verify.

I should also add that this bug was discovered by syzkaller.

> I am not a big fan of your patch and would prefer a solution without the
> loop.

Agreed.