netdev - Re: [PATCH net-next] net: bail on sock_wfree, sock_rfree when we have a TCP

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Date:	Thu, 2 Jul 2015 01:26:42 +0200
From:	Eric Dumazet <edumazet@...gle.com>
To:	Alex Gartrell <alexgartrell@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	"agartrell@...com" <agartrell@...com>,
	netdev <netdev@...r.kernel.org>, kernel-team <kernel-team@...com>
Subject: Re: [PATCH net-next] net: bail on sock_wfree, sock_rfree when we have
 a TCP_TIMEWAIT sk

On Thu, Jul 2, 2015 at 1:18 AM, Alex Gartrell <alexgartrell@...il.com> wrote:
> On Wednesday, July 1, 2015, Eric Dumazet <edumazet@...gle.com> wrote:
>>
>> On Wed, Jul 1, 2015 at 11:14 PM, David Miller <davem@...emloft.net> wrote:
>> > From: Alex Gartrell <agartrell@...com>
>> > Date: Wed, 1 Jul 2015 13:13:09 -0700
>> >
>> >> If we early-demux bind a TCP_TIMEWAIT socket to an skb and then orphan
>> >> it
>> >> (as we need to do in the ipvs forwarding case), sock_wfree and
>> >> sock_rfree
>> >> are going to reach into the inet_timewait_sock and mess with fields
>> >> that
>> >> don't exist.
>> >>
>> >> Signed-off-by: Alex Gartrell <agartrell@...com>
>> >
>> > If we're forwarding, we should not find a local socket, period.
>>
>> A socket cannot change state to TCP_TIMEWAIT.
>>
>> A new object is allocated and old one is removed from ehash, then
>> freed (rcu rules being applied)
>>
>> Also sock_wfree() has nothing to do with early demux. It is for output
>> path skbs only.
>
>
> Alright I kind of cheated and didn't include full context here. The problem
> is that within ipvs we are getting  packets that have been early demuxed and
> associated with time wait sockets which we then wish to forward immediately
> (ip_vs_xmit.c).  Under normal circumstances it would never be associated
> with any sk at all, but it is because of early demux, so we want to drop the
> relationship by calling skb_orphan.  This invokes the destructor which lands
> us there.
>
> So that is how we reach this illegal "treating a twsk like an sk" state.
>
> If there is a better way to drop the association than skb_orphan I will use
> it.

I think you are mistaken Alex.

socket early demux cannot possibly set skb->destructor to sock_rfree()

If skb->destructor is set by early demux, it correctly points to sock_edemux()

And this one correctly handles all socket variants.

/* All sockets share common refcount, but have different destructors */
void sock_gen_put(struct sock *sk)
{
        if (!atomic_dec_and_test(&sk->sk_refcnt))
                return;

        if (sk->sk_state == TCP_TIME_WAIT)
                inet_twsk_free(inet_twsk(sk));
        else if (sk->sk_state == TCP_NEW_SYN_RECV)
                reqsk_free(inet_reqsk(sk));
        else
                sk_free(sk);
}
EXPORT_SYMBOL_GPL(sock_gen_put);

void sock_edemux(struct sk_buff *skb)
{
        sock_gen_put(skb->sk);
}
EXPORT_SYMBOL(sock_edemux);
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