netdev - Re: [PATCH net] net: ensure all external references are released in deferred skbuffs

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Date:   Wed, 22 Jun 2022 21:04:08 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Ilya Maximets <i.maximets@....org>
Cc:     Steffen Klassert <steffen.klassert@...unet.com>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Florian Westphal <fw@...len.de>,
        netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>, dev@...nvswitch.org,
        LKML <linux-kernel@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: [PATCH net] net: ensure all external references are released in
 deferred skbuffs

On Wed, Jun 22, 2022 at 8:19 PM Ilya Maximets <i.maximets@....org> wrote:
>
> On 6/22/22 19:03, Eric Dumazet wrote:
> > On Wed, Jun 22, 2022 at 6:47 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >>
> >> On Wed, Jun 22, 2022 at 6:39 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >>>
> >>> On Wed, Jun 22, 2022 at 6:29 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >>>>
> >>>> On Wed, Jun 22, 2022 at 4:26 PM Ilya Maximets <i.maximets@....org> wrote:
> >>>>>
> >>>>> On 6/22/22 13:43, Eric Dumazet wrote:
> >>>>
> >>>>>
> >>>>> I tested the patch below and it seems to fix the issue seen
> >>>>> with OVS testsuite.  Though it's not obvious for me why this
> >>>>> happens.  Can you explain a bit more?
> >>>>
> >>>> Anyway, I am not sure we can call nf_reset_ct(skb) that early.
> >>>>
> >>>> git log seems to say that xfrm check needs to be done before
> >>>> nf_reset_ct(skb), I have no idea why.
> >>>
> >>> Additional remark: In IPv6 side, xfrm6_policy_check() _is_ called
> >>> after nf_reset_ct(skb)
> >>>
> >>> Steffen, do you have some comments ?
> >>>
> >>> Some context:
> >>> commit b59c270104f03960069596722fea70340579244d
> >>> Author: Patrick McHardy <kaber@...sh.net>
> >>> Date:   Fri Jan 6 23:06:10 2006 -0800
> >>>
> >>>     [NETFILTER]: Keep conntrack reference until IPsec policy checks are done
> >>>
> >>>     Keep the conntrack reference until policy checks have been performed for
> >>>     IPsec NAT support. The reference needs to be dropped before a packet is
> >>>     queued to avoid having the conntrack module unloadable.
> >>>
> >>>     Signed-off-by: Patrick McHardy <kaber@...sh.net>
> >>>     Signed-off-by: David S. Miller <davem@...emloft.net>
> >>>
> >>
> >> Oh well... __xfrm_policy_check() has :
> >>
> >> nf_nat_decode_session(skb, &fl, family);
> >>
> >> This  answers my questions.
> >>
> >> This means we are probably missing at least one XFRM check in TCP
> >> stack in some cases.
> >> (Only after adding this XFRM check we can call nf_reset_ct(skb))
> >>
> >
> > Maybe this will help ?
>
> I tested this patch and it seems to fix the OVS problem.
> I did not test the xfrm part of it.
>
> Will you post an official patch?

Yes I will. I need to double check we do not leak either the req, or the child.

Maybe the XFRM check should be done even earlier, on the listening socket ?

Or if we assume the SYNACK packet has been sent after the XFRM test
has been applied to the SYN,
maybe we could just call nf_reset_ct(skb) to lower risk of regressions.

With the last patch, it would be strange that we accept the 3WHS and
establish a socket,
but drop the payload in the 3rd packet...

>
> >
> > diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> > index fe8f23b95d32ca4a35d05166d471327bc608fa91..49c1348e40b6c7b6a98b54d716f29c948e00ba33
> > 100644
> > --- a/net/ipv4/tcp_ipv4.c
> > +++ b/net/ipv4/tcp_ipv4.c
> > @@ -2019,12 +2019,19 @@ int tcp_v4_rcv(struct sk_buff *skb)
> >                 if (nsk == sk) {
> >                         reqsk_put(req);
> >                         tcp_v4_restore_cb(skb);
> > -               } else if (tcp_child_process(sk, nsk, skb)) {
> > -                       tcp_v4_send_reset(nsk, skb);
> > -                       goto discard_and_relse;
> >                 } else {
> > -                       sock_put(sk);
> > -                       return 0;
> > +                       if (!xfrm4_policy_check(nsk, XFRM_POLICY_IN, skb)) {
> > +                               drop_reason = SKB_DROP_REASON_XFRM_POLICY;
> > +                               goto discard_and_relse;
> > +                       }
> > +                       nf_reset_ct(skb);
> > +                       if (tcp_child_process(sk, nsk, skb)) {
> > +                               tcp_v4_send_reset(nsk, skb);
> > +                               goto discard_and_relse;
> > +                       } else {
> > +                               sock_put(sk);
> > +                               return 0;
> > +                       }
> >                 }
> >         }
>