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

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Message-ID: <CANn89i+en=eU3L1kCcn41+-MRuoge0KHwcLHY3ah8TRmLMaMvg@mail.gmail.com>
Date:   Wed, 22 Jun 2022 21:27:11 +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 9:04 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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...

Ilya, can you test the following patch ?
I think it makes more sense to let XFRM reject the packet earlier, and
not complete the 3WHS,
if for some reason this happens.

Thanks !

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index fe8f23b95d32ca4a35d05166d471327bc608fa91..da5a3c44c4fb70f1d3ecc596e694a86267f1c44a
100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1964,7 +1964,10 @@ int tcp_v4_rcv(struct sk_buff *skb)
                struct sock *nsk;

                sk = req->rsk_listener;
-               drop_reason = tcp_inbound_md5_hash(sk, skb,
+               if (!xfrm4_policy_check(sk, XFRM_POLICY_IN, skb))
+                       drop_reason = SKB_DROP_REASON_XFRM_POLICY;
+               else
+                       drop_reason = tcp_inbound_md5_hash(sk, skb,
                                                   &iph->saddr, &iph->daddr,
                                                   AF_INET, dif, sdif);
                if (unlikely(drop_reason)) {
@@ -2016,6 +2019,7 @@ int tcp_v4_rcv(struct sk_buff *skb)
                        }
                        goto discard_and_relse;
                }
+               nf_reset_ct(skb);
                if (nsk == sk) {
                        reqsk_put(req);
                        tcp_v4_restore_cb(skb);