netdev - Re: [PATCH net] ipv4: tcp: Fix SO

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Message-ID: <20200630234540.2em5gcjthb2lh3x6@kafai-mbp.dhcp.thefacebook.com>
Date:   Tue, 30 Jun 2020 16:45:40 -0700
From:   Martin KaFai Lau <kafai@...com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
CC:     Network Development <netdev@...r.kernel.org>,
        David Miller <davem@...emloft.net>,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH net] ipv4: tcp: Fix SO_MARK in RST and ACK packet

On Tue, Jun 30, 2020 at 07:20:46PM -0400, Willem de Bruijn wrote:
> On Tue, Jun 30, 2020 at 6:18 PM Martin KaFai Lau <kafai@...com> wrote:
> >
> > When testing a recent kernel (5.6 in our case), the skb->mark of the
> > IPv4 TCP RST pkt does not carry the mark from sk->sk_mark.  It is
> > discovered by the bpf@tc that depends on skb->mark to work properly.
> > The same bpf prog has been working in the earlier kernel version.
> > After reverting commit c6af0c227a22 ("ip: support SO_MARK cmsg"),
> > the skb->mark is set and seen by bpf@tc properly.
> >
> > We have noticed that in IPv4 TCP RST but it should also
> > happen to the ACK based on tcp_v4_send_ack() is also depending
> > on ip_send_unicast_reply().
> >
> > This patch tries to fix it by initializing the ipc.sockc.mark to
> > fl4.flowi4_mark.
> >
> > Fixes: c6af0c227a22 ("ip: support SO_MARK cmsg")
> > Cc: Willem de Bruijn <willemb@...gle.com>
> > Signed-off-by: Martin KaFai Lau <kafai@...com>
> > ---
> >  net/ipv4/ip_output.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c
> > index 090d3097ee15..033512f719ec 100644
> > --- a/net/ipv4/ip_output.c
> > +++ b/net/ipv4/ip_output.c
> > @@ -1703,6 +1703,7 @@ void ip_send_unicast_reply(struct sock *sk, struct sk_buff *skb,
> >         sk->sk_bound_dev_if = arg->bound_dev_if;
> >         sk->sk_sndbuf = sysctl_wmem_default;
> >         sk->sk_mark = fl4.flowi4_mark;
> > +       ipc.sockc.mark = fl4.flowi4_mark;
> >         err = ip_append_data(sk, &fl4, ip_reply_glue_bits, arg->iov->iov_base,
> >                              len, 0, &ipc, &rt, MSG_DONTWAIT);
> >         if (unlikely(err)) {
> 
> Yes, this total sense. I missed these cases.
> 
> Slight modification, the line above then no longer needs to be set.
> That line was added in commit bf99b4ded5f8 ("tcp: fix mark propagation
> with fwmark_reflect enabled"). Basically, it pretends that the socket
> has a mark associated, but sk here is always the (netns) global
> control sock. So your BPF program was depending on fwmark_reflect?
Make sense.  I was also tempting to remove the line above.
Thanks for the commit pointer.

No, the BPF program does not depend on fwmark_reflect.  It depends
on the sk->sk_mark set by a user space process.

I was also considering to do ipcm_init_sk() but then rolled back
because of the global control sock here.

> 
> ipv6 seems to work differently enough not to have this problem,
> tcp_v6_send_response passing fl6.flowi6_mark directly to ip6_xmit.
> This was added in commit commit 92e55f412cff ("tcp: don't annotate
> mark on control socket from tcp_v6_send_response()").
Correct. IPv6 does it differently, so the same problem is
not observed in IPv6.

> 
> But I do see the same pattern where a socket mark is set from a
> reflected value in icmp_reply and __icmp_send. Those almost certainly
> need updating too. I can do that separately if you prefer. I even
> placed ipcm_init right below this sk_mark initialization without
> considering ipcm_init_sk. D'oh.
Good point.  I think it will only be a few lines change altogether,
so it makes little sense to break up the fix.  I will toss mine and
wait for yours ;)

Thanks for your help!