Re: [RFC PATCH 2/3] ipv6: Run a reverse sk_lookup on sendmsg.

[Tiago Lam <tiagolam@cloudflare.com>]

On Sat, Sep 14, 2024 at 01:40:25PM +0200, Eric Dumazet wrote:
> On Fri, Sep 13, 2024 at 11:39 AM Tiago Lam <tiagolam@cloudflare.com> wrote:
> >
> > This follows the same rationale provided for the ipv4 counterpart, where
> > it now runs a reverse socket lookup when source addresses and/or ports
> > are changed, on sendmsg, to check whether egress traffic should be
> > allowed to go through or not.
> >
> > As with ipv4, the ipv6 sendmsg path is also extended here to support the
> > IPV6_ORIGDSTADDR ancilliary message to be able to specify a source
> > address/port.
> >
> > Suggested-by: Jakub Sitnicki <jakub@cloudflare.com>
> > Signed-off-by: Tiago Lam <tiagolam@cloudflare.com>
> > ---
> >  net/ipv6/datagram.c | 76 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  net/ipv6/udp.c      |  8 ++++--
> >  2 files changed, 82 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/ipv6/datagram.c b/net/ipv6/datagram.c
> > index fff78496803d..4214dda1c320 100644
> > --- a/net/ipv6/datagram.c
> > +++ b/net/ipv6/datagram.c
> > @@ -756,6 +756,27 @@ void ip6_datagram_recv_ctl(struct sock *sk, struct msghdr *msg,
> >  }
> >  EXPORT_SYMBOL_GPL(ip6_datagram_recv_ctl);
> >
> > +static inline bool reverse_sk_lookup(struct flowi6 *fl6, struct sock *sk,
> > +                                    struct in6_addr *saddr, __be16 sport)
> > +{
> > +       if (static_branch_unlikely(&bpf_sk_lookup_enabled) &&
> > +           (saddr && sport) &&
> > +           (ipv6_addr_cmp(&sk->sk_v6_rcv_saddr, saddr) || inet_sk(sk)->inet_sport != sport)) {
> > +               struct sock *sk_egress;
> > +
> > +               bpf_sk_lookup_run_v6(sock_net(sk), IPPROTO_UDP, &fl6->daddr, fl6->fl6_dport,
> > +                                    saddr, ntohs(sport), 0, &sk_egress);
> > +               if (!IS_ERR_OR_NULL(sk_egress) &&
> > +                   atomic64_read(&sk_egress->sk_cookie) == atomic64_read(&sk->sk_cookie))
> 
> I do not understand this.
> 
> 1) sk_cookie is not always initialized. It is done on demand, when/if
> __sock_gen_cookie() was called.
> 
> 2) if sk1 and sk2 share the same sk_cookie, then sk1 == sk2 ???
> 
> So why not simply testing sk_egress == sk ?
> 

Oh, yes, you're right. I'll include this in my next revision, thanks.