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

[Tiago Lam <tiagolam@cloudflare.com>]

On Fri, Sep 13, 2024 at 11:24:09AM -0700, Martin KaFai Lau wrote:
> On 9/13/24 2:39 AM, Tiago Lam 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);
> 
> iirc, in the ingress path, the sk could also be selected by a tc bpf prog
> doing bpf_sk_assign. Then this re-run on sk_lookup may give an incorrect
> result?
> 

If it does give the incorrect result, we still fallback to the normal
egress path.

> In general, is it necessary to rerun any bpf prog if the user space has
> specified the IP[v6]_ORIGDSTADDR.
> 

More generally, wouldn't that also be the case if someone calls
bpf_sk_assign() in both TC and sk_lookup on ingress? It can lead to some
interference between the two.

It seems like the interesting cases are:
1. Calling bpf_sk_assign() on both TC and sk_lookup ingress: if this
happens sk_lookup on egress should match the correct socket when doing
the reverse lookup;
2. Calling bpf_sk_assign() only on ingress TC: in this case it will
depend if an sk_lookup program is attached or not:
  a. If not, there's no reverse lookup on egress either;
  b. But if yes, although the reverse sk_lookup here won't match the
  initial socket assigned at ingress TC, the packets will still fallback
  to the normal egress path;

You're right in that case 2b above will continue with the same
restrictions as before.

Tiago.