Subject: Re: [PATCH bpf 4/5] bpf: Set flow flag to allow any source IP in bpf_tunnel_key

From: Paul Chaignon <[email-masked]>
Date: Mon, 18 Jul 2022 18:04:03 +0200

On Fri, Jul 15, 2022 at 11:21:08AM -0700, Yonghong Song wrote:
> On 7/15/22 8:02 AM, Paul Chaignon wrote:
> > Commit 26101f5ab6bd ("bpf: Add source ip in "struct bpf_tunnel_key"")
> > added support for getting and setting the outer source IP of encapsulated
> > packets via the bpf_skb_{get,set}_tunnel_key BPF helper. This change
> > allows BPF programs to set any IP address as the source, including for
> > example the IP address of a container running on the same host.
> > 
> > In that last case, however, the encapsulated packets are dropped when
> > looking up the route because the source IP address isn't assigned to any
> > interface on the host. To avoid this, we need to set the
> > FLOWI_FLAG_ANYSRC flag.
> > 
> > Fixes: 26101f5ab6bd ("bpf: Add source ip in "struct bpf_tunnel_key"")
> > Signed-off-by: Paul Chaignon <[email-masked]>
> > ---
> >   net/core/filter.c | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/net/core/filter.c b/net/core/filter.c
> > index 5d16d66727fc..6d9c800cdab9 100644
> > --- a/net/core/filter.c
> > +++ b/net/core/filter.c
> > @@ -4641,6 +4641,7 @@ BPF_CALL_4(bpf_skb_set_tunnel_key, struct sk_buff *, skb,
> >   	info->key.tun_id = cpu_to_be64(from->tunnel_id);
> >   	info->key.tos = from->tunnel_tos;
> >   	info->key.ttl = from->tunnel_ttl;
> > +	info->key.flow_flags = FLOWI_FLAG_ANYSRC;
> 
> Can we set FLOWI_FLAG_ANYSRC in all conditions?
> In lwt_bpf.c, func bpf_lwt_xmit_reroute(), FLOWI_FLAG_ANYSRC
> is set for ipv4 but not for ipv6. I am wondering whether
> FLOWI_FLAG_ANYSRC needs to be set for ipv6 packet or not
> in bpf_skb_set_tunnel_key().

I've confirmed that IPv6-encapsulated packets are not dropped even if
the flag is not set and the outer source IP address is not assigned to
the host. This is also expected given we never check for
FLOWI_FLAG_ANYSRC on the IPv6 path. I must have been confused by the
fact we sometimes still set FLOWI_FLAG_ANYSRC for IPv6.

I've sent a v2 without the changes to IPv6 code paths.

> 
> >   	if (flags & BPF_F_TUNINFO_IPV6) {
> >   		info->mode |= IP_TUNNEL_INFO_IPV6;