Re: [PATCH v3 bpf-next 4/5] net: netfilter: add kfunc helper to add a new ct entry

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Kumar Kartikeya Dwivedi <memxor@gmail.com> writes:

> On Thu, May 19, 2022 at 03:44:58AM IST, Alexei Starovoitov wrote:
>> On Wed, May 18, 2022 at 2:09 PM Lorenzo Bianconi
>> <lorenzo.bianconi@redhat.com> wrote:
>> >
>> > > Lorenzo Bianconi <lorenzo@kernel.org> writes:
>> > >
>> > > > Introduce bpf_xdp_ct_add and bpf_skb_ct_add kfunc helpers in order to
>> > > > add a new entry to ct map from an ebpf program.
>> > > > Introduce bpf_nf_ct_tuple_parse utility routine.
>> > > >
>> > > > Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
>> > > > ---
>> > > >  net/netfilter/nf_conntrack_bpf.c | 212 +++++++++++++++++++++++++++----
>> > > >  1 file changed, 189 insertions(+), 23 deletions(-)
>> > > >
>> > > > diff --git a/net/netfilter/nf_conntrack_bpf.c b/net/netfilter/nf_conntrack_bpf.c
>> > > > index a9271418db88..3d31b602fdf1 100644
>> > > > --- a/net/netfilter/nf_conntrack_bpf.c
>> > > > +++ b/net/netfilter/nf_conntrack_bpf.c
>> > > > @@ -55,41 +55,114 @@ enum {
>> > > >     NF_BPF_CT_OPTS_SZ = 12,
>> > > >  };
>> > > >
>> > > > -static struct nf_conn *__bpf_nf_ct_lookup(struct net *net,
>> > > > -                                     struct bpf_sock_tuple *bpf_tuple,
>> > > > -                                     u32 tuple_len, u8 protonum,
>> > > > -                                     s32 netns_id, u8 *dir)
>> > > > +static int bpf_nf_ct_tuple_parse(struct bpf_sock_tuple *bpf_tuple,
>> > > > +                            u32 tuple_len, u8 protonum, u8 dir,
>> > > > +                            struct nf_conntrack_tuple *tuple)
>> > > >  {
>> > > > -   struct nf_conntrack_tuple_hash *hash;
>> > > > -   struct nf_conntrack_tuple tuple;
>> > > > -   struct nf_conn *ct;
>> > > > +   union nf_inet_addr *src = dir ? &tuple->dst.u3 : &tuple->src.u3;
>> > > > +   union nf_inet_addr *dst = dir ? &tuple->src.u3 : &tuple->dst.u3;
>> > > > +   union nf_conntrack_man_proto *sport = dir ? (void *)&tuple->dst.u
>> > > > +                                             : &tuple->src.u;
>> > > > +   union nf_conntrack_man_proto *dport = dir ? &tuple->src.u
>> > > > +                                             : (void *)&tuple->dst.u;
>> > > >
>> > > >     if (unlikely(protonum != IPPROTO_TCP && protonum != IPPROTO_UDP))
>> > > > -           return ERR_PTR(-EPROTO);
>> > > > -   if (unlikely(netns_id < BPF_F_CURRENT_NETNS))
>> > > > -           return ERR_PTR(-EINVAL);
>> > > > +           return -EPROTO;
>> > > > +
>> > > > +   memset(tuple, 0, sizeof(*tuple));
>> > > >
>> > > > -   memset(&tuple, 0, sizeof(tuple));
>> > > >     switch (tuple_len) {
>> > > >     case sizeof(bpf_tuple->ipv4):
>> > > > -           tuple.src.l3num = AF_INET;
>> > > > -           tuple.src.u3.ip = bpf_tuple->ipv4.saddr;
>> > > > -           tuple.src.u.tcp.port = bpf_tuple->ipv4.sport;
>> > > > -           tuple.dst.u3.ip = bpf_tuple->ipv4.daddr;
>> > > > -           tuple.dst.u.tcp.port = bpf_tuple->ipv4.dport;
>> > > > +           tuple->src.l3num = AF_INET;
>> > > > +           src->ip = bpf_tuple->ipv4.saddr;
>> > > > +           sport->tcp.port = bpf_tuple->ipv4.sport;
>> > > > +           dst->ip = bpf_tuple->ipv4.daddr;
>> > > > +           dport->tcp.port = bpf_tuple->ipv4.dport;
>> > > >             break;
>> > > >     case sizeof(bpf_tuple->ipv6):
>> > > > -           tuple.src.l3num = AF_INET6;
>> > > > -           memcpy(tuple.src.u3.ip6, bpf_tuple->ipv6.saddr, sizeof(bpf_tuple->ipv6.saddr));
>> > > > -           tuple.src.u.tcp.port = bpf_tuple->ipv6.sport;
>> > > > -           memcpy(tuple.dst.u3.ip6, bpf_tuple->ipv6.daddr, sizeof(bpf_tuple->ipv6.daddr));
>> > > > -           tuple.dst.u.tcp.port = bpf_tuple->ipv6.dport;
>> > > > +           tuple->src.l3num = AF_INET6;
>> > > > +           memcpy(src->ip6, bpf_tuple->ipv6.saddr, sizeof(bpf_tuple->ipv6.saddr));
>> > > > +           sport->tcp.port = bpf_tuple->ipv6.sport;
>> > > > +           memcpy(dst->ip6, bpf_tuple->ipv6.daddr, sizeof(bpf_tuple->ipv6.daddr));
>> > > > +           dport->tcp.port = bpf_tuple->ipv6.dport;
>> > > >             break;
>> > > >     default:
>> > > > -           return ERR_PTR(-EAFNOSUPPORT);
>> > > > +           return -EAFNOSUPPORT;
>> > > >     }
>> > > > +   tuple->dst.protonum = protonum;
>> > > > +   tuple->dst.dir = dir;
>> > > > +
>> > > > +   return 0;
>> > > > +}
>> > > >
>> > > > -   tuple.dst.protonum = protonum;
>> > > > +struct nf_conn *
>> > > > +__bpf_nf_ct_alloc_entry(struct net *net, struct bpf_sock_tuple *bpf_tuple,
>> > > > +                   u32 tuple_len, u8 protonum, s32 netns_id, u32 timeout)
>> > > > +{
>> > > > +   struct nf_conntrack_tuple otuple, rtuple;
>> > > > +   struct nf_conn *ct;
>> > > > +   int err;
>> > > > +
>> > > > +   if (unlikely(netns_id < BPF_F_CURRENT_NETNS))
>> > > > +           return ERR_PTR(-EINVAL);
>> > > > +
>> > > > +   err = bpf_nf_ct_tuple_parse(bpf_tuple, tuple_len, protonum,
>> > > > +                               IP_CT_DIR_ORIGINAL, &otuple);
>> > > > +   if (err < 0)
>> > > > +           return ERR_PTR(err);
>> > > > +
>> > > > +   err = bpf_nf_ct_tuple_parse(bpf_tuple, tuple_len, protonum,
>> > > > +                               IP_CT_DIR_REPLY, &rtuple);
>> > > > +   if (err < 0)
>> > > > +           return ERR_PTR(err);
>> > > > +
>> > > > +   if (netns_id >= 0) {
>> > > > +           net = get_net_ns_by_id(net, netns_id);
>> > > > +           if (unlikely(!net))
>> > > > +                   return ERR_PTR(-ENONET);
>> > > > +   }
>> > > > +
>> > > > +   ct = nf_conntrack_alloc(net, &nf_ct_zone_dflt, &otuple, &rtuple,
>> > > > +                           GFP_ATOMIC);
>> > > > +   if (IS_ERR(ct))
>> > > > +           goto out;
>> > > > +
>> > > > +   ct->timeout = timeout * HZ + jiffies;
>> > > > +   ct->status |= IPS_CONFIRMED;
>> > > > +
>> > > > +   memset(&ct->proto, 0, sizeof(ct->proto));
>> > > > +   if (protonum == IPPROTO_TCP)
>> > > > +           ct->proto.tcp.state = TCP_CONNTRACK_ESTABLISHED;
>> > >
>> > > Hmm, isn't it a bit limiting to hard-code this to ESTABLISHED
>> > > connections? Presumably for TCP you'd want to use this when you see a
>> > > SYN and then rely on conntrack to help with the subsequent state
>> > > tracking for when the SYN-ACK comes back? What's the usecase for
>> > > creating an entry in ESTABLISHED state, exactly?
>> >
>> > I guess we can even add a parameter and pass the state from the caller.
>> > I was not sure if it is mandatory.
>>
>> It's probably cleaner and more flexible to split
>> _alloc and _insert into two kfuncs and let bpf
>> prog populate ct directly.
>
> Right, so we can just whitelist a few fields and allow assignments into those.
> One small problem is that we should probably only permit this for nf_conn
> PTR_TO_BTF_ID obtained from _alloc, and make it rdonly on _insert.
>
> We can do the rw->ro conversion by taking in ref from alloc, and releasing on
> _insert, then returning ref from _insert.

Sounds reasonable enough; I guess _insert would also need to
sanity-check some of the values to prevent injecting invalid state into
the conntrack table.

> For the other part, either return a different shadow PTR_TO_BTF_ID
> with only the fields that can be set, convert insns for it, and then
> on insert return the rdonly PTR_TO_BTF_ID of struct nf_conn, or
> otherwise store the source func in the per-register state and use that
> to deny BPF_WRITE for normal nf_conn. Thoughts?

Hmm, if they're different BTF IDs wouldn't the BPF program have to be
aware of this and use two different structs for the pointer storage?
That seems a bit awkward from an API PoV?

Also, what about updating? For this to be useful with TCP, you'd really
want to be able to update the CT state as the connection is going
through the handshake state transitions...

-Toke