From: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
Date: Mon, 26 Jun 2023 14:08:46 -0700
> From: Lorenz Bauer <lmb@xxxxxxxxxxxxx>
> Date: Mon, 26 Jun 2023 16:09:03 +0100
> > Currently the bpf_sk_assign helper in tc BPF context refuses SO_REUSEPORT
> > sockets. This means we can't use the helper to steer traffic to Envoy,
> > which configures SO_REUSEPORT on its sockets. In turn, we're blocked
> > from removing TPROXY from our setup.
> >
> > The reason that bpf_sk_assign refuses such sockets is that the
> > bpf_sk_lookup helpers don't execute SK_REUSEPORT programs. Instead,
> > one of the reuseport sockets is selected by hash. This could cause
> > dispatch to the "wrong" socket:
> >
> > sk = bpf_sk_lookup_tcp(...) // select SO_REUSEPORT by hash
> > bpf_sk_assign(skb, sk) // SK_REUSEPORT wasn't executed
> >
> > Fixing this isn't as simple as invoking SK_REUSEPORT from the lookup
> > helpers unfortunately. In the tc context, L2 headers are at the start
> > of the skb, while SK_REUSEPORT expects L3 headers instead.
> >
> > Instead, we execute the SK_REUSEPORT program when the assigned socket
> > is pulled out of the skb, further up the stack. This creates some
> > trickiness with regards to refcounting as bpf_sk_assign will put both
> > refcounted and RCU freed sockets in skb->sk. reuseport sockets are RCU
> > freed. We can infer that the sk_assigned socket is RCU freed if the
> > reuseport lookup succeeds, but convincing yourself of this fact isn't
> > straight forward. Therefore we defensively check refcounting on the
> > sk_assign sock even though it's probably not required in practice.
> >
> > Fixes: 8e368dc72e86 ("bpf: Fix use of sk->sk_reuseport from sk_assign")
> > Fixes: cf7fbe660f2d ("bpf: Add socket assign support")
> > Co-developed-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> > Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> > Signed-off-by: Lorenz Bauer <lmb@xxxxxxxxxxxxx>
> > Cc: Joe Stringer <joe@xxxxxxxxx>
> > Link: https://lore.kernel.org/bpf/CACAyw98+qycmpQzKupquhkxbvWK4OFyDuuLMBNROnfWMZxUWeA@xxxxxxxxxxxxxx/
> > ---
> > include/net/inet6_hashtables.h | 59 ++++++++++++++++++++++++++++++++++++++----
> > include/net/inet_hashtables.h | 52 +++++++++++++++++++++++++++++++++++--
> > include/net/sock.h | 7 +++--
> > include/uapi/linux/bpf.h | 3 ---
> > net/core/filter.c | 2 --
> > net/ipv4/udp.c | 8 ++++--
> > net/ipv6/udp.c | 10 ++++---
> > tools/include/uapi/linux/bpf.h | 3 ---
> > 8 files changed, 122 insertions(+), 22 deletions(-)
> >
> > diff --git a/include/net/inet6_hashtables.h b/include/net/inet6_hashtables.h
> > index 4d2a1a3c0be7..4d300af6ccb6 100644
> > --- a/include/net/inet6_hashtables.h
> > +++ b/include/net/inet6_hashtables.h
> > @@ -103,6 +103,49 @@ static inline struct sock *__inet6_lookup(struct net *net,
> > daddr, hnum, dif, sdif);
> > }
> >
> > +static inline
> > +struct sock *inet6_steal_sock(struct net *net, struct sk_buff *skb, int doff,
> > + const struct in6_addr *saddr, const __be16 sport,
> > + const struct in6_addr *daddr, const __be16 dport,
> > + bool *refcounted, inet6_ehashfn_t ehashfn)
> > +{
> > + struct sock *sk, *reuse_sk;
> > + bool prefetched;
> > +
> > + sk = skb_steal_sock(skb, refcounted, &prefetched);
> > + if (!sk)
> > + return NULL;
> > +
> > + if (!prefetched)
> > + return sk;
> > +
> > + if (sk->sk_protocol == IPPROTO_TCP) {
> > + if (sk->sk_state != TCP_LISTEN)
> > + return sk;
> > + } else if (sk->sk_protocol == IPPROTO_UDP) {
> > + if (sk->sk_state != TCP_CLOSE)
> > + return sk;
> > + } else {
> > + return sk;
> > + }
> > +
> > + reuse_sk = inet6_lookup_reuseport(net, sk, skb, doff,
> > + saddr, sport, daddr, ntohs(dport),
> > + ehashfn);
> > + if (!reuse_sk || reuse_sk == sk)
> > + return sk;
> > +
> > + /* We've chosen a new reuseport sock which is never refcounted.
> > + * sk might be refcounted however, drop the reference if necessary.
> > + */
> > + if (*refcounted) {
> > + sock_put(sk);
> > + *refcounted = false;
> > + }
>
> As *refcounted should be false here (TCP_LISTEN and UDP sk have
> SOCK_RCU_FREE and other sk does not reach here), I prefer adding
> WARN_ON_ONCE() to catch a future bug:
>
> WARN_ON_ONCE(*refcounted);
> sock_put(sk);
Sorry, sock_put(sk) is not needed here.
