Re: [PATCH bpf-next v3 6/7] bpf, net: Support SO_REUSEPORT sockets with bpf_sk_assign

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From: Kuniyuki Iwashima <kuniyu@amazon.com>
Date: Mon, 26 Jun 2023 14:08:46 -0700
> From: Lorenz Bauer <lmb@isovalent.com>
> 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@iogearbox.net>
> > Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
> > Signed-off-by: Lorenz Bauer <lmb@isovalent.com>
> > Cc: Joe Stringer <joe@cilium.io>
> > Link: https://lore.kernel.org/bpf/CACAyw98+qycmpQzKupquhkxbvWK4OFyDuuLMBNROnfWMZxUWeA@mail.gmail.com/
> > ---
> >  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.