bpf, net: Support SO_REUSEPORT sockets with bpf_sk_assign

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>
Cc: Joe Stringer <joe@cilium.io>
Link: https://lore.kernel.org/bpf/CACAyw98+qycmpQzKupquhkxbvWK4OFyDuuLMBNROnfWMZxUWeA@mail.gmail.com/
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Lorenz Bauer <lmb@isovalent.com>
Link: https://lore.kernel.org/r/20230720-so-reuseport-v6-7-7021b683cdae@isovalent.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>

1 files changed, 5 insertions, 2 deletions

diff --git a/include/net/sock.h b/include/net/sock.h
index 7ae44bf866af5c..74cbfb15d28927 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -2815,20 +2815,23 @@ sk_is_refcounted(struct sock *sk)
  * skb_steal_sock - steal a socket from an sk_buff
  * @skb: sk_buff to steal the socket from
  * @refcounted: is set to true if the socket is reference-counted
+ * @prefetched: is set to true if the socket was assigned from bpf
  */
 static inline struct sock *
-skb_steal_sock(struct sk_buff *skb, bool *refcounted)
+skb_steal_sock(struct sk_buff *skb, bool *refcounted, bool *prefetched)
 {
 	if (skb->sk) {
 		struct sock *sk = skb->sk;
 
 		*refcounted = true;
-		if (skb_sk_is_prefetched(skb))
+		*prefetched = skb_sk_is_prefetched(skb);
+		if (*prefetched)
 			*refcounted = sk_is_refcounted(sk);
 		skb->destructor = NULL;
 		skb->sk = NULL;
 		return sk;
 	}
+	*prefetched = false;
 	*refcounted = false;
 	return NULL;
 }