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: 8e368dc72e ("bpf: Fix use of sk->sk_reuseport from sk_assign")
Fixes: cf7fbe660f ("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>

include/net/inet6_hashtables.h

@@ -103,6 +103,46 @@ 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)
+		return sk;
+
+	/* We've chosen a new reuseport sock which is never refcounted. This
+	 * implies that sk also isn't refcounted.
+	 */
+	WARN_ON_ONCE(*refcounted);
+
+	return reuse_sk;
+}
+
 static inline struct sock *__inet6_lookup_skb(struct inet_hashinfo *hashinfo,
 					      struct sk_buff *skb, int doff,
 					      const __be16 sport,
@@ -110,14 +150,20 @@ static inline struct sock *__inet6_lookup_skb(struct inet_hashinfo *hashinfo,
 					      int iif, int sdif,
 					      bool *refcounted)
 {
-	struct sock *sk = skb_steal_sock(skb, refcounted);
+	struct net *net = dev_net(skb_dst(skb)->dev);
+	const struct ipv6hdr *ip6h = ipv6_hdr(skb);
+	struct sock *sk;
 
+	sk = inet6_steal_sock(net, skb, doff, &ip6h->saddr, sport, &ip6h->daddr, dport,
+			      refcounted, inet6_ehashfn);
+	if (IS_ERR(sk))
+		return NULL;
 	if (sk)
 		return sk;
 
-	return __inet6_lookup(dev_net(skb_dst(skb)->dev), hashinfo, skb,
-			      doff, &ipv6_hdr(skb)->saddr, sport,
-			      &ipv6_hdr(skb)->daddr, ntohs(dport),
+	return __inet6_lookup(net, hashinfo, skb,
+			      doff, &ip6h->saddr, sport,
+			      &ip6h->daddr, ntohs(dport),
 			      iif, sdif, refcounted);
 }
 

include/net/inet_hashtables.h

@@ -449,6 +449,46 @@ static inline struct sock *inet_lookup(struct net *net,
 	return sk;
 }
 
+static inline
+struct sock *inet_steal_sock(struct net *net, struct sk_buff *skb, int doff,
+			     const __be32 saddr, const __be16 sport,
+			     const __be32 daddr, const __be16 dport,
+			     bool *refcounted, inet_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 = inet_lookup_reuseport(net, sk, skb, doff,
+					 saddr, sport, daddr, ntohs(dport),
+					 ehashfn);
+	if (!reuse_sk)
+		return sk;
+
+	/* We've chosen a new reuseport sock which is never refcounted. This
+	 * implies that sk also isn't refcounted.
+	 */
+	WARN_ON_ONCE(*refcounted);
+
+	return reuse_sk;
+}
+
 static inline struct sock *__inet_lookup_skb(struct inet_hashinfo *hashinfo,
 					     struct sk_buff *skb,
 					     int doff,
@@ -457,13 +497,18 @@ static inline struct sock *__inet_lookup_skb(struct inet_hashinfo *hashinfo,
 					     const int sdif,
 					     bool *refcounted)
 {
-	struct sock *sk = skb_steal_sock(skb, refcounted);
+	struct net *net = dev_net(skb_dst(skb)->dev);
 	const struct iphdr *iph = ip_hdr(skb);
+	struct sock *sk;
 
+	sk = inet_steal_sock(net, skb, doff, iph->saddr, sport, iph->daddr, dport,
+			     refcounted, inet_ehashfn);
+	if (IS_ERR(sk))
+		return NULL;
 	if (sk)
 		return sk;
 
-	return __inet_lookup(dev_net(skb_dst(skb)->dev), hashinfo, skb,
+	return __inet_lookup(net, hashinfo, skb,
 			     doff, iph->saddr, sport,
 			     iph->daddr, dport, inet_iif(skb), sdif,
 			     refcounted);

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;
 }

include/uapi/linux/bpf.h

@@ -4198,9 +4198,6 @@ union bpf_attr {
  *		**-EOPNOTSUPP** if the operation is not supported, for example
  *		a call from outside of TC ingress.
  *
- *		**-ESOCKTNOSUPPORT** if the socket type is not supported
- *		(reuseport).
- *
  * long bpf_sk_assign(struct bpf_sk_lookup *ctx, struct bpf_sock *sk, u64 flags)
  *	Description
  *		Helper is overloaded depending on BPF program type. This

net/core/filter.c

@@ -7351,8 +7351,6 @@ BPF_CALL_3(bpf_sk_assign, struct sk_buff *, skb, struct sock *, sk, u64, flags)
 		return -EOPNOTSUPP;
 	if (unlikely(dev_net(skb->dev) != sock_net(sk)))
 		return -ENETUNREACH;
-	if (unlikely(sk_fullsock(sk) && sk->sk_reuseport))
-		return -ESOCKTNOSUPPORT;
 	if (sk_unhashed(sk))
 		return -EOPNOTSUPP;
 	if (sk_is_refcounted(sk) &&

net/ipv4/udp.c

@@ -2388,7 +2388,11 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 	if (udp4_csum_init(skb, uh, proto))
 		goto csum_error;
 
-	sk = skb_steal_sock(skb, &refcounted);
+	sk = inet_steal_sock(net, skb, sizeof(struct udphdr), saddr, uh->source, daddr, uh->dest,
+			     &refcounted, udp_ehashfn);
+	if (IS_ERR(sk))
+		goto no_sk;
+
 	if (sk) {
 		struct dst_entry *dst = skb_dst(skb);
 		int ret;
@@ -2409,7 +2413,7 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 	sk = __udp4_lib_lookup_skb(skb, uh->source, uh->dest, udptable);
 	if (sk)
 		return udp_unicast_rcv_skb(sk, skb, uh);
-
+no_sk:
 	if (!xfrm4_policy_check(NULL, XFRM_POLICY_IN, skb))
 		goto drop;
 	nf_reset_ct(skb);

net/ipv6/udp.c

@@ -964,7 +964,11 @@ int __udp6_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 		goto csum_error;
 
 	/* Check if the socket is already available, e.g. due to early demux */
-	sk = skb_steal_sock(skb, &refcounted);
+	sk = inet6_steal_sock(net, skb, sizeof(struct udphdr), saddr, uh->source, daddr, uh->dest,
+			      &refcounted, udp6_ehashfn);
+	if (IS_ERR(sk))
+		goto no_sk;
+
 	if (sk) {
 		struct dst_entry *dst = skb_dst(skb);
 		int ret;
@@ -998,7 +1002,7 @@ int __udp6_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 			goto report_csum_error;
 		return udp6_unicast_rcv_skb(sk, skb, uh);
 	}
-
+no_sk:
 	reason = SKB_DROP_REASON_NO_SOCKET;
 
 	if (!uh->check)

tools/include/uapi/linux/bpf.h

@@ -4198,9 +4198,6 @@ union bpf_attr {
  *		**-EOPNOTSUPP** if the operation is not supported, for example
  *		a call from outside of TC ingress.
  *
- *		**-ESOCKTNOSUPPORT** if the socket type is not supported
- *		(reuseport).
- *
  * long bpf_sk_assign(struct bpf_sk_lookup *ctx, struct bpf_sock *sk, u64 flags)
  *	Description
  *		Helper is overloaded depending on BPF program type. This