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From:   Lorenz Bauer <lmb@isovalent.com>
To:     "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        David Ahern <dsahern@kernel.org>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        Joe Stringer <joe@wand.net.nz>
Cc:     Lorenz Bauer <lmb@isovalent.com>, Joe Stringer <joe@cilium.io>,
        Martin KaFai Lau <kafai@fb.com>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org, bpf@vger.kernel.org
Subject: [PATCH bpf-next 1/2] bpf, net: Support SO_REUSEPORT sockets with bpf_sk_assign
Date:   Thu, 25 May 2023 09:19:22 +0100
Message-Id: <20230525081923.8596-1-lmb@isovalent.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

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: 8e368dc ("bpf: Fix use of sk->sk_reuseport from sk_assign")
Fixes: cf7fbe6 ("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 | 36 +++++++++++++++++++++++++++++-----
 include/net/inet_hashtables.h  | 27 +++++++++++++++++++++++--
 include/net/sock.h             |  7 +++++--
 include/uapi/linux/bpf.h       |  3 ---
 net/core/filter.c              |  2 --
 net/ipv4/inet_hashtables.c     | 15 +++++++-------
 net/ipv4/udp.c                 | 23 +++++++++++++++++++---
 net/ipv6/inet6_hashtables.c    | 19 +++++++++---------
 net/ipv6/udp.c                 | 23 +++++++++++++++++++---
 tools/include/uapi/linux/bpf.h |  3 ---
 10 files changed, 119 insertions(+), 39 deletions(-)

diff --git a/include/net/inet6_hashtables.h b/include/net/inet6_hashtables.h
index 56f1286583d3..3ba4dc2703da 100644
--- a/include/net/inet6_hashtables.h
+++ b/include/net/inet6_hashtables.h
@@ -48,6 +48,13 @@ struct sock *__inet6_lookup_established(struct net *net,
 					const u16 hnum, const int dif,
 					const int sdif);
 
+struct sock *inet6_lookup_reuseport(struct net *net, struct sock *sk,
+				    struct sk_buff *skb, int doff,
+				    const struct in6_addr *saddr,
+				    __be16 sport,
+				    const struct in6_addr *daddr,
+				    unsigned short hnum);
+
 struct sock *inet6_lookup_listener(struct net *net,
 				   struct inet_hashinfo *hashinfo,
 				   struct sk_buff *skb, int doff,
@@ -85,14 +92,33 @@ 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);
-
+	bool prefetched;
+	struct sock *sk = skb_steal_sock(skb, refcounted, &prefetched);
+	struct net *net = dev_net(skb_dst(skb)->dev);
+	const struct ipv6hdr *ip6h = ipv6_hdr(skb);
+
+	if (prefetched) {
+		struct sock *reuse_sk = inet6_lookup_reuseport(net, sk, skb, doff,
+							       &ip6h->saddr, sport,
+							       &ip6h->daddr, ntohs(dport));
+		if (reuse_sk) {
+			if (reuse_sk != sk) {
+				if (*refcounted) {
+					sock_put(sk);
+					*refcounted = false;
+				}
+				if (IS_ERR(reuse_sk))
+					return NULL;
+			}
+			return reuse_sk;
+		}
+	}
 	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);
 }
 
diff --git a/include/net/inet_hashtables.h b/include/net/inet_hashtables.h
index 99bd823e97f6..c2af195ca71f 100644
--- a/include/net/inet_hashtables.h
+++ b/include/net/inet_hashtables.h
@@ -379,6 +379,11 @@ struct sock *__inet_lookup_established(struct net *net,
 				       const __be32 daddr, const u16 hnum,
 				       const int dif, const int sdif);
 
+struct sock *inet_lookup_reuseport(struct net *net, struct sock *sk,
+				   struct sk_buff *skb, int doff,
+				   __be32 saddr, __be16 sport,
+				   __be32 daddr, unsigned short hnum);
+
 static inline struct sock *
 	inet_lookup_established(struct net *net, struct inet_hashinfo *hashinfo,
 				const __be32 saddr, const __be16 sport,
@@ -436,13 +441,31 @@ static inline struct sock *__inet_lookup_skb(struct inet_hashinfo *hashinfo,
 					     const int sdif,
 					     bool *refcounted)
 {
-	struct sock *sk = skb_steal_sock(skb, refcounted);
+	bool prefetched;
+	struct sock *sk = skb_steal_sock(skb, refcounted, &prefetched);
+	struct net *net = dev_net(skb_dst(skb)->dev);
 	const struct iphdr *iph = ip_hdr(skb);
 
+	if (prefetched) {
+		struct sock *reuse_sk = inet_lookup_reuseport(net, sk, skb, doff,
+							      iph->saddr, sport,
+							      iph->daddr, ntohs(dport));
+		if (reuse_sk) {
+			if (reuse_sk != sk) {
+				if (*refcounted) {
+					sock_put(sk);
+					*refcounted = false;
+				}
+				if (IS_ERR(reuse_sk))
+					return NULL;
+			}
+			return reuse_sk;
+		}
+	}
 	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);
diff --git a/include/net/sock.h b/include/net/sock.h
index 656ea89f60ff..5645570c2a64 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -2806,20 +2806,23 @@ 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;
 }
diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 1bb11a6ee667..2af606a525db 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -4144,9 +4144,6 @@ 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
diff --git a/net/core/filter.c b/net/core/filter.c
index 968139f4a1ac..5f451260849b 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -7265,8 +7265,6 @@ 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_is_refcounted(sk) &&
 	    unlikely(!refcount_inc_not_zero(&sk->sk_refcnt)))
 		return -ENOENT;
diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index e7391bf310a7..920131e4a65d 100644
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c
@@ -332,10 +332,10 @@ static inline int compute_score(struct sock *sk, struct net *net,
 	return score;
 }
 
-static inline struct sock *lookup_reuseport(struct net *net, struct sock *sk,
-					    struct sk_buff *skb, int doff,
-					    __be32 saddr, __be16 sport,
-					    __be32 daddr, unsigned short hnum)
+struct sock *inet_lookup_reuseport(struct net *net, struct sock *sk,
+				   struct sk_buff *skb, int doff,
+				   __be32 saddr, __be16 sport,
+				   __be32 daddr, unsigned short hnum)
 {
 	struct sock *reuse_sk = NULL;
 	u32 phash;
@@ -346,6 +346,7 @@ static inline struct sock *lookup_reuseport(struct net *net, struct sock *sk,
 	}
 	return reuse_sk;
 }
+EXPORT_SYMBOL_GPL(inet_lookup_reuseport);
 
 /*
  * Here are some nice properties to exploit here. The BSD API
@@ -369,8 +370,8 @@ static struct sock *inet_lhash2_lookup(struct net *net,
 	sk_nulls_for_each_rcu(sk, node, &ilb2->nulls_head) {
 		score = compute_score(sk, net, hnum, daddr, dif, sdif);
 		if (score > hiscore) {
-			result = lookup_reuseport(net, sk, skb, doff,
-						  saddr, sport, daddr, hnum);
+			result = inet_lookup_reuseport(net, sk, skb, doff,
+						       saddr, sport, daddr, hnum);
 			if (result)
 				return result;
 
@@ -399,7 +400,7 @@ static inline struct sock *inet_lookup_run_bpf(struct net *net,
 	if (no_reuseport || IS_ERR_OR_NULL(sk))
 		return sk;
 
-	reuse_sk = lookup_reuseport(net, sk, skb, doff, saddr, sport, daddr, hnum);
+	reuse_sk = inet_lookup_reuseport(net, sk, skb, doff, saddr, sport, daddr, hnum);
 	if (reuse_sk)
 		sk = reuse_sk;
 	return sk;
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 6893fb867529..c67253386a38 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -2426,7 +2426,7 @@ int __udp4_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 	struct rtable *rt = skb_rtable(skb);
 	__be32 saddr, daddr;
 	struct net *net = dev_net(skb->dev);
-	bool refcounted;
+	bool refcounted, prefetched;
 	int drop_reason;
 
 	drop_reason = SKB_DROP_REASON_NOT_SPECIFIED;
@@ -2455,11 +2455,28 @@ 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 = skb_steal_sock(skb, &refcounted, &prefetched);
 	if (sk) {
 		struct dst_entry *dst = skb_dst(skb);
 		int ret;
 
+		if (prefetched) {
+			struct sock *reuse_sk = lookup_reuseport(net, sk, skb,
+								 saddr, uh->source,
+								 daddr, ntohs(uh->dest));
+			if (reuse_sk) {
+				if (reuse_sk != sk) {
+					if (refcounted) {
+						sock_put(sk);
+						refcounted = false;
+					}
+					if (IS_ERR(reuse_sk))
+						goto no_sk;
+				}
+				sk = reuse_sk;
+			}
+		}
+
 		if (unlikely(rcu_dereference(sk->sk_rx_dst) != dst))
 			udp_sk_rx_dst_set(sk, dst);
 
@@ -2476,7 +2493,7 @@ 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);
diff --git a/net/ipv6/inet6_hashtables.c b/net/ipv6/inet6_hashtables.c
index b64b49012655..b7c56867314e 100644
--- a/net/ipv6/inet6_hashtables.c
+++ b/net/ipv6/inet6_hashtables.c
@@ -111,12 +111,12 @@ static inline int compute_score(struct sock *sk, struct net *net,
 	return score;
 }
 
-static inline struct sock *lookup_reuseport(struct net *net, struct sock *sk,
-					    struct sk_buff *skb, int doff,
-					    const struct in6_addr *saddr,
-					    __be16 sport,
-					    const struct in6_addr *daddr,
-					    unsigned short hnum)
+struct sock *inet6_lookup_reuseport(struct net *net, struct sock *sk,
+				    struct sk_buff *skb, int doff,
+				    const struct in6_addr *saddr,
+				    __be16 sport,
+				    const struct in6_addr *daddr,
+				    unsigned short hnum)
 {
 	struct sock *reuse_sk = NULL;
 	u32 phash;
@@ -127,6 +127,7 @@ static inline struct sock *lookup_reuseport(struct net *net, struct sock *sk,
 	}
 	return reuse_sk;
 }
+EXPORT_SYMBOL_GPL(inet6_lookup_reuseport);
 
 /* called with rcu_read_lock() */
 static struct sock *inet6_lhash2_lookup(struct net *net,
@@ -143,8 +144,8 @@ static struct sock *inet6_lhash2_lookup(struct net *net,
 	sk_nulls_for_each_rcu(sk, node, &ilb2->nulls_head) {
 		score = compute_score(sk, net, hnum, daddr, dif, sdif);
 		if (score > hiscore) {
-			result = lookup_reuseport(net, sk, skb, doff,
-						  saddr, sport, daddr, hnum);
+			result = inet6_lookup_reuseport(net, sk, skb, doff,
+							saddr, sport, daddr, hnum);
 			if (result)
 				return result;
 
@@ -175,7 +176,7 @@ static inline struct sock *inet6_lookup_run_bpf(struct net *net,
 	if (no_reuseport || IS_ERR_OR_NULL(sk))
 		return sk;
 
-	reuse_sk = lookup_reuseport(net, sk, skb, doff, saddr, sport, daddr, hnum);
+	reuse_sk = inet6_lookup_reuseport(net, sk, skb, doff, saddr, sport, daddr, hnum);
 	if (reuse_sk)
 		sk = reuse_sk;
 	return sk;
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index e5a337e6b970..3fede8ec95c4 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -949,7 +949,7 @@ int __udp6_lib_rcv(struct sk_buff *skb, struct udp_table *udptable,
 	struct net *net = dev_net(skb->dev);
 	struct udphdr *uh;
 	struct sock *sk;
-	bool refcounted;
+	bool refcounted, prefetched;
 	u32 ulen = 0;
 
 	if (!pskb_may_pull(skb, sizeof(struct udphdr)))
@@ -986,11 +986,28 @@ 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 = skb_steal_sock(skb, &refcounted, &prefetched);
 	if (sk) {
 		struct dst_entry *dst = skb_dst(skb);
 		int ret;
 
+		if (prefetched) {
+			struct sock *reuse_sk = lookup_reuseport(net, sk, skb,
+								 saddr, uh->source,
+								 daddr, ntohs(uh->dest));
+			if (reuse_sk) {
+				if (reuse_sk != sk) {
+					if (refcounted) {
+						sock_put(sk);
+						refcounted = false;
+					}
+					if (IS_ERR(reuse_sk))
+						goto no_sk;
+				}
+				sk = reuse_sk;
+			}
+		}
+
 		if (unlikely(rcu_dereference(sk->sk_rx_dst) != dst))
 			udp6_sk_rx_dst_set(sk, dst);
 
@@ -1020,7 +1037,7 @@ 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)
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 1bb11a6ee667..2af606a525db 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -4144,9 +4144,6 @@ 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
-- 
2.40.1