[PATCH v1 net 1/3] udp: Update reuse->has_conns under reuseport_lock.

[Kuniyuki Iwashima]

When we call connect() for a UDP socket in a reuseport group, we have
to update sk->sk_reuseport_cb->has_conns to 1.  Otherwise, the kernel
could select a unconnected socket wrongly for packets sent to the
connected socket.

However, the current way to set has_conns is illegal and possible to
trigger that problem.  reuseport_has_conns() changes has_conns under
rcu_read_lock(), which upgrades the RCU reader to the updater.  Then,
it must do the update under the updater's lock, reuseport_lock, but
it doesn't for now.

For this reason, there is a race below where we fail to set has_conns
resulting in the wrong socket selection.  To avoid the race, let's split
the reader and updater with proper locking.

 cpu1                               cpu2
+----+                             +----+

__ip[46]_datagram_connect()        reuseport_grow()
.                                  .
|- reuseport_has_conns(sk, true)   |- more_reuse = __reuseport_alloc(more_socks_size)
|  .                               |
|  |- rcu_read_lock()
|  |- reuse = rcu_dereference(sk->sk_reuseport_cb)
|  |
|  |                               |  /* reuse->has_conns == 0 here */
|  |                               |- more_reuse->has_conns = reuse->has_conns
|  |- reuse->has_conns = 1         |  /* more_reuse->has_conns SHOULD BE 1 HERE */
|  |                               |
|  |                               |- rcu_assign_pointer(reuse->socks[i]->sk_reuseport_cb,
|  |                               |                     more_reuse)
|  `- rcu_read_unlock()            `- kfree_rcu(reuse, rcu)
|
|- sk->sk_state = TCP_ESTABLISHED

Fixes: acdcecc61285 ("udp: correct reuseport selection with connected sockets")
Signed-off-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>
---
 include/net/sock_reuseport.h | 23 +++++++++++++++++------
 net/ipv4/datagram.c          |  2 +-
 net/ipv4/udp.c               |  2 +-
 net/ipv6/datagram.c          |  2 +-
 net/ipv6/udp.c               |  2 +-
 5 files changed, 21 insertions(+), 10 deletions(-)

diff --git a/include/net/sock_reuseport.h b/include/net/sock_reuseport.h
index 473b0b0fa4ab..fe9779e6d90f 100644
--- a/include/net/sock_reuseport.h
+++ b/include/net/sock_reuseport.h
@@ -43,21 +43,32 @@ struct sock *reuseport_migrate_sock(struct sock *sk,
 extern int reuseport_attach_prog(struct sock *sk, struct bpf_prog *prog);
 extern int reuseport_detach_prog(struct sock *sk);
 
-static inline bool reuseport_has_conns(struct sock *sk, bool set)
+static inline bool reuseport_has_conns(struct sock *sk)
 {
 	struct sock_reuseport *reuse;
 	bool ret = false;
 
 	rcu_read_lock();
 	reuse = rcu_dereference(sk->sk_reuseport_cb);
-	if (reuse) {
-		if (set)
-			reuse->has_conns = 1;
-		ret = reuse->has_conns;
-	}
+	if (reuse && reuse->has_conns)
+		ret = true;
 	rcu_read_unlock();
 
 	return ret;
 }
 
+static inline void reuseport_has_conns_set(struct sock *sk)
+{
+	struct sock_reuseport *reuse;
+
+	if (!rcu_access_pointer(sk->sk_reuseport_cb))
+		return;
+
+	spin_lock(&reuseport_lock);
+	reuse = rcu_dereference_protected(sk->sk_reuseport_cb,
+					  lockdep_is_held(&reuseport_lock));
+	reuse->has_conns = 1;
+	spin_unlock(&reuseport_lock);
+}
+
 #endif  /* _SOCK_REUSEPORT_H */
diff --git a/net/ipv4/datagram.c b/net/ipv4/datagram.c
index 405a8c2aea64..5e66add7befa 100644
--- a/net/ipv4/datagram.c
+++ b/net/ipv4/datagram.c
@@ -70,7 +70,7 @@ int __ip4_datagram_connect(struct sock *sk, struct sockaddr *uaddr, int addr_len
 	}
 	inet->inet_daddr = fl4->daddr;
 	inet->inet_dport = usin->sin_port;
-	reuseport_has_conns(sk, true);
+	reuseport_has_conns_set(sk);
 	sk->sk_state = TCP_ESTABLISHED;
 	sk_set_txhash(sk);
 	inet->inet_id = prandom_u32();
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index d63118ce5900..29228231b058 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -448,7 +448,7 @@ static struct sock *udp4_lib_lookup2(struct net *net,
 			result = lookup_reuseport(net, sk, skb,
 						  saddr, sport, daddr, hnum);
 			/* Fall back to scoring if group has connections */
-			if (result && !reuseport_has_conns(sk, false))
+			if (result && !reuseport_has_conns(sk))
 				return result;
 
 			result = result ? : sk;
diff --git a/net/ipv6/datagram.c b/net/ipv6/datagram.c
index df665d4e8f0f..5ecb56522f9d 100644
--- a/net/ipv6/datagram.c
+++ b/net/ipv6/datagram.c
@@ -256,7 +256,7 @@ int __ip6_datagram_connect(struct sock *sk, struct sockaddr *uaddr,
 		goto out;
 	}
 
-	reuseport_has_conns(sk, true);
+	reuseport_has_conns_set(sk);
 	sk->sk_state = TCP_ESTABLISHED;
 	sk_set_txhash(sk);
 out:
diff --git a/net/ipv6/udp.c b/net/ipv6/udp.c
index 91e795bb9ade..56e4523a3004 100644
--- a/net/ipv6/udp.c
+++ b/net/ipv6/udp.c
@@ -182,7 +182,7 @@ static struct sock *udp6_lib_lookup2(struct net *net,
 			result = lookup_reuseport(net, sk, skb,
 						  saddr, sport, daddr, hnum);
 			/* Fall back to scoring if group has connections */
-			if (result && !reuseport_has_conns(sk, false))
+			if (result && !reuseport_has_conns(sk))
 				return result;
 
 			result = result ? : sk;
-- 
2.30.2