udp: Update reuse->has_conns under reuseport_lock.

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

Note the likely(reuse) in reuseport_has_conns_set() is always true,
but we put the test there for ease of review.  [0]

For the record, usually, sk_reuseport_cb is changed under lock_sock().
The only exception is reuseport_grow() & TCP reqsk migration case.

  1) shutdown() TCP listener, which is moved into the latter part of
     reuse->socks[] to migrate reqsk.

  2) New listen() overflows reuse->socks[] and call reuseport_grow().

  3) reuse->max_socks overflows u16 with the new listener.

  4) reuseport_grow() pops the old shutdown()ed listener from the array
     and update its sk->sk_reuseport_cb as NULL without lock_sock().

shutdown()ed TCP sk->sk_reuseport_cb can be changed without lock_sock(),
but, reuseport_has_conns_set() is called only for UDP under lock_sock(),
so likely(reuse) never be false in reuseport_has_conns_set().

[0]: https://lore.kernel.org/netdev/CANn89iLja=eQHbsM_Ta2sQF0tOGU8vAGrh_izRuuHjuO1ouUag@mail.gmail.com/

Fixes: acdcecc612 ("udp: correct reuseport selection with connected sockets")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20221014182625.89913-1-kuniyu@amazon.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

include/net/sock_reuseport.h

@@ -43,21 +43,20 @@ 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;
 }
 
+void reuseport_has_conns_set(struct sock *sk);
+
 #endif  /* _SOCK_REUSEPORT_H */

net/core/sock_reuseport.c

@@ -21,6 +21,22 @@ static DEFINE_IDA(reuseport_ida);
 static int reuseport_resurrect(struct sock *sk, struct sock_reuseport *old_reuse,
 			       struct sock_reuseport *reuse, bool bind_inany);
 
+void reuseport_has_conns_set(struct sock *sk)
+{
+	struct sock_reuseport *reuse;
+
+	if (!rcu_access_pointer(sk->sk_reuseport_cb))
+		return;
+
+	spin_lock_bh(&reuseport_lock);
+	reuse = rcu_dereference_protected(sk->sk_reuseport_cb,
+					  lockdep_is_held(&reuseport_lock));
+	if (likely(reuse))
+		reuse->has_conns = 1;
+	spin_unlock_bh(&reuseport_lock);
+}
+EXPORT_SYMBOL(reuseport_has_conns_set);
+
 static int reuseport_sock_index(struct sock *sk,
 				const struct sock_reuseport *reuse,
 				bool closed)

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();

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;

net/ipv6/datagram.c

@@ -256,7 +256,7 @@ ipv4_connected:
 		goto out;
 	}
 
-	reuseport_has_conns(sk, true);
+	reuseport_has_conns_set(sk);
 	sk->sk_state = TCP_ESTABLISHED;
 	sk_set_txhash(sk);
 out:

net/ipv6/udp.c

@@ -195,7 +195,7 @@ 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;