af_unix: Set sk->sk_state under unix_state_lock() for truly disconencted peer.

When a SOCK_DGRAM socket connect()s to another socket, the both sockets'
sk->sk_state are changed to TCP_ESTABLISHED so that we can register them
to BPF SOCKMAP.

When the socket disconnects from the peer by connect(AF_UNSPEC), the state
is set back to TCP_CLOSE.

Then, the peer's state is also set to TCP_CLOSE, but the update is done
locklessly and unconditionally.

Let's say socket A connect()ed to B, B connect()ed to C, and A disconnects
from B.

After the first two connect()s, all three sockets' sk->sk_state are
TCP_ESTABLISHED:

  $ ss -xa
  Netid State  Recv-Q Send-Q  Local Address:Port  Peer Address:PortProcess
  u_dgr ESTAB  0      0       @A 641              * 642
  u_dgr ESTAB  0      0       @B 642              * 643
  u_dgr ESTAB  0      0       @C 643              * 0

And after the disconnect, B's state is TCP_CLOSE even though it's still
connected to C and C's state is TCP_ESTABLISHED.

  $ ss -xa
  Netid State  Recv-Q Send-Q  Local Address:Port  Peer Address:PortProcess
  u_dgr UNCONN 0      0       @A 641              * 0
  u_dgr UNCONN 0      0       @B 642              * 643
  u_dgr ESTAB  0      0       @C 643              * 0

In this case, we cannot register B to SOCKMAP.

So, when a socket disconnects from the peer, we should not set TCP_CLOSE to
the peer if the peer is connected to yet another socket, and this must be
done under unix_state_lock().

Note that we use WRITE_ONCE() for sk->sk_state as there are many lockless
readers.  These data-races will be fixed in the following patches.

Fixes: 83301b53 ("af_unix: Set TCP_ESTABLISHED for datagram sockets too")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

net/unix/af_unix.c

@@ -570,7 +570,6 @@ static void unix_dgram_disconnected(struct sock *sk, struct sock *other)
 			sk_error_report(other);
 		}
 	}
-	other->sk_state = TCP_CLOSE;
 }
 
 static void unix_sock_destructor(struct sock *sk)
@@ -1424,8 +1423,15 @@ static int unix_dgram_connect(struct socket *sock, struct sockaddr *addr,
 
 		unix_state_double_unlock(sk, other);
 
-		if (other != old_peer)
+		if (other != old_peer) {
 			unix_dgram_disconnected(sk, old_peer);
+
+			unix_state_lock(old_peer);
+			if (!unix_peer(old_peer))
+				WRITE_ONCE(old_peer->sk_state, TCP_CLOSE);
+			unix_state_unlock(old_peer);
+		}
+
 		sock_put(old_peer);
 	} else {
 		unix_peer(sk) = other;