soreuseport: fix NULL ptr dereference SO_REUSEPORT after bind

Marc Dionne discovered a NULL pointer dereference when setting
SO_REUSEPORT on a socket after it is bound.
This patch removes the assumption that at least one socket in the
reuseport group is bound with the SO_REUSEPORT option before other
bind calls occur.

Fixes: e32ea7e74727 ("soreuseport: fast reuseport UDP socket selection")
Reported-by: Marc Dionne <[e-mail]>
Signed-off-by: Craig Gallek <[e-mail]>
Tested-by: Marc Dionne <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/sock_reuseport.h

···
 };
 
 extern int reuseport_alloc(struct sock *sk);
-extern int reuseport_add_sock(struct sock *sk, const struct sock *sk2);
+extern int reuseport_add_sock(struct sock *sk, struct sock *sk2);
 extern void reuseport_detach_sock(struct sock *sk);
 extern struct sock *reuseport_select_sock(struct sock *sk,
 					  u32 hash,

net/core/sock_reuseport.c

···
  *  @sk2: Socket belonging to the existing reuseport group.
  *  May return ENOMEM and not add socket to group under memory pressure.
  */
-int reuseport_add_sock(struct sock *sk, const struct sock *sk2)
+int reuseport_add_sock(struct sock *sk, struct sock *sk2)
 {
 	struct sock_reuseport *reuse;
+
+	if (!rcu_access_pointer(sk2->sk_reuseport_cb)) {
+		int err = reuseport_alloc(sk2);
+
+		if (err)
+			return err;
+	}
 
 	spin_lock_bh(&reuseport_lock);
 	reuse = rcu_dereference_protected(sk2->sk_reuseport_cb,