net: sockmap: avoid race between sock_map_destroy() and sk_psock_put()

Syzbot has triggered the following race condition:

On CPU0, 'sk_psock_drop()' (most likely scheduled from 'sock_map_unref()'
called by 'sock_map_update_common()') is running at [1]:

void sk_psock_drop(struct sock *sk, struct sk_psock *psock)
{
        write_lock_bh(&sk->sk_callback_lock);
        sk_psock_restore_proto(sk, psock);                              [1]
        rcu_assign_sk_user_data(sk, NULL);                              [2]
        ...
}

If 'sock_map_destroy()' is scheduled on CPU1 at the same time, psock is
always NULL at [3]. But, since [1] may be is in progress during [4], the
value of 'saved_destroy' at this point is undefined:

void sock_map_destroy(struct sock *sk)
{
        void (*saved_destroy)(struct sock *sk);
        struct sk_psock *psock;

        rcu_read_lock();
        psock = sk_psock_get(sk);                                       [3]
        if (unlikely(!psock)) {
                rcu_read_unlock();
                saved_destroy = READ_ONCE(sk->sk_prot)->destroy;        [4]
        } else {
                saved_destroy = psock->saved_destroy;                   [5]
                sock_map_remove_links(sk, psock);
                rcu_read_unlock();
                sk_psock_stop(psock);
                sk_psock_put(sk, psock);
        }
        if (WARN_ON_ONCE(saved_destroy == sock_map_destroy))
                return;
        if (saved_destroy)
                saved_destroy(sk);
}

Fix this issue in 3 steps:

1. Prefer 'sk_psock()' over 'sk_psock_get()' at [3]. Since zero
   refcount is ignored, 'psock' is non-NULL until [2] is completed.

2. Add read lock around [5], to make sure that [1] is not in progress
   when the former is executed.

3. Since 'sk_psock()' does not adjust reference counting, drop
   'sk_psock_put()' and redundant 'sk_psock_stop()' (which is
   executed by 'sk_psock_drop()' anyway).

Fixes: 5b4a79b ("bpf, sockmap: Don't let sock_map_{close,destroy,unhash} call itself")
Reported-by: [email protected]
Closes: https://syzkaller.appspot.com/bug?extid=f363afac6b0ace576f45
Signed-off-by: Dmitry Antipov <[email protected]>
Signed-off-by: NipaLocal <nipa@local>

net/core/sock_map.c

@@ -1650,16 +1650,16 @@ void sock_map_destroy(struct sock *sk)
 	struct sk_psock *psock;
 
 	rcu_read_lock();
-	psock = sk_psock_get(sk);
+	psock = sk_psock(sk);
 	if (unlikely(!psock)) {
 		rcu_read_unlock();
 		saved_destroy = READ_ONCE(sk->sk_prot)->destroy;
 	} else {
+		read_lock_bh(&sk->sk_callback_lock);
 		saved_destroy = psock->saved_destroy;
+		read_unlock_bh(&sk->sk_callback_lock);
 		sock_map_remove_links(sk, psock);
 		rcu_read_unlock();
-		sk_psock_stop(psock);
-		sk_psock_put(sk, psock);
 	}
 	if (WARN_ON_ONCE(saved_destroy == sock_map_destroy))
 		return;