bpf, sockmap: avoid using sk_socket after free when sending

The sk->sk_socket is not locked or referenced, and during the call to
skb_send_sock(), there is a race condition with the release of sk_socket.
All types of sockets(tcp/udp/unix/vsock) will be affected.

Race conditions:
'''
CPU0                               CPU1
skb_send_sock
  sendmsg_unlocked
    sock_sendmsg
      sock_sendmsg_nosec
                                   close(fd):
                                     ...
                                   ops->release()
                                     sock_map_close()
                                   sk_socket->ops = NULL
                                   free(socket)
      sock->ops->sendmsg
            ^
            panic here
'''

Based on the fact that we already wait for the workqueue to finish in
sock_map_close() if psock is held, we simply increase the psock
reference count to avoid race conditions.
'''
void sock_map_close()
{
    ...
    if (likely(psock)) {
    ...
    psock = sk_psock_get(sk);
    if (unlikely(!psock))
        goto no_psock; <=== Control usually jumps here via goto
        ...
        cancel_delayed_work_sync(&psock->work); <=== not executed
        sk_psock_put(sk, psock);
        ...
}
'''

The panic I catched:
'''
Workqueue: events sk_psock_backlog
RIP: 0010:sock_sendmsg+0x21d/0x440
RAX: 0000000000000000 RBX: ffffc9000521fad8 RCX: 0000000000000001
...
Call Trace:
 <TASK>
 ? die_addr+0x40/0xa0
 ? exc_general_protection+0x14c/0x230
 ? asm_exc_general_protection+0x26/0x30
 ? sock_sendmsg+0x21d/0x440
 ? sock_sendmsg+0x3e0/0x440
 ? __pfx_sock_sendmsg+0x10/0x10
 __skb_send_sock+0x543/0xb70
 sk_psock_backlog+0x247/0xb80
...
'''

Reported-by: Michal Luczaj <[email protected]>
Fixes: 799aa7f ("skmsg: Avoid lock_sock() in sk_psock_backlog()")
Signed-off-by: Jiayuan Chen <[email protected]>

Author: mrpre

net/core/skmsg.c

@@ -655,6 +655,14 @@ static void sk_psock_backlog(struct work_struct *work)
 	bool ingress;
 	int ret;
 
+	/* Increment the psock refcnt to synchronize with close(fd) path in
+	 * sock_map_close(), ensuring we wait for backlog thread completion
+	 * before sk_socket freed. If refcnt increment fails, it indicates
+	 * sock_map_close() completed with sk_socket potentially already freed.
+	 */
+	if (!sk_psock_get(psock->sk))
+		return;
+
 	mutex_lock(&psock->work_mutex);
 	if (unlikely(state->len)) {
 		len = state->len;
@@ -702,6 +710,7 @@ static void sk_psock_backlog(struct work_struct *work)
 	}
 end:
 	mutex_unlock(&psock->work_mutex);
+	sk_psock_put(psock->sk, psock);
 }
 
 struct sk_psock *sk_psock_init(struct sock *sk, int node)