Re: [PATCH net v2] unix: Fix race in SOCK_SEQPACKET's unix_dgram_sendmsg()

[Kuniyuki Iwashima <kuniyu@amazon.com>]

From:   Kirill Tkhai <tkhai@ya.ru>
Date:   Sun, 27 Nov 2022 01:46:51 +0300
> There is a race resulting in alive SOCK_SEQPACKET socket
> may change its state from TCP_ESTABLISHED to TCP_CLOSE:
> 
> unix_release_sock(peer)                  unix_dgram_sendmsg(sk)
>   sock_orphan(peer)
>     sock_set_flag(peer, SOCK_DEAD)
>                                            sock_alloc_send_pskb()
>                                              if !(sk->sk_shutdown & SEND_SHUTDOWN)
>                                                OK
>                                            if sock_flag(peer, SOCK_DEAD)
>                                              sk->sk_state = TCP_CLOSE
>   sk->sk_shutdown = SHUTDOWN_MASK
> 
> 
> After that socket sk remains almost normal: it is able to connect, listen, accept
> and recvmsg, while it can't sendmsg.
> 
> Since this is the only possibility for alive SOCK_SEQPACKET to change
> the state in such way, we should better fix this strange and potentially
> danger corner case.
> 
> Also, move TCP_CLOSE assignment for SOCK_DGRAM sockets under state lock
> to fix race with unix_dgram_connect():
> 
> unix_dgram_connect(other)            unix_dgram_sendmsg(sk)
>                                        unix_peer(sk) = NULL
>                                        unix_state_unlock(sk)
>   unix_state_double_lock(sk, other)
>   sk->sk_state  = TCP_ESTABLISHED
>   unix_peer(sk) = other
>   unix_state_double_unlock(sk, other)
>                                        sk->sk_state  = TCP_CLOSED
> 
> This patch fixes both of these races.
> 
> Fixes: 83301b5367a9 ("af_unix: Set TCP_ESTABLISHED for datagram sockets too")
> Suggested-by: Kuniyuki Iwashima <kuniyu@amazon.com>
> Signed-off-by: Kirill Tkhai <tkhai@ya.ru>

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>

Thank you, Kirill.


> ---
> v2: Disconnect from peer right there.
> 
>  net/unix/af_unix.c |    7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index b3545fc68097..be40023a61fb 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -2001,11 +2001,14 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
>  		err = 0;
>  		if (unix_peer(sk) == other) {
>  			unix_peer(sk) = NULL;
> -			unix_dgram_peer_wake_disconnect_wakeup(sk, other);
> +
> +			if (sk->sk_type == SOCK_DGRAM) {
> +				unix_dgram_peer_wake_disconnect_wakeup(sk, other);
> +				sk->sk_state = TCP_CLOSE;
> +			}
>  
>  			unix_state_unlock(sk);
>  
> -			sk->sk_state = TCP_CLOSE;
>  			unix_dgram_disconnected(sk, other);
>  			sock_put(other);
>  			err = -ECONNREFUSED;