Subject: Re: [PATCH bpf-next v4 2/5] af_unix: add unix_stream_proto for sockmap

From: Jakub Sitnicki <[email-masked]>
Date: Thu, 05 Aug 2021 20:44:37 +0200

On Thu, Aug 05, 2021 at 07:13 AM CEST, Jiang Wang wrote:

[...]

> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -791,17 +791,35 @@ static void unix_close(struct sock *sk, long timeout)
>  	 */
>  }
>
> -struct proto unix_proto = {
> -	.name			= "UNIX",
> +static void unix_unhash(struct sock *sk)
> +{
> +	/* Nothing to do here, unix socket does not need a ->unhash().
> +	 * This is merely for sockmap.
> +	 */
> +}
> +
> +struct proto unix_dgram_proto = {
> +	.name			= "UNIX-DGRAM",
> +	.owner			= THIS_MODULE,
> +	.obj_size		= sizeof(struct unix_sock),
> +	.close			= unix_close,
> +#ifdef CONFIG_BPF_SYSCALL
> +	.psock_update_sk_prot	= unix_dgram_bpf_update_proto,
> +#endif
> +};
> +
> +struct proto unix_stream_proto = {
> +	.name			= "UNIX-STREAM",
>  	.owner			= THIS_MODULE,
>  	.obj_size		= sizeof(struct unix_sock),
>  	.close			= unix_close,
> +	.unhash			= unix_unhash,
>  #ifdef CONFIG_BPF_SYSCALL
> -	.psock_update_sk_prot	= unix_bpf_update_proto,
> +	.psock_update_sk_prot	= unix_stream_bpf_update_proto,
>  #endif
>  };
>
> -static struct sock *unix_create1(struct net *net, struct socket *sock, int kern)
> +static struct sock *unix_create1(struct net *net, struct socket *sock, int kern, int type)
>  {
>  	struct sock *sk = NULL;
>  	struct unix_sock *u;
> @@ -810,7 +828,11 @@ static struct sock *unix_create1(struct net *net, struct socket *sock, int kern)
>  	if (atomic_long_read(&unix_nr_socks) > 2 * get_max_files())
>  		goto out;
>
> -	sk = sk_alloc(net, PF_UNIX, GFP_KERNEL, &unix_proto, kern);
> +	if (type == SOCK_STREAM)
> +		sk = sk_alloc(net, PF_UNIX, GFP_KERNEL, &unix_stream_proto, kern);
> +	else /*dgram and  seqpacket */
> +		sk = sk_alloc(net, PF_UNIX, GFP_KERNEL, &unix_dgram_proto, kern);
> +
>  	if (!sk)
>  		goto out;
>
> @@ -872,7 +894,7 @@ static int unix_create(struct net *net, struct socket *sock, int protocol,
>  		return -ESOCKTNOSUPPORT;
>  	}
>
> -	return unix_create1(net, sock, kern) ? 0 : -ENOMEM;
> +	return unix_create1(net, sock, kern, sock->type) ? 0 : -ENOMEM;
>  }
>
>  static int unix_release(struct socket *sock)
> @@ -1286,7 +1308,7 @@ static int unix_stream_connect(struct socket *sock, struct sockaddr *uaddr,
>  	err = -ENOMEM;
>
>  	/* create new sock for complete connection */
> -	newsk = unix_create1(sock_net(sk), NULL, 0);
> +	newsk = unix_create1(sock_net(sk), NULL, 0, sock->type);
>  	if (newsk == NULL)
>  		goto out;
>
> @@ -2261,7 +2283,7 @@ static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg, size_t si
>  	struct sock *sk = sock->sk;
>
>  #ifdef CONFIG_BPF_SYSCALL
> -	if (sk->sk_prot != &unix_proto)
> +	if (READ_ONCE(sk->sk_prot) != &unix_dgram_proto)
>  		return sk->sk_prot->recvmsg(sk, msg, size, flags & MSG_DONTWAIT,
>  					    flags & ~MSG_DONTWAIT, NULL);

Notice we have two reads from sk->sk_prot here.  And the value
sk->sk_prot holds might change between reads (that is when we remove the
socket from sockmap). So we want to load it just once.

Otherwise, it seems possible that sk->sk_prot->recvmsg will be called,
when sk->sk_prot == unix_proto. Which means sk->sk_prot->recvmsg is NULL.

>  #endif
> @@ -2580,6 +2602,20 @@ static int unix_stream_read_actor(struct sk_buff *skb,
>  	return ret ?: chunk;
>  }
>
> +int __unix_stream_recvmsg(struct sock *sk, struct msghdr *msg,
> +			  size_t size, int flags)
> +{
> +	struct unix_stream_read_state state = {
> +		.recv_actor = unix_stream_read_actor,
> +		.socket = sk->sk_socket,
> +		.msg = msg,
> +		.size = size,
> +		.flags = flags
> +	};
> +
> +	return unix_stream_read_generic(&state, true);
> +}
> +
>  static int unix_stream_recvmsg(struct socket *sock, struct msghdr *msg,
>  			       size_t size, int flags)
>  {
> @@ -2591,6 +2627,12 @@ static int unix_stream_recvmsg(struct socket *sock, struct msghdr *msg,
>  		.flags = flags
>  	};
>
> +#ifdef CONFIG_BPF_SYSCALL
> +	struct sock *sk = sock->sk;
> +	if (sk->sk_prot != &unix_stream_proto)
> +		return sk->sk_prot->recvmsg(sk, msg, size, flags & MSG_DONTWAIT,
> +					    flags & ~MSG_DONTWAIT, NULL);

Also needs READ_ONCE annotations.

> +#endif
>  	return unix_stream_read_generic(&state, true);
>  }
>
> @@ -2652,6 +2694,7 @@ static int unix_shutdown(struct socket *sock, int mode)
>
>  		int peer_mode = 0;
>
> +		other->sk_prot->unhash(other);

Here as well.

>  		if (mode&RCV_SHUTDOWN)
>  			peer_mode |= SEND_SHUTDOWN;
>  		if (mode&SEND_SHUTDOWN)

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