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From: Kuniyuki Iwashima <kuniyu@amazon.com>
To: <leitao@debian.org>
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Subject: Re: [PATCH net-next] af_unix: Fix data races in unix_release_sock/unix_stream_sendmsg
Date: Wed, 8 May 2024 10:33:24 -0700
Message-ID: <20240508173324.53565-1-kuniyu@amazon.com>
In-Reply-To: <20240508111749.2386649-1-leitao@debian.org>
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From: Breno Leitao <leitao@debian.org>
Date: Wed,  8 May 2024 04:17:45 -0700
> A data-race condition has been identified in af_unix. In one data path,
> the write function unix_release_sock() atomically writes to
> sk->sk_shutdown using WRITE_ONCE. However, on the reader side,
> unix_stream_sendmsg() does not read it atomically. Consequently, this
> issue is causing the following KCSAN splat to occur:
> 
> 	BUG: KCSAN: data-race in unix_release_sock / unix_stream_sendmsg
> 
> 	write (marked) to 0xffff88867256ddbb of 1 bytes by task 7270 on cpu 28:
> 	unix_release_sock (net/unix/af_unix.c:640)
> 	unix_release (net/unix/af_unix.c:1050)
> 	sock_close (net/socket.c:659 net/socket.c:1421)
> 	__fput (fs/file_table.c:422)
> 	__fput_sync (fs/file_table.c:508)
> 	__se_sys_close (fs/open.c:1559 fs/open.c:1541)
> 	__x64_sys_close (fs/open.c:1541)
> 	x64_sys_call (arch/x86/entry/syscall_64.c:33)
> 	do_syscall_64 (arch/x86/entry/common.c:?)
> 	entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
> 
> 	read to 0xffff88867256ddbb of 1 bytes by task 989 on cpu 14:
> 	unix_stream_sendmsg (net/unix/af_unix.c:2273)
> 	__sock_sendmsg (net/socket.c:730 net/socket.c:745)
> 	____sys_sendmsg (net/socket.c:2584)
> 	__sys_sendmmsg (net/socket.c:2638 net/socket.c:2724)
> 	__x64_sys_sendmmsg (net/socket.c:2753 net/socket.c:2750 net/socket.c:2750)
> 	x64_sys_call (arch/x86/entry/syscall_64.c:33)
> 	do_syscall_64 (arch/x86/entry/common.c:?)
> 	entry_SYSCALL_64_after_hwframe (arch/x86/entry/entry_64.S:130)
> 
> 	value changed: 0x01 -> 0x03
> 
> The line numbers are related to commit dd5a440a31fa ("Linux 6.9-rc7").
> 
> Commit e1d09c2c2f57 ("af_unix: Fix data races around sk->sk_shutdown.")
> addressed a comparable issue in the past regarding sk->sk_shutdown.
> However, it overlooked resolving this particular data path.
> 
> To prevent potential race conditions in the future, all read accesses to
> sk->sk_shutdown in af_unix need be marked with READ_ONCE().

Let's not add READ_ONCE() if not needed.  Othwewise, someone reading
the code would assess wrongly that the value could be updated locklessly
elsewhere.

You can find all writers of sk->sk_shutdown do that update under
unix_state_lock().


> Although
> there are additional reads in other->sk_shutdown without atomic reads,
> I'm excluding them as I'm uncertain about their potential parallel
> execution.
> 
> Signed-off-by: Breno Leitao <leitao@debian.org>
> ---
>  net/unix/af_unix.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 9a6ad5974dff..74795e6d13c6 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -2270,7 +2270,7 @@ static int unix_stream_sendmsg(struct socket *sock, struct msghdr *msg,
>  			goto out_err;
>  	}
>  
> -	if (sk->sk_shutdown & SEND_SHUTDOWN)
> +	if (READ_ONCE(sk->sk_shutdown) & SEND_SHUTDOWN)
>  		goto pipe_err;
>  
>  	while (sent < len) {
> @@ -2446,7 +2446,7 @@ int __unix_dgram_recvmsg(struct sock *sk, struct msghdr *msg, size_t size,
>  		unix_state_lock(sk);
>  		/* Signal EOF on disconnected non-blocking SEQPACKET socket. */
>  		if (sk->sk_type == SOCK_SEQPACKET && err == -EAGAIN &&
> -		    (sk->sk_shutdown & RCV_SHUTDOWN))
> +		    (READ_ONCE(sk->sk_shutdown) & RCV_SHUTDOWN))

Here we locked unix_state_lock() just before accessing sk_shutdown,
so no need for READ_ONCE().


>  			err = 0;
>  		unix_state_unlock(sk);
>  		goto out;
> @@ -2566,7 +2566,7 @@ static long unix_stream_data_wait(struct sock *sk, long timeo,
>  		if (tail != last ||
>  		    (tail && tail->len != last_len) ||
>  		    sk->sk_err ||
> -		    (sk->sk_shutdown & RCV_SHUTDOWN) ||
> +		    (READ_ONCE(sk->sk_shutdown) & RCV_SHUTDOWN) ||
>  		    signal_pending(current) ||
>  		    !timeo)
>  			break;

Same here,


> @@ -2764,7 +2764,7 @@ static int unix_stream_read_generic(struct unix_stream_read_state *state,
>  			err = sock_error(sk);
>  			if (err)
>  				goto unlock;
> -			if (sk->sk_shutdown & RCV_SHUTDOWN)
> +			if (READ_ONCE(sk->sk_shutdown) & RCV_SHUTDOWN)
>  				goto unlock;
>  
>  			unix_state_unlock(sk);

and here.

Could you update the changelog and repost v2 for unix_stream_sendmsg()
targetting net tree with this Fixes tag ?

  Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")

Thanks!

> -- 
> 2.43.0