netdev - Re: [PATCH v1 net] af_unix: Annotate data-race around unix

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Date: Wed, 22 May 2024 10:52:19 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Kuniyuki Iwashima <kuniyu@...zon.com>, "David S. Miller"
	 <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	 <kuba@...nel.org>
Cc: Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org,
 syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH v1 net] af_unix: Annotate data-race around
 unix_sk(sk)->addr.

On Sat, 2024-05-18 at 09:01 +0900, Kuniyuki Iwashima wrote:
> Once unix_sk(sk)->addr is assigned under net->unx.table.locks,
> *(unix_sk(sk)->addr) and unix_sk(sk)->path are fully set up, and
> unix_sk(sk)->addr is never changed.
> 
> unix_getname() and unix_copy_addr() access the two fields locklessly,
> and commit ae3b564179bf ("missing barriers in some of unix_sock ->addr
> and ->path accesses") added smp_store_release() and smp_load_acquire()
> pairs.
> 
> In other functions, we still read unix_sk(sk)->addr locklessly to check
> if the socket is bound, and KCSAN complains about it.  [0]
> 
> Given these functions have no dependency for *(unix_sk(sk)->addr) and
> unix_sk(sk)->path, READ_ONCE() is enough to annotate the data-race.
> 
> [0]:
> BUG: KCSAN: data-race in unix_bind / unix_listen
> 
> write (marked) to 0xffff88805f8d1840 of 8 bytes by task 13723 on cpu 0:
>  __unix_set_addr_hash net/unix/af_unix.c:329 [inline]
>  unix_bind_bsd net/unix/af_unix.c:1241 [inline]
>  unix_bind+0x881/0x1000 net/unix/af_unix.c:1319
>  __sys_bind+0x194/0x1e0 net/socket.c:1847
>  __do_sys_bind net/socket.c:1858 [inline]
>  __se_sys_bind net/socket.c:1856 [inline]
>  __x64_sys_bind+0x40/0x50 net/socket.c:1856
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0x4f/0x110 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x46/0x4e
> 
> read to 0xffff88805f8d1840 of 8 bytes by task 13724 on cpu 1:
>  unix_listen+0x72/0x180 net/unix/af_unix.c:734
>  __sys_listen+0xdc/0x160 net/socket.c:1881
>  __do_sys_listen net/socket.c:1890 [inline]
>  __se_sys_listen net/socket.c:1888 [inline]
>  __x64_sys_listen+0x2e/0x40 net/socket.c:1888
>  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
>  do_syscall_64+0x4f/0x110 arch/x86/entry/common.c:83
>  entry_SYSCALL_64_after_hwframe+0x46/0x4e
> 
> value changed: 0x0000000000000000 -> 0xffff88807b5b1b40
> 
> Reported by Kernel Concurrency Sanitizer on:
> CPU: 1 PID: 13724 Comm: syz-executor.4 Not tainted 6.8.0-12822-gcd51db110a7e #12
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.0-0-gd239552ce722-prebuilt.qemu.org 04/01/2014
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Reported-by: syzkaller <syzkaller@...glegroups.com>
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>
> ---
>  net/unix/af_unix.c | 10 ++++++----
>  1 file changed, 6 insertions(+), 4 deletions(-)
> 
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index ca101690e740..92a88ac070ca 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -731,7 +731,7 @@ static int unix_listen(struct socket *sock, int backlog)
>  	if (sock->type != SOCK_STREAM && sock->type != SOCK_SEQPACKET)
>  		goto out;	/* Only stream/seqpacket sockets accept */
>  	err = -EINVAL;
> -	if (!u->addr)
> +	if (!READ_ONCE(u->addr))
>  		goto out;	/* No listens on an unbound socket */
>  	unix_state_lock(sk);
>  	if (sk->sk_state != TCP_CLOSE && sk->sk_state != TCP_LISTEN)
> @@ -1369,7 +1369,7 @@ static int unix_dgram_connect(struct socket *sock, struct sockaddr *addr,
>  
>  		if ((test_bit(SOCK_PASSCRED, &sock->flags) ||
>  		     test_bit(SOCK_PASSPIDFD, &sock->flags)) &&
> -		    !unix_sk(sk)->addr) {
> +		    !READ_ONCE(unix_sk(sk)->addr)) {
>  			err = unix_autobind(sk);
>  			if (err)
>  				goto out;
> @@ -1481,7 +1481,8 @@ static int unix_stream_connect(struct socket *sock, struct sockaddr *uaddr,
>  		goto out;
>  
>  	if ((test_bit(SOCK_PASSCRED, &sock->flags) ||
> -	     test_bit(SOCK_PASSPIDFD, &sock->flags)) && !u->addr) {
> +	     test_bit(SOCK_PASSPIDFD, &sock->flags)) &&
> +	    !READ_ONCE(u->addr)) {
>  		err = unix_autobind(sk);
>  		if (err)
>  			goto out;
> @@ -1951,7 +1952,8 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
>  	}
>  
>  	if ((test_bit(SOCK_PASSCRED, &sock->flags) ||
> -	     test_bit(SOCK_PASSPIDFD, &sock->flags)) && !u->addr) {
> +	     test_bit(SOCK_PASSPIDFD, &sock->flags)) &&
> +	    !READ_ONCE(u->addr)) {
>  		err = unix_autobind(sk);
>  		if (err)
>  			goto out;

There are a few other places where ->addr is accessed lockless (under
the bindlock, but prior to acquiring the table spinlock, e.g.
unix_bind_* and unix_autobind. The latter is suspect as it's called
right after the touched code. Why are such spots not relevant here?

Also the  newu->addr/otheru->addr handling in unix_stream_connect()
looks suspect.

Thanks,

Paolo