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From: Breno Leitao <leitao@debian.org>
To: kuniyu@amazon.com,
	"David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>,
	Paolo Abeni <pabeni@redhat.com>
Cc: netdev@vger.kernel.org,
	horms@kernel.org,
	paulmck@kernel.org,
	David Howells <dhowells@redhat.com>,
	Alexander Mikhalitsyn <alexander@mihalicyn.com>,
	John Fastabend <john.fastabend@gmail.com>,
	Daan De Meyer <daan.j.demeyer@gmail.com>,
	linux-kernel@vger.kernel.org (open list)
Subject: [PATCH net v2] af_unix: Fix data races in unix_release_sock/unix_stream_sendmsg
Date: Thu,  9 May 2024 01:14:46 -0700
Message-ID: <20240509081459.2807828-1-leitao@debian.org>
Precedence: bulk
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit

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.
This patch only offending unix_stream_sendmsg() function, since the
other reads seem to be protected by unix_state_lock() as discussed in
Link: https://lore.kernel.org/all/20240508173324.53565-1-kuniyu@amazon.com/

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Breno Leitao <leitao@debian.org>
---
Changelog:

v2:
	* Only fix the usecase reported by KCSAN
	* Targeting net instead of net-next
---
 net/unix/af_unix.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index dc1651541723..fa906ec5e657 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -2224,7 +2224,7 @@ 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) {
-- 
2.43.0