netlink: annotate data races around nlk->bound

While existing code is correct, KCSAN is reporting
a data-race in netlink_insert / netlink_sendmsg [1]

It is correct to read nlk->bound without a lock, as netlink_autobind()
will acquire all needed locks.

[1]
BUG: KCSAN: data-race in netlink_insert / netlink_sendmsg

write to 0xffff8881031c8b30 of 1 bytes by task 18752 on cpu 0:
 netlink_insert+0x5cc/0x7f0 net/netlink/af_netlink.c:597
 netlink_autobind+0xa9/0x150 net/netlink/af_netlink.c:842
 netlink_sendmsg+0x479/0x7c0 net/netlink/af_netlink.c:1892
 sock_sendmsg_nosec net/socket.c:703 [inline]
 sock_sendmsg net/socket.c:723 [inline]
 ____sys_sendmsg+0x360/0x4d0 net/socket.c:2392
 ___sys_sendmsg net/socket.c:2446 [inline]
 __sys_sendmsg+0x1ed/0x270 net/socket.c:2475
 __do_sys_sendmsg net/socket.c:2484 [inline]
 __se_sys_sendmsg net/socket.c:2482 [inline]
 __x64_sys_sendmsg+0x42/0x50 net/socket.c:2482
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff8881031c8b30 of 1 bytes by task 18751 on cpu 1:
 netlink_sendmsg+0x270/0x7c0 net/netlink/af_netlink.c:1891
 sock_sendmsg_nosec net/socket.c:703 [inline]
 sock_sendmsg net/socket.c:723 [inline]
 __sys_sendto+0x2a8/0x370 net/socket.c:2019
 __do_sys_sendto net/socket.c:2031 [inline]
 __se_sys_sendto net/socket.c:2027 [inline]
 __x64_sys_sendto+0x74/0x90 net/socket.c:2027
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x00 -> 0x01

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 18751 Comm: syz-executor.0 Not tainted 5.14.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

Fixes: da314c99 ("netlink: Replace rhash_portid with bound")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -594,7 +594,10 @@ static int netlink_insert(struct sock *sk, u32 portid)
 
 	/* We need to ensure that the socket is hashed and visible. */
 	smp_wmb();
-	nlk_sk(sk)->bound = portid;
+	/* Paired with lockless reads from netlink_bind(),
+	 * netlink_connect() and netlink_sendmsg().
+	 */
+	WRITE_ONCE(nlk_sk(sk)->bound, portid);
 
 err:
 	release_sock(sk);
@@ -1012,7 +1015,8 @@ static int netlink_bind(struct socket *sock, struct sockaddr *addr,
 	if (nlk->ngroups < BITS_PER_LONG)
 		groups &= (1UL << nlk->ngroups) - 1;
 
-	bound = nlk->bound;
+	/* Paired with WRITE_ONCE() in netlink_insert() */
+	bound = READ_ONCE(nlk->bound);
 	if (bound) {
 		/* Ensure nlk->portid is up-to-date. */
 		smp_rmb();
@@ -1098,8 +1102,9 @@ static int netlink_connect(struct socket *sock, struct sockaddr *addr,
 
 	/* No need for barriers here as we return to user-space without
 	 * using any of the bound attributes.
+	 * Paired with WRITE_ONCE() in netlink_insert().
 	 */
-	if (!nlk->bound)
+	if (!READ_ONCE(nlk->bound))
 		err = netlink_autobind(sock);
 
 	if (err == 0) {
@@ -1888,7 +1893,8 @@ static int netlink_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
 		dst_group = nlk->dst_group;
 	}
 
-	if (!nlk->bound) {
+	/* Paired with WRITE_ONCE() in netlink_insert() */
+	if (!READ_ONCE(nlk->bound)) {
 		err = netlink_autobind(sock);
 		if (err)
 			goto out;