netlink: annotate data-races around sk->sk_err

syzbot caught another data-race in netlink when
setting sk->sk_err.

Annotate all of them for good measure.

BUG: KCSAN: data-race in netlink_recvmsg / netlink_recvmsg

write to 0xffff8881613bb220 of 4 bytes by task 28147 on cpu 0:
netlink_recvmsg+0x448/0x780 net/netlink/af_netlink.c:1994
sock_recvmsg_nosec net/socket.c:1027 [inline]
sock_recvmsg net/socket.c:1049 [inline]
__sys_recvfrom+0x1f4/0x2e0 net/socket.c:2229
__do_sys_recvfrom net/socket.c:2247 [inline]
__se_sys_recvfrom net/socket.c:2243 [inline]
__x64_sys_recvfrom+0x78/0x90 net/socket.c:2243
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

write to 0xffff8881613bb220 of 4 bytes by task 28146 on cpu 1:
netlink_recvmsg+0x448/0x780 net/netlink/af_netlink.c:1994
sock_recvmsg_nosec net/socket.c:1027 [inline]
sock_recvmsg net/socket.c:1049 [inline]
__sys_recvfrom+0x1f4/0x2e0 net/socket.c:2229
__do_sys_recvfrom net/socket.c:2247 [inline]
__se_sys_recvfrom net/socket.c:2243 [inline]
__x64_sys_recvfrom+0x78/0x90 net/socket.c:2243
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x41/0xc0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

value changed: 0x00000000 -> 0x00000016

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 28146 Comm: syz-executor.0 Not tainted 6.6.0-rc3-syzkaller-00055-g9ed22ae6be81 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/06/2023

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/20231003183455.3410550-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/netlink/af_netlink.c

@@ -352,7 +352,7 @@ static void netlink_overrun(struct sock *sk)
 	if (!nlk_test_bit(RECV_NO_ENOBUFS, sk)) {
 		if (!test_and_set_bit(NETLINK_S_CONGESTED,
 				      &nlk_sk(sk)->state)) {
-			sk->sk_err = ENOBUFS;
+			WRITE_ONCE(sk->sk_err, ENOBUFS);
 			sk_error_report(sk);
 		}
 	}
@@ -1605,7 +1605,7 @@ static int do_one_set_err(struct sock *sk, struct netlink_set_err_data *p)
 		goto out;
 	}
 
-	sk->sk_err = p->code;
+	WRITE_ONCE(sk->sk_err, p->code);
 	sk_error_report(sk);
 out:
 	return ret;
@@ -1991,7 +1991,7 @@ static int netlink_recvmsg(struct socket *sock, struct msghdr *msg, size_t len,
 	    atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf / 2) {
 		ret = netlink_dump(sk);
 		if (ret) {
-			sk->sk_err = -ret;
+			WRITE_ONCE(sk->sk_err, -ret);
 			sk_error_report(sk);
 		}
 	}
@@ -2511,7 +2511,7 @@ void netlink_ack(struct sk_buff *in_skb, struct nlmsghdr *nlh, int err,
 err_bad_put:
 	nlmsg_free(skb);
 err_skb:
-	NETLINK_CB(in_skb).sk->sk_err = ENOBUFS;
+	WRITE_ONCE(NETLINK_CB(in_skb).sk->sk_err, ENOBUFS);
 	sk_error_report(NETLINK_CB(in_skb).sk);
 }
 EXPORT_SYMBOL(netlink_ack);