net-timestamp: convert sk->sk_tskey to atomic_t

UDP sendmsg() can be lockless, this is causing all kinds
of data races.

This patch converts sk->sk_tskey to remove one of these races.

BUG: KCSAN: data-race in __ip_append_data / __ip_append_data

read to 0xffff8881035d4b6c of 4 bytes by task 8877 on cpu 1:
 __ip_append_data+0x1c1/0x1de0 net/ipv4/ip_output.c:994
 ip_make_skb+0x13f/0x2d0 net/ipv4/ip_output.c:1636
 udp_sendmsg+0x12bd/0x14c0 net/ipv4/udp.c:1249
 inet_sendmsg+0x5f/0x80 net/ipv4/af_inet.c:819
 sock_sendmsg_nosec net/socket.c:705 [inline]
 sock_sendmsg net/socket.c:725 [inline]
 ____sys_sendmsg+0x39a/0x510 net/socket.c:2413
 ___sys_sendmsg net/socket.c:2467 [inline]
 __sys_sendmmsg+0x267/0x4c0 net/socket.c:2553
 __do_sys_sendmmsg net/socket.c:2582 [inline]
 __se_sys_sendmmsg net/socket.c:2579 [inline]
 __x64_sys_sendmmsg+0x53/0x60 net/socket.c:2579
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

wr...

include/net/sock.h

@@ -507,7 +507,7 @@ struct sock {
 #endif
 	u16			sk_tsflags;
 	u8			sk_shutdown;
-	u32			sk_tskey;
+	atomic_t		sk_tskey;
 	atomic_t		sk_zckey;
 
 	u8			sk_clockid;
@@ -2667,7 +2667,7 @@ static inline void _sock_tx_timestamp(struct sock *sk, __u16 tsflags,
 		__sock_tx_timestamp(tsflags, tx_flags);
 		if (tsflags & SOF_TIMESTAMPING_OPT_ID && tskey &&
 		    tsflags & SOF_TIMESTAMPING_TX_RECORD_MASK)
-			*tskey = sk->sk_tskey++;
+			*tskey = atomic_inc_return(&sk->sk_tskey) - 1;
 	}
 	if (unlikely(sock_flag(sk, SOCK_WIFI_STATUS)))
 		*tx_flags |= SKBTX_WIFI_STATUS;

net/can/j1939/transport.c

@@ -2006,7 +2006,7 @@ struct j1939_session *j1939_tp_send(struct j1939_priv *priv,
 		/* set the end-packet for broadcast */
 		session->pkt.last = session->pkt.total;
 
-	skcb->tskey = session->sk->sk_tskey++;
+	skcb->tskey = atomic_inc_return(&session->sk->sk_tskey) - 1;
 	session->tskey = skcb->tskey;
 
 	return session;

net/core/skbuff.c

@@ -4730,7 +4730,7 @@ static void __skb_complete_tx_timestamp(struct sk_buff *skb,
 	if (sk->sk_tsflags & SOF_TIMESTAMPING_OPT_ID) {
 		serr->ee.ee_data = skb_shinfo(skb)->tskey;
 		if (sk_is_tcp(sk))
-			serr->ee.ee_data -= sk->sk_tskey;
+			serr->ee.ee_data -= atomic_read(&sk->sk_tskey);
 	}
 
 	err = sock_queue_err_skb(sk, skb);

net/core/sock.c

@@ -879,9 +879,9 @@ int sock_set_timestamping(struct sock *sk, int optname,
 			if ((1 << sk->sk_state) &
 			    (TCPF_CLOSE | TCPF_LISTEN))
 				return -EINVAL;
-			sk->sk_tskey = tcp_sk(sk)->snd_una;
+			atomic_set(&sk->sk_tskey, tcp_sk(sk)->snd_una);
 		} else {
-			sk->sk_tskey = 0;
+			atomic_set(&sk->sk_tskey, 0);
 		}
 	}
 

net/ipv4/ip_output.c

@@ -991,7 +991,7 @@ static int __ip_append_data(struct sock *sk,
 
 	if (cork->tx_flags & SKBTX_ANY_SW_TSTAMP &&
 	    sk->sk_tsflags & SOF_TIMESTAMPING_OPT_ID)
-		tskey = sk->sk_tskey++;
+		tskey = atomic_inc_return(&sk->sk_tskey) - 1;
 
 	hh_len = LL_RESERVED_SPACE(rt->dst.dev);
 

net/ipv6/ip6_output.c

@@ -1465,7 +1465,7 @@ static int __ip6_append_data(struct sock *sk,
 
 	if (cork->tx_flags & SKBTX_ANY_SW_TSTAMP &&
 	    sk->sk_tsflags & SOF_TIMESTAMPING_OPT_ID)
-		tskey = sk->sk_tskey++;
+		tskey = atomic_inc_return(&sk->sk_tskey) - 1;
 
 	hh_len = LL_RESERVED_SPACE(rt->dst.dev);