net: annotate races around sk->sk_bound_dev_if

UDP sendmsg() is lockless, and reads sk->sk_bound_dev_if while
this field can be changed by another thread.

Adds minimal annotations to avoid KCSAN splats for UDP.
Following patches will add more annotations to potential lockless readers.

BUG: KCSAN: data-race in __ip6_datagram_connect / udpv6_sendmsg

write to 0xffff888136d47a94 of 4 bytes by task 7681 on cpu 0:
 __ip6_datagram_connect+0x6e2/0x930 net/ipv6/datagram.c:221
 ip6_datagram_connect+0x2a/0x40 net/ipv6/datagram.c:272
 inet_dgram_connect+0x107/0x190 net/ipv4/af_inet.c:576
 __sys_connect_file net/socket.c:1900 [inline]
 __sys_connect+0x197/0x1b0 net/socket.c:1917
 __do_sys_connect net/socket.c:1927 [inline]
 __se_sys_connect net/socket.c:1924 [inline]
 __x64_sys_connect+0x3d/0x50 net/socket.c:1924
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x2b/0x50 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888136d47a94 of 4 bytes by task 7670 on cpu 1:
 udpv6_sendmsg+0xc60/0x16e0 net/ipv6/udp.c:1436
 inet6_sendmsg+0x5f/0x80 net/ipv6/af_inet6.c:652
 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+0x2b/0x50 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x00000000 -> 0xffffff9b

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 7670 Comm: syz-executor.3 Tainted: G        W         5.18.0-rc1-syzkaller-dirty #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

I chose to not add Fixes: tag because race has minor consequences
and stable teams busy enough.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip.h

@@ -93,7 +93,7 @@ static inline void ipcm_init_sk(struct ipcm_cookie *ipcm,
 
 	ipcm->sockc.mark = inet->sk.sk_mark;
 	ipcm->sockc.tsflags = inet->sk.sk_tsflags;
-	ipcm->oif = inet->sk.sk_bound_dev_if;
+	ipcm->oif = READ_ONCE(inet->sk.sk_bound_dev_if);
 	ipcm->addr = inet->inet_saddr;
 }
 

include/net/sock.h

@@ -2875,13 +2875,14 @@ static inline void sk_pacing_shift_update(struct sock *sk, int val)
  */
 static inline bool sk_dev_equal_l3scope(struct sock *sk, int dif)
 {
+	int bound_dev_if = READ_ONCE(sk->sk_bound_dev_if);
 	int mdif;
 
-	if (!sk->sk_bound_dev_if || sk->sk_bound_dev_if == dif)
+	if (!bound_dev_if || bound_dev_if == dif)
 		return true;
 
 	mdif = l3mdev_master_ifindex_by_index(sock_net(sk), dif);
-	if (mdif && mdif == sk->sk_bound_dev_if)
+	if (mdif && mdif == bound_dev_if)
 		return true;
 
 	return false;

net/ipv6/datagram.c

@@ -218,11 +218,11 @@ int __ip6_datagram_connect(struct sock *sk, struct sockaddr *uaddr,
 				err = -EINVAL;
 				goto out;
 			}
-			sk->sk_bound_dev_if = usin->sin6_scope_id;
+			WRITE_ONCE(sk->sk_bound_dev_if, usin->sin6_scope_id);
 		}
 
 		if (!sk->sk_bound_dev_if && (addr_type & IPV6_ADDR_MULTICAST))
-			sk->sk_bound_dev_if = np->mcast_oif;
+			WRITE_ONCE(sk->sk_bound_dev_if, np->mcast_oif);
 
 		/* Connect to link-local address requires an interface */
 		if (!sk->sk_bound_dev_if) {
@@ -798,7 +798,7 @@ int ip6_datagram_send_ctl(struct net *net, struct sock *sk,
 			if (src_idx) {
 				if (fl6->flowi6_oif &&
 				    src_idx != fl6->flowi6_oif &&
-				    (sk->sk_bound_dev_if != fl6->flowi6_oif ||
+				    (READ_ONCE(sk->sk_bound_dev_if) != fl6->flowi6_oif ||
 				     !sk_dev_equal_l3scope(sk, src_idx)))
 					return -EINVAL;
 				fl6->flowi6_oif = src_idx;

net/ipv6/udp.c

@@ -105,7 +105,7 @@ static int compute_score(struct sock *sk, struct net *net,
 			 const struct in6_addr *daddr, unsigned short hnum,
 			 int dif, int sdif)
 {
-	int score;
+	int bound_dev_if, score;
 	struct inet_sock *inet;
 	bool dev_match;
 
@@ -132,10 +132,11 @@ static int compute_score(struct sock *sk, struct net *net,
 		score++;
 	}
 
-	dev_match = udp_sk_bound_dev_eq(net, sk->sk_bound_dev_if, dif, sdif);
+	bound_dev_if = READ_ONCE(sk->sk_bound_dev_if);
+	dev_match = udp_sk_bound_dev_eq(net, bound_dev_if, dif, sdif);
 	if (!dev_match)
 		return -1;
-	if (sk->sk_bound_dev_if)
+	if (bound_dev_if)
 		score++;
 
 	if (READ_ONCE(sk->sk_incoming_cpu) == raw_smp_processor_id())
@@ -789,7 +790,7 @@ static bool __udp_v6_is_mcast_sock(struct net *net, struct sock *sk,
 	    (inet->inet_dport && inet->inet_dport != rmt_port) ||
 	    (!ipv6_addr_any(&sk->sk_v6_daddr) &&
 		    !ipv6_addr_equal(&sk->sk_v6_daddr, rmt_addr)) ||
-	    !udp_sk_bound_dev_eq(net, sk->sk_bound_dev_if, dif, sdif) ||
+	    !udp_sk_bound_dev_eq(net, READ_ONCE(sk->sk_bound_dev_if), dif, sdif) ||
 	    (!ipv6_addr_any(&sk->sk_v6_rcv_saddr) &&
 		    !ipv6_addr_equal(&sk->sk_v6_rcv_saddr, loc_addr)))
 		return false;
@@ -1433,7 +1434,7 @@ int udpv6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	}
 
 	if (!fl6->flowi6_oif)
-		fl6->flowi6_oif = sk->sk_bound_dev_if;
+		fl6->flowi6_oif = READ_ONCE(sk->sk_bound_dev_if);
 
 	if (!fl6->flowi6_oif)
 		fl6->flowi6_oif = np->sticky_pktinfo.ipi6_ifindex;