net: silence KCSAN warnings around sk_add_backlog() calls

sk_add_backlog() callers usually read sk->sk_rcvbuf without
owning the socket lock. This means sk_rcvbuf value can
be changed by other cpus, and KCSAN complains.

Add READ_ONCE() annotations to document the lockless nature
of these reads.

Note that writes over sk_rcvbuf should also use WRITE_ONCE(),
but this will be done in separate patches to ease stable
backports (if we decide this is relevant for stable trees).

BUG: KCSAN: data-race in tcp_add_backlog / tcp_recvmsg

write to 0xffff88812ab369f8 of 8 bytes by interrupt on cpu 1:
__sk_add_backlog include/net/sock.h:902 [inline]
sk_add_backlog include/net/sock.h:933 [inline]
tcp_add_backlog+0x45a/0xcc0 net/ipv4/tcp_ipv4.c:1737
tcp_v4_rcv+0x1aba/0x1bf0 net/ipv4/tcp_ipv4.c:1925
ip_protocol_deliver_rcu+0x51/0x470 net/ipv4/ip_input.c:204
ip_local_deliver_finish+0x110/0x140 net/ipv4/ip_input.c:231
NF_HOOK include/linux/netfilter.h:305 [inline]
NF_HOOK include/linux/netfilter.h:299 [inline]
ip_local_deliver+0x133/0x210 net/ipv4/ip_input.c:252
dst_input include/net/dst.h:442 [inline]
ip_rcv_finish+0x121/0x160 net/ipv4/ip_input.c:413
NF_HOOK include/linux/netfilter.h:305 [inline]
NF_HOOK include/linux/netfilter.h:299 [inline]
ip_rcv+0x18f/0x1a0 net/ipv4/ip_input.c:523
__netif_receive_skb_one_core+0xa7/0xe0 net/core/dev.c:5004
__netif_receive_skb+0x37/0xf0 net/core/dev.c:5118
netif_receive_skb_internal+0x59/0x190 net/core/dev.c:5208
napi_skb_finish net/core/dev.c:5671 [inline]
napi_gro_receive+0x28f/0x330 net/core/dev.c:5704
receive_buf+0x284/0x30b0 drivers/net/virtio_net.c:1061
virtnet_receive drivers/net/virtio_net.c:1323 [inline]
virtnet_poll+0x436/0x7d0 drivers/net/virtio_net.c:1428
napi_poll net/core/dev.c:6352 [inline]
net_rx_action+0x3ae/0xa50 net/core/dev.c:6418

read to 0xffff88812ab369f8 of 8 bytes by task 7271 on cpu 0:
tcp_recvmsg+0x470/0x1a30 net/ipv4/tcp.c:2047
inet_recvmsg+0xbb/0x250 net/ipv4/af_inet.c:838
sock_recvmsg_nosec net/socket.c:871 [inline]
sock_recvmsg net/socket.c:889 [inline]
sock_recvmsg+0x92/0xb0 net/socket.c:885
sock_read_iter+0x15f/0x1e0 net/socket.c:967
call_read_iter include/linux/fs.h:1864 [inline]
new_sync_read+0x389/0x4f0 fs/read_write.c:414
__vfs_read+0xb1/0xc0 fs/read_write.c:427
vfs_read fs/read_write.c:461 [inline]
vfs_read+0x143/0x2c0 fs/read_write.c:446
ksys_read+0xd5/0x1b0 fs/read_write.c:587
__do_sys_read fs/read_write.c:597 [inline]
__se_sys_read fs/read_write.c:595 [inline]
__x64_sys_read+0x4c/0x60 fs/read_write.c:595
do_syscall_64+0xcf/0x2f0 arch/x86/entry/common.c:296
entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 7271 Comm: syz-fuzzer Not tainted 5.3.0+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

Signed-off-by: Eric Dumazet <[e-mail]>
Reported-by: syzbot <[e-mail]>
Signed-off-by: Jakub Kicinski <[e-mail]>

net/core/sock.c

···
 		rc = sk_backlog_rcv(sk, skb);
 
 		mutex_release(&sk->sk_lock.dep_map, 1, _RET_IP_);
-	} else if (sk_add_backlog(sk, skb, sk->sk_rcvbuf)) {
+	} else if (sk_add_backlog(sk, skb, READ_ONCE(sk->sk_rcvbuf))) {
 		bh_unlock_sock(sk);
 		atomic_inc(&sk->sk_drops);
 		goto discard_and_relse;

net/ipv4/tcp_ipv4.c

···
 
 bool tcp_add_backlog(struct sock *sk, struct sk_buff *skb)
 {
-	u32 limit = sk->sk_rcvbuf + sk->sk_sndbuf;
+	u32 limit = READ_ONCE(sk->sk_rcvbuf) + READ_ONCE(sk->sk_sndbuf);
 	struct skb_shared_info *shinfo;
 	const struct tcphdr *th;
 	struct tcphdr *thtail;

net/llc/llc_conn.c

···
 	else {
 		dprintk("%s: adding to backlog...\n", __func__);
 		llc_set_backlog_type(skb, LLC_PACKET);
-		if (sk_add_backlog(sk, skb, sk->sk_rcvbuf))
+		if (sk_add_backlog(sk, skb, READ_ONCE(sk->sk_rcvbuf)))
 			goto drop_unlock;
 	}
 out:

net/sctp/input.c

···
 		bh_lock_sock(sk);
 
 		if (sock_owned_by_user(sk) || !sctp_newsk_ready(sk)) {
-			if (sk_add_backlog(sk, skb, sk->sk_rcvbuf))
+			if (sk_add_backlog(sk, skb, READ_ONCE(sk->sk_rcvbuf)))
 				sctp_chunk_free(chunk);
 			else
 				backloged = 1;
···
 			return 0;
 	} else {
 		if (!sctp_newsk_ready(sk)) {
-			if (!sk_add_backlog(sk, skb, sk->sk_rcvbuf))
+			if (!sk_add_backlog(sk, skb, READ_ONCE(sk->sk_rcvbuf)))
 				return 0;
 			sctp_chunk_free(chunk);
 		} else {
···
 	struct sctp_ep_common *rcvr = chunk->rcvr;
 	int ret;
 
-	ret = sk_add_backlog(sk, skb, sk->sk_rcvbuf);
+	ret = sk_add_backlog(sk, skb, READ_ONCE(sk->sk_rcvbuf));
 	if (!ret) {
 		/* Hold the assoc/ep while hanging on the backlog queue.
 		 * This way, we know structures we need will not disappear

net/tipc/socket.c

···
 	struct tipc_msg *hdr = buf_msg(skb);
 
 	if (unlikely(msg_in_group(hdr)))
-		return sk->sk_rcvbuf;
+		return READ_ONCE(sk->sk_rcvbuf);
 
 	if (unlikely(!msg_connected(hdr)))
-		return sk->sk_rcvbuf << msg_importance(hdr);
+		return READ_ONCE(sk->sk_rcvbuf) << msg_importance(hdr);
 
 	if (likely(tsk->peer_caps & TIPC_BLOCK_FLOWCTL))
-		return sk->sk_rcvbuf;
+		return READ_ONCE(sk->sk_rcvbuf);
 
 	return FLOWCTL_MSG_LIM;
 }

net/x25/x25_dev.c

···
 		if (!sock_owned_by_user(sk)) {
 			queued = x25_process_rx_frame(sk, skb);
 		} else {
-			queued = !sk_add_backlog(sk, skb, sk->sk_rcvbuf);
+			queued = !sk_add_backlog(sk, skb, READ_ONCE(sk->sk_rcvbuf));
 		}
 		bh_unlock_sock(sk);
 		sock_put(sk);