ipv4: fib: annotate races around nh->nh_saddr_genid and nh->nh_saddr

syzbot reported a data-race while accessing nh->nh_saddr_genid [1]

Add annotations, but leave the code lazy as intended.

[1]
BUG: KCSAN: data-race in fib_select_path / fib_select_path

write to 0xffff8881387166f0 of 4 bytes by task 6778 on cpu 1:
fib_info_update_nhc_saddr net/ipv4/fib_semantics.c:1334 [inline]
fib_result_prefsrc net/ipv4/fib_semantics.c:1354 [inline]
fib_select_path+0x292/0x330 net/ipv4/fib_semantics.c:2269
ip_route_output_key_hash_rcu+0x659/0x12c0 net/ipv4/route.c:2810
ip_route_output_key_hash net/ipv4/route.c:2644 [inline]
__ip_route_output_key include/net/route.h:134 [inline]
ip_route_output_flow+0xa6/0x150 net/ipv4/route.c:2872
send4+0x1f5/0x520 drivers/net/wireguard/socket.c:61
wg_socket_send_skb_to_peer+0x94/0x130 drivers/net/wireguard/socket.c:175
wg_socket_send_buffer_to_peer+0xd6/0x100 drivers/net/wireguard/socket.c:200
wg_packet_send_handshake_initiation drivers/net/wireguard/send.c:40 [inline]
wg_packet_handshake_send_worker+0x10c/0x150 drivers/net/wireguard/send.c:51
process_one_work kernel/workqueue.c:2630 [inline]
process_scheduled_works+0x5b8/0xa30 kernel/workqueue.c:2703
worker_thread+0x525/0x730 kernel/workqueue.c:2784
kthread+0x1d7/0x210 kernel/kthread.c:388
ret_from_fork+0x48/0x60 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:304

read to 0xffff8881387166f0 of 4 bytes by task 6759 on cpu 0:
fib_result_prefsrc net/ipv4/fib_semantics.c:1350 [inline]
fib_select_path+0x1cb/0x330 net/ipv4/fib_semantics.c:2269
ip_route_output_key_hash_rcu+0x659/0x12c0 net/ipv4/route.c:2810
ip_route_output_key_hash net/ipv4/route.c:2644 [inline]
__ip_route_output_key include/net/route.h:134 [inline]
ip_route_output_flow+0xa6/0x150 net/ipv4/route.c:2872
send4+0x1f5/0x520 drivers/net/wireguard/socket.c:61
wg_socket_send_skb_to_peer+0x94/0x130 drivers/net/wireguard/socket.c:175
wg_socket_send_buffer_to_peer+0xd6/0x100 drivers/net/wireguard/socket.c:200
wg_packet_send_handshake_initiation drivers/net/wireguard/send.c:40 [inline]
wg_packet_handshake_send_worker+0x10c/0x150 drivers/net/wireguard/send.c:51
process_one_work kernel/workqueue.c:2630 [inline]
process_scheduled_works+0x5b8/0xa30 kernel/workqueue.c:2703
worker_thread+0x525/0x730 kernel/workqueue.c:2784
kthread+0x1d7/0x210 kernel/kthread.c:388
ret_from_fork+0x48/0x60 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x11/0x20 arch/x86/entry/entry_64.S:304

value changed: 0x959d3217 -> 0x959d3218

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 6759 Comm: kworker/u4:15 Not tainted 6.6.0-rc4-syzkaller-00029-gcbf3a2cb #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 09/06/2023
Workqueue: wg-kex-wg1 wg_packet_handshake_send_worker

Fixes: 436c3b66 ("ipv4: Invalidate nexthop cache nh_saddr more correctly.")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20231017192304.82626-1-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/fib_semantics.c

@@ -1325,15 +1325,18 @@ __be32 fib_info_update_nhc_saddr(struct net *net, struct fib_nh_common *nhc,
 				 unsigned char scope)
 {
 	struct fib_nh *nh;
+	__be32 saddr;
 
 	if (nhc->nhc_family != AF_INET)
 		return inet_select_addr(nhc->nhc_dev, 0, scope);
 
 	nh = container_of(nhc, struct fib_nh, nh_common);
-	nh->nh_saddr = inet_select_addr(nh->fib_nh_dev, nh->fib_nh_gw4, scope);
-	nh->nh_saddr_genid = atomic_read(&net->ipv4.dev_addr_genid);
+	saddr = inet_select_addr(nh->fib_nh_dev, nh->fib_nh_gw4, scope);
 
-	return nh->nh_saddr;
+	WRITE_ONCE(nh->nh_saddr, saddr);
+	WRITE_ONCE(nh->nh_saddr_genid, atomic_read(&net->ipv4.dev_addr_genid));
+
+	return saddr;
 }
 
 __be32 fib_result_prefsrc(struct net *net, struct fib_result *res)
@@ -1347,8 +1350,9 @@ __be32 fib_result_prefsrc(struct net *net, struct fib_result *res)
 		struct fib_nh *nh;
 
 		nh = container_of(nhc, struct fib_nh, nh_common);
-		if (nh->nh_saddr_genid == atomic_read(&net->ipv4.dev_addr_genid))
-			return nh->nh_saddr;
+		if (READ_ONCE(nh->nh_saddr_genid) ==
+		    atomic_read(&net->ipv4.dev_addr_genid))
+			return READ_ONCE(nh->nh_saddr);
 	}
 
 	return fib_info_update_nhc_saddr(net, nhc, res->fi->fib_scope);