ipv6: annotate data-race in ndisc_router_discovery()

syzbot found that ndisc_router_discovery() could read and write
in6_dev->ra_mtu without holding a lock [1]

This looks fine, IFLA_INET6_RA_MTU is best effort.

Add READ_ONCE()/WRITE_ONCE() to document the race.

Note that we might also reject illegal MTU values
(mtu < IPV6_MIN_MTU || mtu > skb->dev->mtu) in a future patch.

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

read to 0xffff888119809c20 of 4 bytes by task 25817 on cpu 1:
  ndisc_router_discovery+0x151d/0x1c90 net/ipv6/ndisc.c:1558
  ndisc_rcv+0x2ad/0x3d0 net/ipv6/ndisc.c:1841
  icmpv6_rcv+0xe5a/0x12f0 net/ipv6/icmp.c:989
  ip6_protocol_deliver_rcu+0xb2a/0x10d0 net/ipv6/ip6_input.c:438
  ip6_input_finish+0xf0/0x1d0 net/ipv6/ip6_input.c:489
  NF_HOOK include/linux/netfilter.h:318 [inline]
  ip6_input+0x5e/0x140 net/ipv6/ip6_input.c:500
  ip6_mc_input+0x27c/0x470 net/ipv6/ip6_input.c:590
  dst_input include/net/dst.h:474 [inline]
  ip6_rcv_finish+0x336/0x340 net/ipv6/ip6_input.c:79
...

write to 0xffff888119809c20 of 4 bytes by task 25816 on cpu 0:
  ndisc_router_discovery+0x155a/0x1c90 net/ipv6/ndisc.c:1559
  ndisc_rcv+0x2ad/0x3d0 net/ipv6/ndisc.c:1841
  icmpv6_rcv+0xe5a/0x12f0 net/ipv6/icmp.c:989
  ip6_protocol_deliver_rcu+0xb2a/0x10d0 net/ipv6/ip6_input.c:438
  ip6_input_finish+0xf0/0x1d0 net/ipv6/ip6_input.c:489
  NF_HOOK include/linux/netfilter.h:318 [inline]
  ip6_input+0x5e/0x140 net/ipv6/ip6_input.c:500
  ip6_mc_input+0x27c/0x470 net/ipv6/ip6_input.c:590
  dst_input include/net/dst.h:474 [inline]
  ip6_rcv_finish+0x336/0x340 net/ipv6/ip6_input.c:79
...

value changed: 0x00000000 -> 0xe5400659

Fixes: 49b99da ("ipv6: add IFLA_INET6_RA_MTU to expose mtu value")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Rocco Yue <rocco.yue@mediatek.com>
Link: https://patch.msgid.link/20260118152941.2563857-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/ipv6/ndisc.c

@@ -1555,8 +1555,8 @@ static enum skb_drop_reason ndisc_router_discovery(struct sk_buff *skb)
 		memcpy(&n, ((u8 *)(ndopts.nd_opts_mtu+1))+2, sizeof(mtu));
 		mtu = ntohl(n);
 
-		if (in6_dev->ra_mtu != mtu) {
-			in6_dev->ra_mtu = mtu;
+		if (READ_ONCE(in6_dev->ra_mtu) != mtu) {
+			WRITE_ONCE(in6_dev->ra_mtu, mtu);
 			send_ifinfo_notify = true;
 		}