ipv6: ioam: block BH from ioam6_output()

As explained in commit 1378817 ("tipc: block BH
before using dst_cache"), net/core/dst_cache.c
helpers need to be called with BH disabled.

Disabling preemption in ioam6_output() is not good enough,
because ioam6_output() is called from process context,
lwtunnel_output() only uses rcu_read_lock().

We might be interrupted by a softirq, re-enter ioam6_output()
and corrupt dst_cache data structures.

Fix the race by using local_bh_disable() instead of
preempt_disable().

Fixes: 8cb3bf8 ("ipv6: ioam: Add support for the ip6ip6 encapsulation")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Justin Iurman <justin.iurman@uliege.be>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Link: https://lore.kernel.org/r/20240531132636.2637995-2-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv6/ioam6_iptunnel.c

@@ -351,9 +351,9 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 		goto drop;
 
 	if (!ipv6_addr_equal(&orig_daddr, &ipv6_hdr(skb)->daddr)) {
-		preempt_disable();
+		local_bh_disable();
 		dst = dst_cache_get(&ilwt->cache);
-		preempt_enable();
+		local_bh_enable();
 
 		if (unlikely(!dst)) {
 			struct ipv6hdr *hdr = ipv6_hdr(skb);
@@ -373,9 +373,9 @@ static int ioam6_output(struct net *net, struct sock *sk, struct sk_buff *skb)
 				goto drop;
 			}
 
-			preempt_disable();
+			local_bh_disable();
 			dst_cache_set_ip6(&ilwt->cache, dst, &fl6.saddr);
-			preempt_enable();
+			local_bh_enable();
 		}
 
 		skb_dst_drop(skb);