ipv6: sr: block BH in seg6_output_core() and seg6_input_core()

As explained in commit 1378817486d6 ("tipc: block BH
before using dst_cache"), net/core/dst_cache.c
helpers need to be called with BH disabled.

Disabling preemption in seg6_output_core() is not good enough,
because seg6_output_core() is called from process context,
lwtunnel_output() only uses rcu_read_lock().

We might be interrupted by a softirq, re-enter seg6_output_core()
and corrupt dst_cache data structures.

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

Apply a similar change in seg6_input_core().

Fixes: fa79581ea66c ("ipv6: sr: fix several BUGs when preemption is enabled")
Fixes: 6c8702c60b88 ("ipv6: sr: add support for SRH encapsulation and injection with lwtunnels")
Signed-off-by: Eric Dumazet <[e-mail]>
Cc: David Lebrun <[e-mail]>
Acked-by: Paolo Abeni <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/ipv6/seg6_iptunnel.c

···
 
 	slwt = seg6_lwt_lwtunnel(orig_dst->lwtstate);
 
-	preempt_disable();
+	local_bh_disable();
 	dst = dst_cache_get(&slwt->cache);
-	preempt_enable();
 
 	if (!dst) {
 		ip6_route_input(skb);
 		dst = skb_dst(skb);
 		if (!dst->error) {
-			preempt_disable();
 			dst_cache_set_ip6(&slwt->cache, dst,
 					  &ipv6_hdr(skb)->saddr);
-			preempt_enable();
 		}
 	} else {
 		skb_dst_drop(skb);
 		skb_dst_set(skb, dst);
 	}
+	local_bh_enable();
 
 	err = skb_cow_head(skb, LL_RESERVED_SPACE(dst->dev));
 	if (unlikely(err))
···
 
 	slwt = seg6_lwt_lwtunnel(orig_dst->lwtstate);
 
-	preempt_disable();
+	local_bh_disable();
 	dst = dst_cache_get(&slwt->cache);
-	preempt_enable();
+	local_bh_enable();
 
 	if (unlikely(!dst)) {
 		struct ipv6hdr *hdr = ipv6_hdr(skb);
···
 			goto drop;
 		}
 
-		preempt_disable();
+		local_bh_disable();
 		dst_cache_set_ip6(&slwt->cache, dst, &fl6.saddr);
-		preempt_enable();
+		local_bh_enable();
 	}
 
 	skb_dst_drop(skb);