net: ipv6: send pkttoobig immediately if orig frag size > mtu

If conntrack defragments incoming ipv6 frags it stores largest original
frag size in ip6cb and sets ->local_df.

We must thus first test the largest original frag size vs. mtu, and not
vice versa.

Without this patch PKTTOOBIG is still generated in ip6_fragment() later
in the stack, but

1) IPSTATS_MIB_INTOOBIGERRORS won't increment
2) packet did (needlessly) traverse netfilter postrouting hook.

Fixes: fe6cc55f3a9 ("net: ip, ipv6: handle gso skbs in forwarding path")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 5 insertions, 1 deletions

diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c
index 40e7581374f7..31a38bde69ef 100644
--- a/net/ipv6/ip6_output.c
+++ b/net/ipv6/ip6_output.c
@@ -344,12 +344,16 @@ static unsigned int ip6_dst_mtu_forward(const struct dst_entry *dst)
 
 static bool ip6_pkt_too_big(const struct sk_buff *skb, unsigned int mtu)
 {
-        if (skb->len <= mtu || skb->local_df)
+        if (skb->len <= mtu)
                 return false;
 
+        /* ipv6 conntrack defrag sets max_frag_size + local_df */
         if (IP6CB(skb)->frag_max_size && IP6CB(skb)->frag_max_size > mtu)
                 return true;
 
+        if (skb->local_df)
+                return false;
+
         if (skb_is_gso(skb) && skb_gso_network_seglen(skb) <= mtu)
                 return false;