ip: reject too-big defragmented DF-skb when forwarding

Send icmp pmtu error if we find that the largest fragment of df-skb
exceeded the output path mtu.

The ip output path will still catch this later on but we can avoid the
forward/postrouting hook traversal by rejecting right away.

This is what ipv6 already does.

Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/ip_forward.c b/net/ipv4/ip_forward.c
index 3674484..2d3aa40 100644 (file)
--- a/net/ipv4/ip_forward.c
+++ b/net/ipv4/ip_forward.c
@@ -39,17 +39,21 @@
 #include <net/route.h>
 #include <net/xfrm.h>
 
-static bool ip_may_fragment(const struct sk_buff *skb)
-{
-       return unlikely((ip_hdr(skb)->frag_off & htons(IP_DF)) == 0) ||
-               skb->ignore_df;
-}
-
 static bool ip_exceeds_mtu(const struct sk_buff *skb, unsigned int mtu)
 {
        if (skb->len <= mtu)
                return false;
 
+       if (unlikely((ip_hdr(skb)->frag_off & htons(IP_DF)) == 0))
+               return false;
+
+       /* original fragment exceeds mtu and DF is set */
+       if (unlikely(IPCB(skb)->frag_max_size > mtu))
+               return true;
+
+       if (skb->ignore_df)
+               return false;
+
        if (skb_is_gso(skb) && skb_gso_network_seglen(skb) <= mtu)
                return false;
 
@@ -114,7 +118,7 @@ int ip_forward(struct sk_buff *skb)
 
        IPCB(skb)->flags |= IPSKB_FORWARDED;
        mtu = ip_dst_mtu_maybe_forward(&rt->dst, true);
-       if (!ip_may_fragment(skb) && ip_exceeds_mtu(skb, mtu)) {
+       if (ip_exceeds_mtu(skb, mtu)) {
                IP_INC_STATS(dev_net(rt->dst.dev), IPSTATS_MIB_FRAGFAILS);
                icmp_send(skb, ICMP_DEST_UNREACH, ICMP_FRAG_NEEDED,
                          htonl(mtu));