[PATCH nf-next v2 1/1] ipv6: defrag: drop non-last frags smaller than min mtu

[PATCH nf-next v2 1/1] ipv6: defrag: drop non-last frags smaller than min mtu

[Florian Westphal]

don't bother with pathological cases, they only waste cycles.
IPv6 requires a minimum MTU of 1280 so we should never see fragments
smaller than this (except last frag).

v2: drop IPv4 part, which added same check w. IPV4_MIN_MTU (68).
    There were concerns that there could be even smaller frags
    generated by intermediate nodes, e.g. on radio networks.

Cc: Peter Oskolkov <posk@google.com>
Cc: Eric Dumazet <edumazet@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
---
 net/ipv6/netfilter/nf_conntrack_reasm.c | 4 ++++
 net/ipv6/reassembly.c                   | 4 ++++
 2 files changed, 8 insertions(+)

diff --git a/net/ipv6/netfilter/nf_conntrack_reasm.c b/net/ipv6/netfilter/nf_conntrack_reasm.c
index 0610bdab721c..c121d534d321 100644
--- a/net/ipv6/netfilter/nf_conntrack_reasm.c
+++ b/net/ipv6/netfilter/nf_conntrack_reasm.c
@@ -557,6 +557,10 @@ int nf_ct_frag6_gather(struct net *net, struct sk_buff *skb, u32 user)
 	hdr = ipv6_hdr(skb);
 	fhdr = (struct frag_hdr *)skb_transport_header(skb);
 
+	if (-skb_network_offset(skb) + skb->len < IPV6_MIN_MTU &&
+	    fhdr->frag_off & htons(IP6_MF))
+		return -EINVAL;
+
 	skb_orphan(skb);
 	fq = fq_find(net, fhdr->identification, user, hdr,
 		     skb->dev ? skb->dev->ifindex : 0);
diff --git a/net/ipv6/reassembly.c b/net/ipv6/reassembly.c
index 6edd2ac8ae4b..ff00ada6128f 100644
--- a/net/ipv6/reassembly.c
+++ b/net/ipv6/reassembly.c
@@ -455,6 +455,10 @@ static int ipv6_frag_rcv(struct sk_buff *skb)
 		return 1;
 	}
 
+	if (-skb_network_offset(skb) + skb->len < IPV6_MIN_MTU &&
+	    fhdr->frag_off & htons(IP6_MF))
+		goto fail_hdr;
+
 	iif = skb->dev ? skb->dev->ifindex : 0;
 	fq = fq_find(net, fhdr->identification, hdr, iif);
 	if (fq) {
-- 
2.16.4

Re: [PATCH nf-next v2 1/1] ipv6: defrag: drop non-last frags smaller than min mtu

[Stephen Hemminger]

On Fri,  3 Aug 2018 02:05:39 +0200
Florian Westphal <fw@strlen.de> wrote:

> +	if (-skb_network_offset(skb) + skb->len < IPV6_MIN_MTU &&

Is there a reason to write the arithmetic expression in this order?
Why not:
	if (skb->len - skb_network_offset(skb) < IPV6_MIN_MTU &&