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Commit 7b4fdf77 authored Mar 03, 2017 by Florian Westphal Committed by Pablo Neira Ayuso Mar 08, 2017

netfilter: don't track fragmented packets



Andrey reports syzkaller splat caused by

NF_CT_ASSERT(!ip_is_fragment(ip_hdr(skb)));

in ipv4 nat.  But this assertion (and the comment) are wrong, this function
does see fragments when IP_NODEFRAG setsockopt is used.

As conntrack doesn't track packets without complete l4 header, only the
first fragment is tracked.

Because applying nat to first packet but not the rest makes no sense this
also turns off tracking of all fragments.

Reported-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

parent 8d70eeb8

net/ipv4/netfilter/nf_conntrack_l3proto_ipv4.c

+4 −0

Original line number	Diff line number	Diff line
		@@ -165,6 +165,10 @@ static unsigned int ipv4_conntrack_local(void *priv,
		if (skb->len < sizeof(struct iphdr) \|\|
		ip_hdrlen(skb) < sizeof(struct iphdr))
		return NF_ACCEPT;

		if (ip_is_fragment(ip_hdr(skb))) /* IP_NODEFRAG setsockopt set */
		return NF_ACCEPT;

		return nf_conntrack_in(state->net, PF_INET, state->hook, skb);
		}

net/ipv4/netfilter/nf_nat_l3proto_ipv4.c

+0 −5

Original line number	Diff line number	Diff line
		@@ -255,11 +255,6 @@ nf_nat_ipv4_fn(void priv, struct sk_buff skb,
		/* maniptype == SRC for postrouting. */
		enum nf_nat_manip_type maniptype = HOOK2MANIP(state->hook);

		/* We never see fragments: conntrack defrags on pre-routing
		* and local-out, and nf_nat_out protects post-routing.
		*/
		NF_CT_ASSERT(!ip_is_fragment(ip_hdr(skb)));

		ct = nf_ct_get(skb, &ctinfo);
		/* Can't track? It's not due to stress, or conntrack would
		* have dropped it. Hence it's the user's responsibilty to