CVE-2016-9755: Out-of-bounds Write (CWE-787)
value: The netfilter subsystem in the Linux kernel before 4.9 mishandles IPv6 reassembly, which allows local users to cause a denial of service (integer overflow, out-of-bounds write, and GPF) or possibly have unspecified other impact via a crafted application that makes socket, connect, and writev system calls, related to net/ipv6/netfilter/nf_conntrack_reasm.c and net/ipv6/netfilter/nf_defrag_ipv6_hooks.c.
CWE description: The product writes data past the end, or before the beginning, of the intended buffer.
CVSS v3 base score: 7.8
Published: 2016-12-28T07:59Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: net/ipv6/netfilter/nf_conntrack_reasm.c, net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
Commit date: 2016-11-29 20:23:58 +0100
Commit message: netfilter: ipv6: nf_defrag: drop mangled skb on ream error

Dmitry Vyukov reported GPF in network stack that Andrey traced down to
negative nh offset in nf_ct_frag6_queue().

Problem is that all network headers before fragment header are pulled.
Normal ipv6 reassembly will drop the skb when errors occur further down
the line.

netfilter doesn't do this, and instead passed the original fragment
along.  That was also fine back when netfilter ipv6 defrag worked with
cloned fragments, as the original, pristine fragment was passed on.

So we either have to undo the pull op, or discard such fragments.
Since they're malformed after all (e.g. overlapping fragment) it seems
preferrable to just drop them.

Same for temporary errors -- it doesn't make sense to accept (and
perhaps forward!) only some fragments of same datagram.

Fixes: 029f7f3b8701cc7ac ("netfilter: ipv6: nf_defrag: avoid/free clone operations")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Debugged-by: Andrey Konovalov <andreyknvl@google.com>
Diagnosed-by: Eric Dumazet <Eric Dumazet <edumazet@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

Diff: diff --git a/net/ipv6/netfilter/nf_conntrack_reasm.c b/net/ipv6/netfilter/nf_conntrack_reasm.c
index e4347aeb2e6533..9948b5ce52dad3 100644
--- a/net/ipv6/netfilter/nf_conntrack_reasm.c
+++ b/net/ipv6/netfilter/nf_conntrack_reasm.c
@@ -574,15 +574,15 @@ int nf_ct_frag6_gather(struct net *net, struct sk_buff *skb, u32 user)
 	u8 prevhdr;
 
 	/* Jumbo payload inhibits frag. header */
 	if (ipv6_hdr(skb)->payload_len == 0) {
 		pr_debug("payload len = 0\n");
-		return -EINVAL;
+		return 0;
 	}
 
 	if (find_prev_fhdr(skb, &prevhdr, &nhoff, &fhoff) < 0)
-		return -EINVAL;
+		return 0;
 
 	if (!pskb_may_pull(skb, fhoff + sizeof(*fhdr)))
 		return -ENOMEM;
 
 	skb_set_transport_header(skb, fhoff);
diff --git a/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c b/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
index f7aab5ab93a558..f06b0471f39fc0 100644
--- a/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
+++ b/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
@@ -67,11 +67,11 @@ static unsigned int ipv6_defrag(void *priv,
 				 nf_ct6_defrag_user(state->hook, skb));
 	/* queued */
 	if (err == -EINPROGRESS)
 		return NF_STOLEN;
 
-	return NF_ACCEPT;
+	return err == 0 ? NF_ACCEPT : NF_DROP;
 }
 
 static struct nf_hook_ops ipv6_defrag_ops[] = {
 	{
 		.hook		= ipv6_defrag,

Vulnerable code: 		return -EINVAL;
		return -EINVAL;
	return NF_ACCEPT;
Fixed code: 		return 0;
		return 0;
	return err == 0 ? NF_ACCEPT : NF_DROP;