[CVE-2016-9755][PATCH Yakkety] netfilter: ipv6: nf_defrag: drop mangled skb on ream error

[Po-Hsu Lin]

From: Florian Westphal <fw at strlen.de>

CVE-2016-9755

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 at google.com>
Debugged-by: Andrey Konovalov <andreyknvl at google.com>
Diagnosed-by: Eric Dumazet <Eric Dumazet <edumazet at google.com>
Signed-off-by: Florian Westphal <fw at strlen.de>
Acked-by: Eric Dumazet <edumazet at google.com>
Signed-off-by: Pablo Neira Ayuso <pablo at netfilter.org>
(cherry picked from commit 9b57da0630c9fd36ed7a20fc0f98dc82cc0777fa)

Signed-off-by: Po-Hsu Lin <po-hsu.lin at canonical.com>
---
 net/ipv6/netfilter/nf_conntrack_reasm.c   |    4 ++--
 net/ipv6/netfilter/nf_defrag_ipv6_hooks.c |    2 +-
 2 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/ipv6/netfilter/nf_conntrack_reasm.c b/net/ipv6/netfilter/nf_conntrack_reasm.c
index e4347ae..9948b5c 100644
--- a/net/ipv6/netfilter/nf_conntrack_reasm.c
+++ b/net/ipv6/netfilter/nf_conntrack_reasm.c
@@ -576,11 +576,11 @@ int nf_ct_frag6_gather(struct net *net, struct sk_buff *skb, u32 user)
 	/* 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;
diff --git a/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c b/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
index f7aab5a..f06b047 100644
--- a/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
+++ b/net/ipv6/netfilter/nf_defrag_ipv6_hooks.c
@@ -69,7 +69,7 @@ static unsigned int ipv6_defrag(void *priv,
 	if (err == -EINPROGRESS)
 		return NF_STOLEN;
 
-	return NF_ACCEPT;
+	return err == 0 ? NF_ACCEPT : NF_DROP;
 }
 
 static struct nf_hook_ops ipv6_defrag_ops[] = {
-- 
1.7.9.5