netfilter: ipv6: fix use-after-free Write in nf_nat_ipv6_manip_pkt

l4proto->manip_pkt() can cause reallocation of skb head so pointer
to the ipv6 header must be reloaded.

Reported-and-tested-by: <syzbot+10005f4292fc9cc89de7@syzkaller.appspotmail.com>
Fixes: 58a317f1061c89 ("netfilter: ipv6: add IPv6 NAT support")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

diff --git a/net/ipv6/netfilter/nf_nat_l3proto_ipv6.c b/net/ipv6/netfilter/nf_nat_l3proto_ipv6.c
index bed57ee..6b7f075 100644 (file)
--- a/net/ipv6/netfilter/nf_nat_l3proto_ipv6.c
+++ b/net/ipv6/netfilter/nf_nat_l3proto_ipv6.c
@@ -99,6 +99,10 @@ static bool nf_nat_ipv6_manip_pkt(struct sk_buff *skb,
            !l4proto->manip_pkt(skb, &nf_nat_l3proto_ipv6, iphdroff, hdroff,
                                target, maniptype))
                return false;
+
+       /* must reload, offset might have changed */
+       ipv6h = (void *)skb->data + iphdroff;
+
 manip_addr:
        if (maniptype == NF_NAT_MANIP_SRC)
                ipv6h->saddr = target->src.u3.in6;