From: Florian Westphal <fw@strlen.de>
Date: Mon, 9 Jul 2018 13:43:38 +0200
Subject: netfilter: ipv6: nf_defrag: drop skb dst before queueing
Patch-mainline: v4.18-rc6
Git-commit: 84379c9afe011020e797e3f50a662b08a6355dcf
References: CVE-2018-5391 bsc#1103097

Eric Dumazet reports:
 Here is a reproducer of an annoying bug detected by syzkaller on our production kernel
 [..]
 ./b78305423 enable_conntrack
 Then :
 sleep 60
 dmesg | tail -10
 [  171.599093] unregister_netdevice: waiting for lo to become free. Usage count = 2
 [  181.631024] unregister_netdevice: waiting for lo to become free. Usage count = 2
 [  191.687076] unregister_netdevice: waiting for lo to become free. Usage count = 2
 [  201.703037] unregister_netdevice: waiting for lo to become free. Usage count = 2
 [  211.711072] unregister_netdevice: waiting for lo to become free. Usage count = 2
 [  221.959070] unregister_netdevice: waiting for lo to become free. Usage count = 2

Reproducer sends ipv6 fragment that hits nfct defrag via LOCAL_OUT hook.
skb gets queued until frag timer expiry -- 1 minute.

Normally nf_conntrack_reasm gets called during prerouting, so skb has
no dst yet which might explain why this wasn't spotted earlier.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Reported-by: John Sperbeck <jsperbeck@google.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Tested-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Michal Kubecek <mkubecek@suse.cz>

---
 net/ipv6/netfilter/nf_conntrack_reasm.c | 2 ++
 1 file changed, 2 insertions(+)

--- a/net/ipv6/netfilter/nf_conntrack_reasm.c
+++ b/net/ipv6/netfilter/nf_conntrack_reasm.c
@@ -590,6 +590,8 @@ int nf_ct_frag6_gather(struct net *net, struct sk_buff *skb, u32 user)
 	    fq->q.meat == fq->q.len &&
 	    nf_ct_frag6_reasm(fq, skb, dev))
 		ret = 0;
+	else
+		skb_dst_drop(skb);
 
 out_unlock:
 	spin_unlock_bh(&fq->q.lock);