[PATCH 4.17 074/101] netfilter: ipv6: nf_defrag: drop skb dst before queueing

[Greg Kroah-Hartman]

4.17-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Florian Westphal <fw@xxxxxxxxx>

commit 84379c9afe011020e797e3f50a662b08a6355dcf upstream.

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@xxxxxxxxx>
Reported-by: John Sperbeck <jsperbeck@xxxxxxxxxx>
Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
Tested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Reported-by: syzbot <syzkaller@xxxxxxxxxxxxxxxx>
Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 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
@@ -585,6 +585,8 @@ int nf_ct_frag6_gather(struct net *net,
 	    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);