netdev - Re: [PATCH net] ppp: don't override sk->sk_state in pppoe_flush

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Date:	Tue, 6 Oct 2015 04:46:04 +0000
From:	Matt Bennett <Matt.Bennett@...iedtelesis.co.nz>
To:	"g.nault@...halink.fr" <g.nault@...halink.fr>
CC:	"core@....lg.ua" <core@....lg.ua>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>,
	"paulus@...ba.org" <paulus@...ba.org>,
	"nuclearcat@...learcat.com" <nuclearcat@...learcat.com>
Subject: Re: [PATCH net] ppp: don't override sk->sk_state in
 pppoe_flush_dev()

> > The second one seems to be trickier. It looks like a race wrt. PADT
> > message reception. Reproducing the bug will probably require to
> > generate some PADT flooding to a host that creates and releases PPPoE
> > connections.

Ok I think I can see the potential race here, specifically the PADT
frame is received while the pppoe interface is being deleted. (I will
have a go inducing this with msleep() in the code tomorrow)

1. pppoe_flush_dev() - sk->sk_state = PPPOX_DEAD, po->pppoe_dev = NULL

2. pppoe_connect() - sk->sk_state = PPPOX_NONE, po->pppoe_dev = NULL

3. pppoe_disc_rcv() - sk->sk_state = PPPOX_ZOMBIE po->pppoe_dev = NULL

4. pppoe_release() - dev_put(po->pppoe_dev) ----> Oops


Either in pppoe_disc_rcv() we add the condition:

@@ -496,7 +499,8 @@ static int pppoe_disc_rcv(struct sk_buff *skb,
struct net_device *dev,
                        /* We're no longer connect at the PPPOE layer,
                         * and must wait for ppp channel to disconnect
us.
                         */
-                       sk->sk_state = PPPOX_ZOMBIE;
+                       if (sk->sk_state & PPPOX_CONNECTED)
+                               sk->sk_state = PPPOX_ZOMBIE;
                }

Or perhaps we remove the assumption that the state PPPOX_ZOMBIE has a
non-null pppoe_dev on it.

I don't know why the code isn't like the following anyway.

-if (sk->sk_state & (PPPOX_CONNECTED | PPPOX_BOUND | PPPOX_ZOMBIE)) {
+if (po->pppoe_dev) {
	dev_put(po->pppoe_dev);
	po->pppoe_dev = NULL;
}