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Date: Tue, 6 Oct 2015 11:46:54 +0200 From: Guillaume Nault <g.nault@...halink.fr> To: Matt Bennett <Matt.Bennett@...iedtelesis.co.nz> 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() On Tue, Oct 06, 2015 at 04:46:04AM +0000, Matt Bennett wrote: > > > 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 > Again, I don't know why you introduce pppoe_connect() into the mix. But anyway, you got the point. Note that pppoe_flush_dev() could be replaced by other calls since we just need to reset po->pppoe_dev (another pppoe_unbind_sock_work() call, due to duplicated PADT, would also trigger the bug). Note also that pppoe_release() needs to be run before pppoe_unbind_sock_work() gets scheduled (or at least before it locks the socket). > 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 think adding complexity in the socket state management would be a good think. Actually I event think about dropping the PPPOX_ZOMBIE state altogether. But that's probably something for net-next. > 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; > } I was thinking about that same approach. pppoe_release() is the only function making that assumption. Other parts of the code seem to only require that PPPOX_CONNECTED => pppoe_dev != NULL. But I think the original condition was valid. Adding PPPOX_ZOMBIE into the test and resetting pppoe_dev upon reception of PADT have changed the relationship between sk_state and pppoe_dev, which is where the problem stands. -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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