Subject: Re: [PATCH v3 net-next 1/2] ppp: add PPPIOCBRIDGECHAN and PPPIOCUNBRIDGECHAN ioctls

From: Guillaume Nault <[email-masked]>
Date: Thu, 10 Dec 2020 16:05:51 +0100

On Thu, Dec 10, 2020 at 02:46:23PM +0000, Tom Parkin wrote:
> On  Mon, Dec 07, 2020 at 17:22:28 +0100, Guillaume Nault wrote:
> > On Fri, Dec 04, 2020 at 04:36:55PM +0000, Tom Parkin wrote:
> > > +		case PPPIOCBRIDGECHAN:
> > > +			if (get_user(unit, p))
> > > +				break;
> > > +			err = -ENXIO;
> > > +			pn = ppp_pernet(current->nsproxy->net_ns);
> > > +			spin_lock_bh(&pn->all_channels_lock);
> > > +			pchb = ppp_find_channel(pn, unit);
> > > +			/* Hold a reference to prevent pchb being freed while
> > > +			 * we establish the bridge.
> > > +			 */
> > > +			if (pchb)
> > > +				refcount_inc(&pchb->file.refcnt);
> > 
> > The !pchb case isn't handled. With this code, if ppp_find_channel()
> > returns NULL, ppp_bridge_channels() will crash when trying to lock
> > pchb->upl.
> 
> Bleh :-(
> 
> Apologies for this.  I have stepped up my tests for "unhappy" code
> paths, and I'll try to run syzkaller at a v4 prior to re-submitting.

No problem, sorry for not having spotted the problem in your previous
version. BTW, note that net-next is probably about to close.

> > > +			spin_unlock_bh(&pn->all_channels_lock);
> > > +			err = ppp_bridge_channels(pch, pchb);
> > > +			/* Drop earlier refcount now bridge establishment is complete */
> > > +			if (refcount_dec_and_test(&pchb->file.refcnt))
> > > +				ppp_destroy_channel(pchb);
> > > +			break;
> > > +
> > 
> > The rest looks good to me.
> 
> Thanks!