netdev - Re: [PATCH net] pppoe: check sockaddr length in pppoe

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Date:   Fri, 27 Apr 2018 11:51:31 -0400
From:   Kevin Easton <kevin@...rana.org>
To:     Guillaume Nault <g.nault@...halink.fr>
Cc:     netdev@...r.kernel.org, Michal Ostrowski <mostrows@...thlink.net>
Subject: Re: [PATCH net] pppoe: check sockaddr length in pppoe_connect()

On Fri, Apr 27, 2018 at 05:39:06PM +0200, Guillaume Nault wrote:
> On Fri, Apr 27, 2018 at 08:23:16AM -0400, Kevin Easton wrote:
...
> > There's another bug here - pppoe_connect() should also be validating
> > sp->sa_family.  My suggested patch was going to be:
> > 
> > diff --git a/drivers/net/ppp/pppoe.c b/drivers/net/ppp/pppoe.c
> > index 1483bc7..90eb3fd 100644
> > --- a/drivers/net/ppp/pppoe.c
> > +++ b/drivers/net/ppp/pppoe.c
> > @@ -620,6 +620,14 @@ static int pppoe_connect(struct socket *sock, struct sockaddr *uservaddr,
> >         lock_sock(sk);
> >  
> >         error = -EINVAL;
> > +       if (sockaddr_len < sizeof(struct sockaddr_pppox))
> > +               goto end;
> > +
> > +       error = -EAFNOSUPPORT;
> > +       if (sp->sa_family != AF_PPPOX)
> > +               goto end;
> > +
> > +       error = -EINVAL;
> >         if (sp->sa_protocol != PX_PROTO_OE)
> >                 goto end;
> >  
> > Should I rework this on top of net.git HEAD?
> > 
> > (The same applies to pppol2tp_connect()).
> > 
> Thanks for the suggestion. But ->sa_family has never been checked.
> Therefore, it has always been possible to connect a PPPoE or L2TP
> socket with an invalid .sa_family field. I'd be surprised if there were
> implementations relying on that, but we never know (for example, an
> implementation could send this field uninitialised). By being stricter
> we'd break such programs. And we don't need this field in the
> connection process, so not checking its value doesn't harm.
> 
> I'm all for being strict and validating user-provided data as much as
> possible, but I'm afraid its too late in this case.

Doesn't the same apply to supplying a bogus sockaddr_len?

I did test the rp-pppoe plugin for pppd with this patch - it does 
correctly set both the sa_family and sockaddr_len.  Checking on
Debian's codesearch also showed that everything in that corpus
that uses PX_PROTO_OE also sets AF_PPPOX.

    - Kevin

>