From: Arnd Bergmann <arnd@arndb.de>
To: =?iso-8859-2?q?Micha=B3_Miros=B3aw?= <mirqus@gmail.com>
Subject: Re: [PATCH] don't allow CAP_NET_ADMIN to load non-netdev kernel modules
Date: Sun, 27 Feb 2011 21:22:52 +0100
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Cc: Ben Hutchings <bhutchings@solarflare.com>,
        David Miller <davem@davemloft.net>, segoon@openwall.com,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        kuznet@ms2.inr.ac.ru, pekkas@netcore.fi, jmorris@namei.org,
        yoshfuji@linux-ipv6.org, kaber@trash.net, eric.dumazet@gmail.com,
        therbert@google.com, xiaosuo@gmail.com, jesse@nicira.com,
        kees.cook@canonical.com, eugene@redhat.com, dan.j.rosenberg@gmail.com,
        akpm@linux-foundation.org
References: <1298660879.2554.23.camel@bwh-desktop> <1298666310.2554.47.camel@bwh-desktop> <AANLkTikmtKksP6o1hecf0=ZnRv0j4+_Q326UEG8ss16k@mail.gmail.com>
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On Friday 25 February 2011, Micha? Miros?aw wrote:
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index 54aaca6..0d09baa 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -1120,8 +1120,20 @@ void dev_load(struct net *net, const char *name)
> >        dev = dev_get_by_name_rcu(net, name);
> >        rcu_read_unlock();
> >
> > -       if (!dev && capable(CAP_NET_ADMIN))
> > -               request_module("%s", name);
> > +       if (!dev && capable(CAP_NET_ADMIN)) {
> > +               /* Check whether the name looks like one that a net
> > +                * driver will generate initially.  If not, require a
> > +                * module alias with a suitable prefix, so that this
> > +                * can't be used to load arbitrary modules.
> > +                */
> > +               if ((strncmp(name, "eth", 3) == 0 &&
> > +                    isdigit((unsigned char)name[3])) ||
> > +                   (strncmp(name, "wlan", 4) == 0 &&
> > +                    isdigit((unsigned char)name[4])))
> > +                       request_module("%s", name);
> > +               else
> > +                       request_module("netdev-%s", name);
> > +       }
> >  }
> >  EXPORT_SYMBOL(dev_load);
> >
> 
> This might be better as:
> 
> if (request_module("netdev-%s", name))
>     ... fallback
> 
> Then after some years the fallback could be removed if announced properly.

The backwards compatibility should mostly be for systems that today don't
use split capabilities, right?

The fallback could therefore rely on CAP_SYS_MODULE as well:

	if (request_module("netdev-%s", name)) {
		if (capable(CAP_SYS_MODULE))
			request_module("%s", name);
	}

Not 100% solution, but should solve the capability escalation nicely without
causing much pain.

	Arnd

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