linux-kernel - Re: [PATCH] don't allow CAP_NET_ADMIN to load non-netdev kernel modules

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Message-Id: <201102272122.52643.arnd@arndb.de>
Date:	Sun, 27 Feb 2011 21:22:52 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	Michał Mirosław <mirqus@...il.com>
Cc:	Ben Hutchings <bhutchings@...arflare.com>,
	David Miller <davem@...emloft.net>, segoon@...nwall.com,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	kuznet@....inr.ac.ru, pekkas@...core.fi, jmorris@...ei.org,
	yoshfuji@...ux-ipv6.org, kaber@...sh.net, eric.dumazet@...il.com,
	therbert@...gle.com, xiaosuo@...il.com, jesse@...ira.com,
	kees.cook@...onical.com, eugene@...hat.com,
	dan.j.rosenberg@...il.com, akpm@...ux-foundation.org
Subject: Re: [PATCH] don't allow CAP_NET_ADMIN to load non-netdev kernel modules

On Friday 25 February 2011, Michał Mirosław 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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