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

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Date:	Mon, 28 Feb 2011 12:29:11 +0300
From:	Michael Tokarev <mjt@....msk.ru>
To:	Arnd Bergmann <arnd@...db.de>
CC:	Michał Mirosław <mirqus@...il.com>,
	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

27.02.2011 23:22, Arnd Bergmann wrote:
> 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.

To me this looks like the best solution so far - trivial and
compatible.

Thanks!

/mjt
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