linux-kernel - Re: request_module vs. modprobe blacklist (and security subsystem implications)

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Date:	Fri, 23 Oct 2009 19:46:38 +1030
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Eric Paris <eparis@...hat.com>
Cc:	Alan Jenkins <sourcejedi.lkml@...glemail.com>,
	linux-kernel@...r.kernel.org, arjan@...radead.org,
	randy.dunlap@...cle.com, andi@...stfloor.org, dhowells@...hat.com,
	akpm@...ux-foundation.org
Subject: Re: request_module vs. modprobe blacklist (and security subsystem implications)

On Fri, 23 Oct 2009 01:00:22 am Eric Paris wrote:
> > If a userspace program tries some security exploit that has been closed, do
> > you want to warn about it?  Because that seems to be the question here.
> 
> I say yes.  Knowing that malicious activity is taking place, even if it
> didn't hurt anything is useful.

Hi Eric,

Your proposal is troubling for three reasons:

1) You would disable logging for things you actually want logged.
2) What *actually* happens when ssh tries to load ipv6 is that
   "modprobe net-pf-10" gets called.
3) Containing modprobe behavior in one set of config files is really nice.

> > Why should ssh not load IPv6?  Because noone should?  Fine, but there's a
> > difference between "I expect it to do this but I won't let it" and "I don't
> > expect it to do this".
> 
> In this case it's because the admin decided not to allow it.  In this
> case it is 'I expect it to do this, and I know that later it would fail,
> so I don't want to complain that it is failing now.'  I want a way to
> move the failure up, and to allow and admin to stop with the useless
> userspace callouts to modprobe.

No.  I anticipate that in the future you will want to do some fairly
sophisticated filtering.  That does not belong in the kernel unless there
are performance concerns, and I don't think there are here.

There are all kinds of things that can be administratively prohibited, and
it would be nice if my security tools would Just Work with that: I don't think
pushing all the different restrictions into the kernel for SELinux's sake
is the way forward.

> I want a way to make the kernel not upcall at all, if I have that I can
> make SELinux do whatever I want.  If I don't have that, all I can do is
> some post failure fragile userspace filtering.

If the kernel is better at filtering than userspace, that is an SELinux
problem that you should address.

Cheers,
Rusty.
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