linux-kernel - Re: [PATCH 2/3] security: introducing security_request

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Date:	Thu, 13 Aug 2009 12:54:05 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Eric Paris <eparis@...hat.com>
Cc:	linux-kernel@...r.kernel.org, selinux@...ho.nsa.gov,
	netdev@...r.kernel.org, linux-security-module@...r.kernel.org,
	sds@...ho.nsa.gov, davem@...emloft.net,
	shemminger@...ux-foundation.org, kees@...ntu.com,
	morgan@...nel.org, casey@...aufler-ca.com, dwalsh@...hat.com
Subject: Re: [PATCH 2/3] security: introducing security_request_module

Quoting Eric Paris (eparis@...hat.com):
> On Thu, 2009-08-13 at 09:03 -0500, Serge E. Hallyn wrote:
> > Quoting Eric Paris (eparis@...hat.com):
> > > Calling request_module() will trigger a userspace upcall which will load a
> > > new module into the kernel.  This can be a dangerous event if the process
> > > able to trigger request_module() is able to control either the modprobe
> > > binary or the module binary.  This patch adds a new security hook to
> > > request_module() which can be used by an LSM to control a processes ability
> > > to call request_module().
> > 
> > Is there a specific case in which you'd want to deny this ability
> > from a real task?
> 
> qemu and any network facing daemon are all programs I don't want to be
> able to even ask the kernel to load a module.  Clearly you are right,

...  What if the network facing daemon might want to use a kernel crypto
module?  What if qemu needs the tun module loaded?

> that the best protection is done by controlling access to modprobe and
> the modules on disk (which we are working to fix vs what happened in the
> xen fb exploit I showed earlier) but stopping it from the other
> direction is, I feel, a useful defense in depth.
> 
> If they can't get modprobe called, they can't take over the system
> directly, even if they did change a module or change modprobe.  I agree
> it's not strong security as if they can change modprobe or modules they
> might be able to just wait until something else calls modprobe (next
> reboot maybe?) to take over the system.  But I'd find it very
> interesting to know that a high threat target tried to do anything which
> attempted to load a module....
> 
> -Eric
> 
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