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

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Date:	Thu, 13 Aug 2009 13:31:24 -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 12:54 -0500, Serge E. Hallyn wrote:
> > 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?
> 
> Loading code into the kernel is a dangerous operation.  We should find
> the places where high risk processes are doing this and either choose to
> accept the security risk or make sure they are loaded before the
> dangerous code is run, aka libvirt knows if the guest needs the tun
> device and it should be allowed to trigger it's loading.  It's shouldn't
> be the guest doing the triggering.

Well, I continue to be dubious, but the hook does no harm so in case
it sounded otherwise, I don't object.

Mind you the right thing to do would be i.e. in the qemu case to have
a more privileged helper load the needed modules and then fire off a
qemu unable to cause module loads.  If userspace is willing to jump
through that rather minimal hoop then this could be useful I suppose.

thanks,
-serge
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