Re: [PATCH 2/2] ima: Support appraise_type=imasig_optional

From: Mimi Zohar
Date: Thu Feb 14 2013 - 11:54:33 EST

On Thu, 2013-02-14 at 10:03 -0500, Vivek Goyal wrote:
> On Wed, Feb 13, 2013 at 05:27:01PM -0500, Mimi Zohar wrote:
> [..]
> > > Yep, I got that. Default policy gets overruled when a new policy is
> > > loaded.
> > >
> > > In secureboot mode, somehow above rule needs to take effect by default.
> > > One option would be that kernel can enforce above rule.
> > > (I guess by adding it to both default_list as well as policy list).
> >
> > The default policy is empty, but can be replaced with boot command line
> > options. The existing options are ima_tcb and/ ima_appraise_tcb.
> > Please feel free to define an additional policy.
> I think just defining a new command line option is not sufficient
> for secureboot use case.
> - One can easily remove kernel command line option without breaking
> booting and easily bypass secureboot restrictions.

> - I guess this is one mandated rule by secureboot. There might still
> be a user policy which can co-exist with this rule.
> So to me this is not a new policy. It is just one mandatory rule which
> gets appended to any policy in secureboot mode. Think of it as mandatory
> rule imposed by kernel for any policy user can define. And in secureboot
> mode a user can not get rid of this rule. (Otherwise it breaks user
> space signing and one can bypass secureboot and boot into unsigned
> kernel).

Your rule allows both signed and unsigned files to be executed. Signed
files will just have more capabilities. The ima_appraise_tcb option
requires all files owned by root to be signed, otherwise access is
denied. The two policies simply can not co-exist.

How about defining your single rule as ima_secureboot and making it the
default policy. Only if ima_appraise_tcb is specified on the kernel
command line, will the default policy be replaced. This type of change,
going from a null policy to an ima_secureboot policy, would require
community approval.



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