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

[Mimi Zohar <zohar@linux.vnet.ibm.com>]

On Mon, 2013-02-18 at 13:21 -0500, Vivek Goyal wrote:
> On Thu, Feb 14, 2013 at 10:30:15AM -0500, Mimi Zohar wrote:
> > 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.
> 
> Thinking loud. I guess we might have to extend ima policy/rules to allow
> multiple appraise rules to co-exist. And access permission will finally
> depend on if all the rules in same category return success.

ima_appraise_tcb rules:
dont_appraise  fsmagic=PROC_SUPER_MAGIC
.
.
.
dont_appraise fsmagic=SELINUX_MAGIC
appraise fowner=0

ima_secureboot:
appraise fowner=0 func=bprm appraise_type=optional

The ima_appraise_tcb appraise rule includes everything that would match
the ima_secureboot rule.  It isn't possible to combine the two policies.

Mimi