Re: IMA: How to manage user space signing policy with others

Vivek Goyal
2013-03-01T18:40:33Z

On Fri, Mar 01, 2013 at 10:28:40AM -0500, Vivek Goyal wrote:
> On Fri, Mar 01, 2013 at 07:15:07AM -0500, Mimi Zohar wrote:
> > On Thu, 2013-02-28 at 20:49 -0500, Mimi Zohar wrote:
> > > On Thu, 2013-02-28 at 17:20 -0500, Eric Paris wrote:
> >
> > > > The ima_tcb policy was meant to be larger than needed to determine a
> > > > trusted computing base, but it is clearly not a superset of what he is
> > > > hoping to accomplish.
> >
> > The builtin measurement and appraisal policies are different. In order
> > not to miss a measurement, the measurement policy measures everything
> > read/executed by root. Userspace can constrain the policy by defining
> > rules based on LSM labels. The appraisal policy measures everything
> > owned by root. Userspace might want to add rules to appraise additional
> > files.
> >
> > We can not OR the measurement builtin and userspace policies, as the
> > userspace policy constrains the builtin policy, but for appraisal we
> > could. Perhaps we should define two rule chains, one for the builtin
> > appraisal rules and another for all other rules.
>
> Ok, just to make sure that I understand it right, I will summarize above.
>
> So a user can overide/replace "measure and audit" rules but it can not
> overide replace kernel's "appraise" rules and it can only append to
> existing appraise rules.
>
> So we internally define two rule chanins. All the appraisal rules
> go in one rule chain and all other rules (measure and audit) go in
> separate chain.
>
> When user writes an "appraise" rule to "policy" file, it gets *appended*
> to internal appraise rule chain and if user writes a "measure or audit"
> rule to "policy" file, it replaces the kernel's rules with user's rules.
>
> Given the fact that policy file ABI is still in testing we should be
> able to change semantics. (As currently user's appraise rules override
> kernel's appraisal rules).
>
> >
> > When secure boot is defined, instead of having a NULL policy, the
> > default policy would be the secureboot integrity policy. These rules
> > would be added to the builtin appraisal rule chain. If the
> > 'ima_appraise_tcb' boot commandline option is specified, these rules
> > would also be added to the builtin appraisal rule chain, but at the head
> > of the chain, as they are more restrictive than the secureboot policy
> > for root owned files.
> >
> > Vivek, would this work?
>
> This should work except the result caching issue. If we are running a
> partially signed user space, then unsigned process can write to disk
> directly (of course with right permisions). So secureboot policy can not
> cache appraisal results.
>
> In fact thinking more about it, I think ima_appraise_tcb policy also
> is vulnerable. This policy will not appraise files which are not
> owned by root. And users belonging to group "disk" have write permission
> to disks.
>
> So if I create a user "foo" and add it to group "disk", it can now launch
> its own processes and write to disk. And write to root owned files and
> ima_appraise_tcb policy will not detect the change.
>
> Hence, if ima_appraise_tcb rules are put in front of secureboot rules,
> caching appraisal results opens a security hole.

To avoid clashes between multiple built-in policies can we keep it
simpler. And that is only one built-in appraisal policy can be effective
a time. So if secureboot policy is effective, one can not use
ima_appraise_tcb.

We can provide one command line option to disable secureboot policy
(which works only if platform has secureboot disabled). So if a user
wants to use ima_appraise_tcb, he needs to pass two command line options.

"ima_apprise_secureboot=disable ima_appraise_tcb".

User can still append its appraise policies using "policy" interface. These
new rules take affect only if existing kernel policy does not apply to the
hook.

Thanks
Vivek