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

Mimi Zohar
2013-03-01T19:39:34Z

On Fri, 2013-03-01 at 13:40 -0500, Vivek Goyal wrote:
> 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.

I was suggesting that a builtin appraise rule chain and everything else
on the other chain. Userspace could replace the other chain with
whatever they wanted, including additional appraisal 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).

The userspace policy could only extend the appraisal rules. We OR the
result of both chains, and use the more restrictive rule.

> > >
> > > 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.

We've already spoken about needing an additional hook or moving the
existing bprm hook. Can we defer the memory caching requirements for
now?

> 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.

After thinking about it some more and discussing it with Dave, the
built-in appraisal policy would be a fixed policy. For now, secureboot
would use it to define their policy. For now, there is no need to
include the ima_appraise_tcb rules in the builtin/fixed chain. They
could continue to be defined in the other chain.

> 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.

Right, so with the changes suggested above, this wouldn't be needed.

> These
> new rules take affect only if existing kernel policy does not apply to the
> hook.

As mentioned above, the more restrictive policy would be used.

thanks,

Mimi