Re: [PATCH v5 6/7] IMA: add critical_data to the built-in policy rules

[Lakshmi Ramasubramanian]

On 11/6/20 7:37 AM, Lakshmi Ramasubramanian wrote:

Hi Mimi,

> > Hi Lakshmi, Tushar,
> >
> > This patch defines a new critical_data builtin policy.  Please update
> > the Subject line.
> >
> > On Sun, 2020-11-01 at 14:26 -0800, Tushar Sugandhi wrote:
> > > From: Lakshmi Ramasubramanian <nramas@xxxxxxxxxxxxxxxxxxx>
> > >
> > > The IMA hook to measure kernel critical data, namely
> > > ima_measure_critical_data(), could be called before a custom IMA policy
> > > is loaded. For example, SELinux calls ima_measure_critical_data() to
> > > measure its state and policy when they are initialized. This occurs
> > > before a custom IMA policy is loaded, and hence IMA hook will not
> > > measure the data. A built-in policy is therefore needed to measure
> > > critical data provided by callers before a custom IMA policy is loaded.
> >
> > ^Define a new critical data builtin policy to allow measuring early
> > kernel integrity critical data before a custom IMA policy is loaded.
>
> I will add the above line in the patch description.
>
> > Either remove the references to SELinux or move this patch after the
> > subsequent patch which measures SELinux critical data.
>
> I will remove the reference to SELinux.
> I think it would be better to have this patch before the SELinux 
> measurement patch.
>
> > > Add CRITICAL_DATA to built-in IMA rules if the kernel command line
> > > contains "ima_policy=critical_data". Set the IMA template for this rule
> > > to "ima-buf" since ima_measure_critical_data() measures a buffer.
> > >
> > > Signed-off-by: Lakshmi Ramasubramanian <nramas@xxxxxxxxxxxxxxxxxxx>
> >
> > > ---
> > >   security/integrity/ima/ima_policy.c | 32 +++++++++++++++++++++++++++++
> > >   1 file changed, 32 insertions(+)
> > >
> > > diff --git a/security/integrity/ima/ima_policy.c 
> > > b/security/integrity/ima/ima_policy.c
> > > index ec99e0bb6c6f..dc8fe969d3fe 100644
> > > --- a/security/integrity/ima/ima_policy.c
> > > +++ b/security/integrity/ima/ima_policy.c
> >
> > > @@ -875,6 +884,29 @@ void __init ima_init_policy(void)
> > >                 ARRAY_SIZE(default_appraise_rules),
> > >                 IMA_DEFAULT_POLICY);
> > > +    if (ima_use_critical_data) {
> > > +        template = lookup_template_desc("ima-buf");
> > > +        if (!template) {
> > > +            ret = -EINVAL;
> > > +            goto out;
> > > +        }
> > > +
> > > +        ret = template_desc_init_fields(template->fmt,
> > > +                        &(template->fields),
> > > +                        &(template->num_fields));
> >
> > The default IMA template when measuring buffer data is "ima_buf".   Is
> > there a reason for allocating and initializing it here and not
> > deferring it until process_buffer_measurement()?
>
> You are right - good catch.
> I will remove the above and validate.

process_buffer_measurement() allocates and initializes "ima-buf" 
template only when the parameter "func" is NONE. Currently, only 
ima_check_blacklist() passes NONE for func when calling 
process_buffer_measurement().

If "func" is anything other than NONE, ima_match_policy() picks
the default IMA template if the IMA policy rule does not specify a template.

We need to add "ima-buf" in the built-in policy for critical_data so 
that the default template is not used for buffer measurement.

Please let me know if I am missing something.

thanks,
 -lakshmi

> > > +        if (ret)
> > > +            goto out;
> > > +
> > > +        critical_data_rules[0].template = template;
> > > +        add_rules(critical_data_rules,
> > > +              ARRAY_SIZE(critical_data_rules),
> > > +              IMA_DEFAULT_POLICY);
> > > +    }
> > > +
> > > +out:
> > > +    if (ret)
> > > +        pr_err("%s failed, result: %d\n", __func__, ret);
> > > +
> > >       ima_update_policy_flag();
> > >   }