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

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

On Wed, 2013-02-13 at 15:13 +0200, Kasatkin, Dmitry wrote:
> On Wed, Feb 13, 2013 at 2:56 PM, Mimi Zohar <zohar@linux.vnet.ibm.com> wrote:
> > On Wed, 2013-02-13 at 14:31 +0200, Kasatkin, Dmitry wrote:
> >> On Mon, Feb 11, 2013 at 10:11 PM, Vivek Goyal <vgoyal@redhat.com> wrote:
> >
> >> > @@ -158,7 +165,8 @@ int ima_appraise_measurement(int func, struct integrity_iint_cache *iint,
> >> >         }
> >> >         switch (xattr_value->type) {
> >> >         case IMA_XATTR_DIGEST:
> >> > -               if (iint->flags & IMA_DIGSIG_REQUIRED) {
> >> > +               if (iint->flags & IMA_DIGSIG_REQUIRED ||
> >> > +                   iint->flags & IMA_DIGSIG_OPTIONAL) {
> >> >                         cause = "IMA signature required";
> >> >                         status = INTEGRITY_FAIL;
> >> >                         break;
> >>
> >> This looks a bit odd... If "optional" signature is missing  - we fail..
> >> It is optional... Why we should fail?
> >
> > 'imasig_optional' does not only mean that the signature is optional, but
> > also implies that it has to be a digital signature, not a hash.  This
> > latter part is what IMA_DIGSIG_REQUIRED means.
> >
> > If 'imasig_optional' set both 'IMA_DIGSIG_OPTIONAL' and
> > 'IMA_DIGSIG_REQUIRED', then this change wouldn't be necessary.
> >
> > IMA_DIGSIG_REQUIRED would enforce that it is a signature.
> > IMA_DIGSIG_OPTIONAL would fail only for files with invalid signatures.
> >
> 
> It sounds odd that optional signature is actually required.
> Optional for me means that it may be there or may be not.
> If it is not there, then it may be hash or nothing.
> 
> I see it is more logical if it is "appraise_type=optional",
> which means that we might have no xattr value, hash or signature.
> It if happens to be a signature, then IMA_DIGSIG flag will be set.

Right, 'appraise_type=' could have been defined either as a comma
separated list of options (eg. appraise_type=imassig,optional) or, as
Vivek implemented, a new option.  Eventually, we will need to extend
'appraise_type=' (eg. required algorithm), but for now I don't have a
problem with the new option.

thanks,

Mimi

> I asked Vivek to send a policy file he uses in his system.
> I would like to see how system configured as a whole...
> 
> - Dmitry