Re: [PATCH v8 4/8] IMA: add policy rule to measure critical data

[Tyler Hicks <tyhicks@linux.microsoft.com>]

On 2020-12-11 17:17:22, Tushar Sugandhi wrote:
> 
> 
> On 2020-12-11 4:25 p.m., Tyler Hicks wrote:
> > On 2020-12-11 15:58:03, Tushar Sugandhi wrote:
> > > A new IMA policy rule is needed for the IMA hook
> > > ima_measure_critical_data() and the corresponding func CRITICAL_DATA for
> > > measuring the input buffer. The policy rule should ensure the buffer
> > > would get measured only when the policy rule allows the action. The
> > > policy rule should also support the necessary constraints (flags etc.)
> > > for integrity critical buffer data measurements.
> > > 
> > > Add a policy rule to define the constraints for restricting integrity
> > > critical data measurements.
> > > 
> > > Signed-off-by: Tushar Sugandhi <tusharsu@linux.microsoft.com>
> > > ---
> > >   Documentation/ABI/testing/ima_policy |  2 +-
> > >   security/integrity/ima/ima_policy.c  | 34 ++++++++++++++++++++++++----
> > >   2 files changed, 31 insertions(+), 5 deletions(-)
> > > 
> > > diff --git a/Documentation/ABI/testing/ima_policy b/Documentation/ABI/testing/ima_policy
> > > index e35263f97fc1..6ec7daa87cba 100644
> > > --- a/Documentation/ABI/testing/ima_policy
> > > +++ b/Documentation/ABI/testing/ima_policy
> > > @@ -32,7 +32,7 @@ Description:
> > >   			func:= [BPRM_CHECK][MMAP_CHECK][CREDS_CHECK][FILE_CHECK]MODULE_CHECK]
> > >   			        [FIRMWARE_CHECK]
> > >   				[KEXEC_KERNEL_CHECK] [KEXEC_INITRAMFS_CHECK]
> > > -				[KEXEC_CMDLINE] [KEY_CHECK]
> > > +				[KEXEC_CMDLINE] [KEY_CHECK] [CRITICAL_DATA]
> > >   			mask:= [[^]MAY_READ] [[^]MAY_WRITE] [[^]MAY_APPEND]
> > >   			       [[^]MAY_EXEC]
> > >   			fsmagic:= hex value
> > > diff --git a/security/integrity/ima/ima_policy.c b/security/integrity/ima/ima_policy.c
> > > index a09d1a41a290..07116ff35c25 100644
> > > --- a/security/integrity/ima/ima_policy.c
> > > +++ b/security/integrity/ima/ima_policy.c
> > > @@ -85,6 +85,7 @@ struct ima_rule_entry {
> > >   	} lsm[MAX_LSM_RULES];
> > >   	char *fsname;
> > >   	struct ima_rule_opt_list *keyrings; /* Measure keys added to these keyrings */
> > > +	struct ima_rule_opt_list *data_source; /* Measure data from this source */
> > 
> > Argh, there are still some more instances of data_source sneaking into
> > this patch too early instead of waiting until the next patch.
> > 
> I kept it purposefully in this patch so that the
> "case CRITICAL_DATA:" could be properly defined.
> 
> Also, my impression was rule->data_source is not part of the user facing
> policy.
> 
> Whereas IMA_DATA_SOURCE, Opt_data_source, data_source=%s are.
> That's why they are part of Patch #5.
> 
> Patch #5 IMA: limit critical data measurement based on a label
> 
> > >   	struct ima_template_desc *template;
> > >   };
> > > @@ -479,6 +480,12 @@ static bool ima_match_rule_data(struct ima_rule_entry *rule,
> > >   		opt_list = rule->keyrings;
> > >   		break;
> > > +	case CRITICAL_DATA:
> > > +		if (!rule->data_source)
> > > +			return true;
> > > +
> > > +		opt_list = rule->data_source;
> > > +		break;
> > 
> > I guess this case should unconditionally return true in this patch and
> > then the include this additional logic in the next patch.
> > 
> > Sorry, I missed these on my last review.
> > 
> No worries.
> 
> As I mentioned above, I kept it purposefully in this patch since
> my impression was rule->data_source is not part of the user facing
> policy.
> 
> But I can simply return true here as you suggested, and move the logic to
> the next patch.

I understand the thinking that it isn't harmful in this patch but I
think it is a bit cleaner to introduce the data_source policy language
element and all of its backend support in the same patch. Please move it
to the next patch. Thanks!

Tyler

> 
> +	case CRITICAL_DATA:
> +		if (!rule->data_source)
> +			return true;
> +
> +		opt_list = rule->data_source;
> +		break;
> 
> 
> ~Tushar
>