[PATCH v2 2/3] powerpc/secvar: Expose secvars relevant to the key management mode

[Srish Srinivasan]

On 5/23/25 11:49 AM, Michal Suchánek wrote:
> Hello,
>
> On Wed, May 21, 2025 at 04:27:58PM +0530, Srish Srinivasan wrote:
>> The PLPKS enabled PowerVM LPAR sysfs exposes all of the secure boot
>> secvars irrespective of the key management mode.
>>
>> The PowerVM LPAR supports static and dynamic key management for secure
>> boot. The key management option can be updated in the management
>> console. Only in the dynamic key mode can the user modify the secure
>> boot secvars db, dbx, grubdb, grubdbx, and sbat, which are exposed via
>> the sysfs interface. But the sysfs interface exposes these secvars even
>> in the static key mode. This could lead to errors when reading them or
>> writing to them in the static key mode.
> would it cause an error when reading these variables or only when
> writing them?
>
> Thanks
>
> Michal

Hi Michal,
Thanks for taking a look.


Yes, when PKS is enabled without enabling dynamic key secure boot, the 
secvars
are NOT yet initialized with the default keys built into the binaries, 
and therefore
reading them will result in an error.


Now, while in static key management mode with PKS enabled, if one tries to
populate secvars that are relevant to dynamic key management, the write does
not fail as long as the "Platform KeyStore Signed Update Infrastructure" 
flag on
the HMC is enabled and the signed updates are authorized by valid PK/KEK 
keys.
However, secvars like db and grubdb populated while in static key management
mode are not used by the Partition Firmware or grub as SB_VERSION is not 
present,
i.e dynamic key secure boot has not been enabled yet. In this case, when 
there is a
transition from static key management to dynamic key management, secvars 
with
the signed update policy bit set will not be overwritten by the 
hypervisor with the
default keys. Now, if the keys written into these secvars were not the 
ones that were
used to sign the grub and kernel, it would fail to verify them.


These are the reasons behind the decision to expose only those secvars 
that are
relevant to the key management mode.

>
>
>> Expose only PK, trustedcadb, and moduledb in the static key mode to
>> enable loading of signed third-party kernel modules.
>>
>> Co-developed-by: Souradeep <soura at imap.linux.ibm.com>
>> Signed-off-by: Souradeep <soura at imap.linux.ibm.com>
>> Signed-off-by: Srish Srinivasan <ssrish at linux.ibm.com>
>> Reviewed-by: Mimi Zohar <zohar at linux.ibm.com>
>> Reviewed-by: Stefan Berger <stefanb at linux.ibm.com>
>> Reviewed-by: Nayna Jain <nayna at linux.ibm.com>
>> Reviewed-by: Andrew Donnellan <ajd at linux.ibm.com>
>> ---
>>   Documentation/ABI/testing/sysfs-secvar        |  6 ++++
>>   arch/powerpc/platforms/pseries/plpks-secvar.c | 28 ++++++++++++++++---
>>   2 files changed, 30 insertions(+), 4 deletions(-)
>>
>> diff --git a/Documentation/ABI/testing/sysfs-secvar b/Documentation/ABI/testing/sysfs-secvar
>> index 45281888e520..948df3446a03 100644
>> --- a/Documentation/ABI/testing/sysfs-secvar
>> +++ b/Documentation/ABI/testing/sysfs-secvar
>> @@ -37,6 +37,12 @@ Description:	Each secure variable is represented as a directory named as
>>   		representation. The data and size can be determined by reading
>>   		their respective attribute files.
>>   
>> +		Only secvars relevant to the key management mode are exposed.
>> +		Only in the dynamic key mode can the user modify the secure boot
>> +		secvars db, dbx, grubdb, grubdbx, and sbat. PK, trustedcadb and
>> +		moduledb are the secvars common to both static and dynamic key
>> +		management modes.
>> +
>>   What:		/sys/firmware/secvar/vars/<variable_name>/size
>>   Date:		August 2019
>>   Contact:	Nayna Jain <nayna at linux.ibm.com>
>> diff --git a/arch/powerpc/platforms/pseries/plpks-secvar.c b/arch/powerpc/platforms/pseries/plpks-secvar.c
>> index 767e5e8c6990..f9e9cc40c9d0 100644
>> --- a/arch/powerpc/platforms/pseries/plpks-secvar.c
>> +++ b/arch/powerpc/platforms/pseries/plpks-secvar.c
>> @@ -59,7 +59,14 @@ static u32 get_policy(const char *name)
>>   		return PLPKS_SIGNEDUPDATE;
>>   }
>>   
>> -static const char * const plpks_var_names[] = {
>> +static const char * const plpks_var_names_static[] = {
>> +	"PK",
>> +	"moduledb",
>> +	"trustedcadb",
>> +	NULL,
>> +};
>> +
>> +static const char * const plpks_var_names_dynamic[] = {
>>   	"PK",
>>   	"KEK",
>>   	"db",
>> @@ -213,21 +220,34 @@ static int plpks_max_size(u64 *max_size)
>>   	return 0;
>>   }
>>   
>> +static const struct secvar_operations plpks_secvar_ops_static = {
>> +	.get = plpks_get_variable,
>> +	.set = plpks_set_variable,
>> +	.format = plpks_secvar_format,
>> +	.max_size = plpks_max_size,
>> +	.config_attrs = config_attrs,
>> +	.var_names = plpks_var_names_static,
>> +};
>>   
>> -static const struct secvar_operations plpks_secvar_ops = {
>> +static const struct secvar_operations plpks_secvar_ops_dynamic = {
>>   	.get = plpks_get_variable,
>>   	.set = plpks_set_variable,
>>   	.format = plpks_secvar_format,
>>   	.max_size = plpks_max_size,
>>   	.config_attrs = config_attrs,
>> -	.var_names = plpks_var_names,
>> +	.var_names = plpks_var_names_dynamic,
>>   };
>>   
>>   static int plpks_secvar_init(void)
>>   {
>> +	u8 mode;
>> +
>>   	if (!plpks_is_available())
>>   		return -ENODEV;
>>   
>> -	return set_secvar_ops(&plpks_secvar_ops);
>> +	mode = plpks_get_sb_keymgmt_mode();
>> +	if (mode)
>> +		return set_secvar_ops(&plpks_secvar_ops_dynamic);
>> +	return set_secvar_ops(&plpks_secvar_ops_static);
>>   }
>>   machine_device_initcall(pseries, plpks_secvar_init);
>> -- 
>> 2.47.1
>>
>>