Re: [PATCH 4/7] KEYS: Introduce a builtin root of trust key flag

[Eric Snowberg]

> On Apr 8, 2022, at 12:49 PM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
> 
> On Fri, 2022-04-08 at 17:34 +0000, Eric Snowberg wrote:
>> 
>>> On Apr 8, 2022, at 10:55 AM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
>>> 
>>> On Fri, 2022-04-08 at 15:27 +0000, Eric Snowberg wrote:
>>>> 
>>>>> On Apr 8, 2022, at 8:40 AM, Mimi Zohar <zohar@xxxxxxxxxxxxx> wrote:
>>>>> 
>>>>> On Tue, 2022-04-05 at 21:53 -0400, Eric Snowberg wrote:
>>>>>> 
>>>>>> The first type of key to use this is X.509.  When a X.509 certificate
>>>>>> is self signed, has the kernCertSign Key Usage set and contains the
>>>>>> CA bit set this new flag is set.
>>>>>> 
>>>>>> Signed-off-by: Eric Snowberg <eric.snowberg@xxxxxxxxxx>
>>>>>> 
>>>>>> diff --git a/include/linux/key.h b/include/linux/key.h
>>>>>> index 7febc4881363..97f6a1f86a27 100644
>>>>>> --- a/include/linux/key.h
>>>>>> +++ b/include/linux/key.h
>>>>>> @@ -230,6 +230,7 @@ struct key {
>>>>>> #define KEY_FLAG_ROOT_CAN_INVAL	7	/* set if key can be invalidated by root without permission */
>>>>>> #define KEY_FLAG_KEEP		8	/* set if key should not be removed */
>>>>>> #define KEY_FLAG_UID_KEYRING	9	/* set if key is a user or user session keyring */
>>>>>> +#define KEY_FLAG_BUILTIN_ROT	10	/* set if key is a builtin Root of Trust key */
>>>>>> 
>>>>>> 	/* the key type and key description string
>>>>>> 	 * - the desc is used to match a key against search criteria
>>>>>> @@ -290,6 +291,7 @@ extern struct key *key_alloc(struct key_type *type,
>>>>>> #define KEY_ALLOC_BYPASS_RESTRICTION	0x0008	/* Override the check on restricted keyrings */
>>>>>> #define KEY_ALLOC_UID_KEYRING		0x0010	/* allocating a user or user session keyring */
>>>>>> #define KEY_ALLOC_SET_KEEP		0x0020	/* Set the KEEP flag on the key/keyring */
>>>>>> +#define KEY_ALLOC_BUILT_IN_ROT		0x0040  /* Add builtin root of trust key */
>>>>> 
>>>>> Since the concept of root of trust is not generic, but limited to
>>>>> specific keyrings, the root CA certificate signing keys on the
>>>>> "machine" keyring need to be identified.  Similar to the
>>>>> KEY_ALLOC_BUILT_IN/KEY_FLAG_BUILTIN, new flags
>>>>> KEY_ALLOC_MACHINE/KEY_FLAG_MACHINE should be defined instead.
>>>> 
>>>> I’m open to renaming these, however this name change seems confusing to me.  
>>>> This flag gets set when the X.509 certificate contains the three CA requirements 
>>>> identified above.  The remaining keys in the machine keyring can be used for 
>>>> anything else.
>>> 
>>> Renaming the flag to KEY_ALLOC_MACHINE/KEY_FLAG_MACHINE differentiates
>>> between the "builtin" keys from the "machine" keys.  The trust models
>>> are very different.
>> 
>> Isn’t the trust model the same for machine and secondary keys?  Both are supplied by 
>> the end-user. That is why I’m confused by naming something _MACHINE when it applies 
>> to more than one keyring.
> 
> True both are supplied by the end-user, but the trust models are
> different.

I think I need more information here, I’m not seeing how they are different trust 
models.

>  In one case the certificates are coming indirectly from
> firmware,

Any kernel signed by a cert in the MokList will boot.  The very thing the machine 
keyring contains.

For example, if a user has a cert (CA bit set false, keyCertSign not set, and it isn’t 
self signed), they can use insert-sys-cert to get it into their kernel.  They can then 
sign the kernel with any key in their MokList.  Why would we want to treat this key 
different if it was injected into the kernel verses coming in through the machine 
keyring? 

I can see the desire to have a root of trust all the way back to the root CA.  What 
I can’t see is if we ignore this for certain keyrings.

> while in the other case the certificates would be limited to
> certificates signed by the initial firmware certificates.  Loading only
> root-CA signing key certificates onto the "machine" keyring highlights
> and enforces the different types of trust.

If the root-CA cert must contain keyCertSign, I don’t see the point in loading only 
root-CA certs either. Why would we want to prevent  a code signing cert with the 
CA bit set from loading into the machine keyring?  A code signing cert should be 
allowed to validate a kernel module, but It should not be allowed to validate other
certs.