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

[Mimi Zohar]

On Tue, 2022-04-05 at 21:53 -0400, Eric Snowberg wrote:
> Some subsystems are interested in knowing if keys within a keyring could
> be used as a foundation of a root of trust.  Introduce a new builtin root
> of trust key flag.

Unfortunately a root of trust is not something that can simply be built
into a certificate.  Roots of trust are normally established based on
HW.  The root of trust for the "builtin_trusted_keys" is established
for systems with secure boot enabled by verifying the signature chain
of trust up to and including the kernel image's signature.  Similarly,
the root of trust for keys on the "secondary_trusted_keys" is based on
all certificates being signed by a key on the "builtin_trusted_keys"
keyring or other keys on the "secondary_trusted_keys" keyring.

Defining a new variable claiming that a root-ca with cert signing usage
on any keyring is a root of trust is just wrong.

> 
> 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.

thanks,

Mimi