Re: [PATCH RFC v2 10/12] KEYS: link system_trusted_keys to mok_trusted_keys

On Aug 5, 2021, at 7:58 AM, Mimi Zohar [off-list ref] wrote:

On Mon, 2021-07-26 at 13:13 -0400, Eric Snowberg wrote:

quoted

diff --git a/certs/system_keyring.c b/certs/system_keyring.c
index dcaf74102ab2..b27ae30eaadc 100644
--- a/certs/system_keyring.c
+++ b/certs/system_keyring.c

@@ -45,6 +45,15 @@ int restrict_link_by_builtin_trusted(struct key *dest_keyring,

				     const union key_payload *payload,
				     struct key *restriction_key)
{
+	/* If the secondary trusted keyring is not enabled, we may link
+	 * through to the mok keyring and the search may follow that link.
+	 */

Refer to section "8) Commenting" of Documentation/process/coding-
style.rst for the format of multi line comments.

Sure, I’ll fix this in the next version.

quoted

+	if (mok_trusted_keys && type == &key_type_keyring &&
+	    dest_keyring == builtin_trusted_keys &&
+	    payload == &mok_trusted_keys->payload)
+		/* Allow the mok keyring to be added to the builtin */
+		return 0;
+

Unless you're changing the meaning of the restriction, then a new
restriction needs to be defined.  In this case, please don't change the
meaning of restrict_link_by_builtin_trusted().  Instead define a new
restriction named restrict_link_by_builtin_and_ca_trusted().

Along with this

quoted

	return restrict_link_by_signature(dest_keyring, type, payload,
					  builtin_trusted_keys);
}

@@ -91,6 +100,15 @@ int restrict_link_by_builtin_and_secondary_trusted(

		/* Allow the builtin keyring to be added to the secondary */
		return 0;

+	/* If we have a secondary trusted keyring, it may contain a link
+	 * through to the mok keyring and the search may follow that link.
+	 */
+	if (mok_trusted_keys && type == &key_type_keyring &&
+	    dest_keyring == secondary_trusted_keys &&
+	    payload == &mok_trusted_keys->payload)
+		/* Allow the mok keyring to be added to the secondary */
+		return 0;
+

Similarly here, please define a new restriction maybe named
restrict_link_by_builtin_secondary_and_ca_trusted().   To avoid code
duplication, the new restriction could be a wrapper around the existing
function.

and this too.

quoted

	return restrict_link_by_signature(dest_keyring, type, payload,
					  secondary_trusted_keys);
}

@@ -321,5 +339,8 @@ void __init set_platform_trusted_keys(struct key *keyring)

void __init set_mok_trusted_keys(struct key *keyring)
{
	mok_trusted_keys = keyring;
+
+	if (key_link(system_trusted_keys, mok_trusted_keys) < 0)
+		panic("Can't link (mok) trusted keyrings\n");
}

From the thread discussion on 00/12:

Only the builtin keys should ever be on the builtin keyring.  The
builtin keyring would need to be linked to the mok keyring.  But in the
secondary keyring case, the mok keyring would be linked to the
secondary keyring, similar to how the builtin keyring is linked to the
secondary keyring.

       if (key_link(secondary_trusted_keys, builtin_trusted_keys) < 0)
               panic("Can't link trusted keyrings\n");

This part is confusing me though.

Here are some of the tests I’m performing with the current series:

Initial setup:
Create and enroll my own key into the MOK.
Sign a kernel, kernel module and IMA key with my new CA key.
Boot with lockdown enabled (to enforce sig validation).

Kernel built with CONFIG_SECONDARY_TRUSTED_KEYRING=y

$ keyctl show %:.secondary_trusted_keys
Keyring
 530463486 ---lswrv      0     0  keyring: .secondary_trusted_keys
 411466727 ---lswrv      0     0   \_ keyring: .builtin_trusted_keys
 979167715 ---lswrv      0     0   |   \_ asymmetric: Build time autogenerated kernel key: 07a56e29cfa1e21379aff2c522efff7d1963202a
 534573591 ---lswrv      0     0   |   \_ asymmetric: Oracle-CA: Oracle certificate signing key: aeefb4bfde095cacaabff81dd266974b1b4e23b8
 968109018 ---lswrv      0     0   \_ keyring: .mok
 857795115 ---lswrv      0     0       \_ asymmetric: Erics-CA: UEK signing key: 9bfa6860483aa46bd83f7fa1289d9fc35799e93b

With this setup I can:
* load a kernel module signed with my CA key
* run "kexec -ls" with the kernel signed with my CA key
* run "kexec -ls" with a kernel signed by a key in the platform keyring
* load another key into the secondary trusted keyring that is signed by my CA key
* load a key into the ima keyring, signed by my CA key

Kernel built without CONFIG_SECONDARY_TRUSTED_KEYRING defined

$ keyctl show %:.builtin_trusted_keys
Keyring
 812785375 ---lswrv      0     0  keyring: .builtin_trusted_keys
 455418681 ---lswrv      0     0   \_ keyring: .mok
 910809006 ---lswrv      0     0   |   \_ asymmetric: Erics-CA: UEK signing key: 9bfa6860483aa46bd83f7fa1289d9fc35799e93b
 115345009 ---lswrv      0     0   \_ asymmetric: Oracle-CA: Oracle certificate signing key: aeefb4bfde095cacaabff81dd266974b1b4e23b8
 513131506 ---lswrv      0     0   \_ asymmetric: Build time autogenerated kernel key: 22353509f203b55b84f15d0aadeddc134b646185

With this setup I can:
* load a kernel module signed with my CA key
* run "kexec -ls" with the kernel signed with my CA key
* run "kexec -ls" with a kernel signed by a key in the platform keyring
* load a key into the ima keyring, signed by my CA key

So why would the linking need to be switched?  Is there a test I’m
missing?  Thanks.