certs: Only allow certs signed by keys on the builtin keyring

Originally the secondary trusted keyring provided a keyring to which extra
keys may be added, provided those keys were not blacklisted and were
vouched for by a key built into the kernel or already in the secondary
trusted keyring.

On systems with the machine keyring configured, additional keys may also
be vouched for by a key on the machine keyring.

Prevent loading additional certificates directly onto the secondary
keyring, vouched for by keys on the machine keyring, yet allow these
certificates to be loaded onto other trusted keyrings.

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

diff --git a/certs/Kconfig b/certs/Kconfig
index 1f109b0..6203697 100644 (file)
--- a/certs/Kconfig
+++ b/certs/Kconfig
@@ -88,7 +88,21 @@ config SECONDARY_TRUSTED_KEYRING
        help
          If set, provide a keyring to which extra keys may be added, provided
          those keys are not blacklisted and are vouched for by a key built
-         into the kernel or already in the secondary trusted keyring.
+         into the kernel, machine keyring (if configured), or already in the
+         secondary trusted keyring.
+
+config SECONDARY_TRUSTED_KEYRING_SIGNED_BY_BUILTIN
+       bool "Only allow additional certs signed by keys on the builtin trusted keyring"
+       depends on SECONDARY_TRUSTED_KEYRING
+       help
+         If set, only certificates signed by keys on the builtin trusted
+         keyring may be loaded onto the secondary trusted keyring.
+
+         Note: The machine keyring, if configured, will be linked to the
+         secondary keyring.  When enabling this option, it is recommended
+         to also configure INTEGRITY_CA_MACHINE_KEYRING_MAX to prevent
+         linking code signing keys with imputed trust to the secondary
+         trusted keyring.
 
 config SYSTEM_BLACKLIST_KEYRING
        bool "Provide system-wide ring of blacklisted keys"

diff --git a/crypto/asymmetric_keys/restrict.c b/crypto/asymmetric_keys/restrict.c
index 6b69ea4..afcd4d1 100644 (file)
--- a/crypto/asymmetric_keys/restrict.c
+++ b/crypto/asymmetric_keys/restrict.c
@@ -102,6 +102,10 @@ int restrict_link_by_signature(struct key *dest_keyring,
 
        if (use_builtin_keys && !test_bit(KEY_FLAG_BUILTIN, &key->flags))
                ret = -ENOKEY;
+       else if (IS_BUILTIN(CONFIG_SECONDARY_TRUSTED_KEYRING_SIGNED_BY_BUILTIN) &&
+                !strcmp(dest_keyring->description, ".secondary_trusted_keys") &&
+                !test_bit(KEY_FLAG_BUILTIN, &key->flags))
+               ret = -ENOKEY;
        else
                ret = verify_signature(key, sig);
        key_put(key);