---
v1: Initial version
v2: Removed secondary keyring references
v3: Removed restrict_link_by_system_trusted_or_ca
    Simplify restrict_link_by_ca - only see if the key is a CA
    Did not add __init in front of restrict_link_by_ca in case
      restriction could be resued in the future
v6: Unmodified from v3
v7: Check for CA restruction in public key
v8: Fix issue found by build bot when asym keys not defined in the config
---
 crypto/asymmetric_keys/restrict.c | 43 +++++++++++++++++++++++++++++++
 include/crypto/public_key.h       | 15 +++++++++++
 2 files changed, 58 insertions(+)

diff --git a/crypto/asymmetric_keys/restrict.c b/crypto/asymmetric_keys/restrict.c
index 84cefe3b3585..a891c598a2aa 100644
--- a/crypto/asymmetric_keys/restrict.c
+++ b/crypto/asymmetric_keys/restrict.c

@@ -108,6 +108,49 @@ int restrict_link_by_signature(struct key *dest_keyring,

        return ret;
 }
 
+/**
+ * restrict_link_by_ca - Restrict additions to a ring of CA keys
+ * @dest_keyring: Keyring being linked to.
+ * @type: The type of key being added.
+ * @payload: The payload of the new key.
+ * @trust_keyring: Unused.
+ *
+ * Check if the new certificate is a CA. If it is a CA, then mark the new
+ * certificate as being ok to link.
+ *
+ * Returns 0 if the new certificate was accepted, -ENOKEY if the
+ * certificate is not a CA. -ENOPKG if the signature uses unsupported
+ * crypto, or some other error if there is a matching certificate but
+ * the signature check cannot be performed.
+ */
+int restrict_link_by_ca(struct key *dest_keyring,
+                       const struct key_type *type,
+                       const union key_payload *payload,
+                       struct key *trust_keyring)
+{
+       const struct public_key_signature *sig;
+       const struct public_key *pkey;
+
+       if (type != &key_type_asymmetric)
+               return -EOPNOTSUPP;
+
+       sig = payload->data[asym_auth];
+       if (!sig)
+               return -ENOPKG;
+
+       if (!sig->auth_ids[0] && !sig->auth_ids[1])
+               return -ENOKEY;
+
+       pkey = payload->data[asym_crypto];
+       if (!pkey)
+               return -ENOPKG;
+
+       if (!pkey->key_is_ca)
+               return -ENOKEY;
+
+       return public_key_verify_signature(pkey, sig);
+}
+
 static bool match_either_id(const struct asymmetric_key_ids *pair,
                            const struct asymmetric_key_id *single)
 {

diff --git a/include/crypto/public_key.h b/include/crypto/public_key.h
index 72dcbc06ef9c..06e34d3340c4 100644
--- a/include/crypto/public_key.h
+++ b/include/crypto/public_key.h

@@ -72,6 +72,21 @@ extern int restrict_link_by_key_or_keyring_chain(struct key *trust_keyring,

                                                 const union key_payload *payload,
                                                 struct key *trusted);
 
+#if IS_REACHABLE(CONFIG_ASYMMETRIC_KEY_TYPE)
+extern int restrict_link_by_ca(struct key *dest_keyring,
+                              const struct key_type *type,
+                              const union key_payload *payload,
+                              struct key *trust_keyring);
+#else
+static inline int restrict_link_by_ca(struct key *dest_keyring,
+                                     const struct key_type *type,
+                                     const union key_payload *payload,
+                                     struct key *trust_keyring)
+{
+       return 0;
+}
+#endif
+
 extern int query_asymmetric_key(const struct kernel_pkey_params *,
                                struct kernel_pkey_query *);