KEYS: X.509: Parse Key Usage

Parse the X.509 Key Usage.  The key usage extension defines the purpose of
the key contained in the certificate.

   id-ce-keyUsage OBJECT IDENTIFIER ::=  { id-ce 15 }

      KeyUsage ::= BIT STRING {
           digitalSignature        (0),
           contentCommitment       (1),
           keyEncipherment         (2),
           dataEncipherment        (3),
           keyAgreement            (4),
           keyCertSign             (5),
           cRLSign                 (6),
           encipherOnly            (7),
           decipherOnly            (8) }

If the keyCertSign or digitalSignature is set, store it in the
public_key structure. Having the purpose of the key being stored
during parsing, allows enforcement on the usage field in the future.
This will be used in a follow on patch that requires knowing the
certificate key usage type.

Link: https://www.rfc-editor.org/rfc/rfc5280#section-4.2.1.3
Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>

1 files changed, 28 insertions, 0 deletions

diff --git a/crypto/asymmetric_keys/x509_cert_parser.c b/crypto/asymmetric_keys/x509_cert_parser.c
index 77547d4bd94d..0a7049b470c1 100644
--- a/crypto/asymmetric_keys/x509_cert_parser.c
+++ b/crypto/asymmetric_keys/x509_cert_parser.c
@@ -579,6 +579,34 @@ int x509_process_extension(void *context, size_t hdrlen,
 		return 0;
 	}
 
+	if (ctx->last_oid == OID_keyUsage) {
+		/*
+		 * Get hold of the keyUsage bit string
+		 * v[1] is the encoding size
+		 *       (Expect either 0x02 or 0x03, making it 1 or 2 bytes)
+		 * v[2] is the number of unused bits in the bit string
+		 *       (If >= 3 keyCertSign is missing when v[1] = 0x02)
+		 * v[3] and possibly v[4] contain the bit string
+		 *
+		 * From RFC 5280 4.2.1.3:
+		 *   0x04 is where keyCertSign lands in this bit string
+		 *   0x80 is where digitalSignature lands in this bit string
+		 */
+		if (v[0] != ASN1_BTS)
+			return -EBADMSG;
+		if (vlen < 4)
+			return -EBADMSG;
+		if (v[2] >= 8)
+			return -EBADMSG;
+		if (v[3] & 0x80)
+			ctx->cert->pub->key_eflags |= 1 << KEY_EFLAG_DIGITALSIG;
+		if (v[1] == 0x02 && v[2] <= 2 && (v[3] & 0x04))
+			ctx->cert->pub->key_eflags |= 1 << KEY_EFLAG_KEYCERTSIGN;
+		else if (vlen > 4 && v[1] == 0x03 && (v[3] & 0x04))
+			ctx->cert->pub->key_eflags |= 1 << KEY_EFLAG_KEYCERTSIGN;
+		return 0;
+	}
+
 	if (ctx->last_oid == OID_authorityKeyIdentifier) {
 		/* Get hold of the CA key fingerprint */
 		ctx->raw_akid = v;