KEYS: X.509: Parse Basic Constraints for CA

Parse the X.509 Basic Constraints.  The basic constraints extension
identifies whether the subject of the certificate is a CA.

BasicConstraints ::= SEQUENCE {
        cA                      BOOLEAN DEFAULT FALSE,
        pathLenConstraint       INTEGER (0..MAX) OPTIONAL }

If the CA is true, store it in the public_key.  This will be used
in a follow on patch that requires knowing if the public key is a CA.

Link: https://www.rfc-editor.org/rfc/rfc5280#section-4.2.1.9
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>

crypto/asymmetric_keys/x509_cert_parser.c

@@ -586,6 +586,28 @@ int x509_process_extension(void *context, size_t hdrlen,
 		return 0;
 	}
 
+	if (ctx->last_oid == OID_basicConstraints) {
+		/*
+		 * Get hold of the basicConstraints
+		 * v[1] is the encoding size
+		 *	(Expect 0x2 or greater, making it 1 or more bytes)
+		 * v[2] is the encoding type
+		 *	(Expect an ASN1_BOOL for the CA)
+		 * v[3] is the contents of the ASN1_BOOL
+		 *      (Expect 1 if the CA is TRUE)
+		 * vlen should match the entire extension size
+		 */
+		if (v[0] != (ASN1_CONS_BIT | ASN1_SEQ))
+			return -EBADMSG;
+		if (vlen < 2)
+			return -EBADMSG;
+		if (v[1] != vlen - 2)
+			return -EBADMSG;
+		if (vlen >= 4 && v[1] != 0 && v[2] == ASN1_BOOL && v[3] == 1)
+			ctx->cert->pub->key_eflags |= 1 << KEY_EFLAG_CA;
+		return 0;
+	}
+
 	return 0;
 }
 

include/crypto/public_key.h

@@ -28,6 +28,8 @@ struct public_key {
 	bool key_is_private;
 	const char *id_type;
 	const char *pkey_algo;
+	unsigned long key_eflags;	/* key extension flags */
+#define KEY_EFLAG_CA		0	/* set if the CA basic constraints is set */
 };
 
 extern void public_key_free(struct public_key *key);