[crypto] Accept OCSP responses containing multiple certificates

RFC2560 mandates that a valid OCSP response will contain exactly one
relevant certificate.  However, some OCSP responders include
extraneous certificates.  iPXE currently assumes that the first
certificate in the OCSP response is the relevant certificate; OCSP
checks will therefore fail if the responder includes the extraneous
certificates before the relevant certificate.

Fix by using the responder ID to identify the relevant certificate.

Reported-by: Christian Stroehmeier <stroemi@mail.uni-paderborn.de>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/crypto/ocsp.c

 #define EINFO_EACCES_STALE						\
 	__einfo_uniqify ( EINFO_EACCES, 0x04,				\
 			  "Stale (or premature) OCSP repsonse" )
+#define EACCES_NO_RESPONDER						\
+	__einfo_error ( EINFO_EACCES_NO_RESPONDER )
+#define EINFO_EACCES_NO_RESPONDER					\
+	__einfo_uniqify ( EINFO_EACCES, 0x05,				\
+			  "Missing OCSP responder certificate" )
+#define ENOTSUP_RESPONSE_TYPE						\
+	__einfo_error ( EINFO_ENOTSUP_RESPONSE_TYPE )
+#define EINFO_ENOTSUP_RESPONSE_TYPE					\
+	__einfo_uniqify ( EINFO_ENOTSUP, 0x01,				\
+			  "Unsupported OCSP response type" )
+#define ENOTSUP_RESPONDER_ID						\
+	__einfo_error ( EINFO_ENOTSUP_RESPONDER_ID )
+#define EINFO_ENOTSUP_RESPONDER_ID					\
+	__einfo_uniqify ( EINFO_ENOTSUP, 0x02,				\
+			  "Unsupported OCSP responder ID" )
 #define EPROTO_MALFORMED_REQUEST					\
 	__einfo_error ( EINFO_EPROTO_MALFORMED_REQUEST )
 #define EINFO_EPROTO_MALFORMED_REQUEST					\
 		DBGC ( ocsp, "OCSP %p \"%s\" response type not supported:\n",
 		       ocsp, ocsp->cert->subject.name );
 		DBGC_HDA ( ocsp, 0, cursor.data, cursor.len );
-		return -ENOTSUP;
+		return -ENOTSUP_RESPONSE_TYPE;
 	}
 
 	return 0;
 }
 
+/**
+ * Compare responder's certificate name
+ *
+ * @v ocsp		OCSP check
+ * @v cert		Certificate
+ * @ret difference	Difference as returned by memcmp()
+ */
+static int ocsp_compare_responder_name ( struct ocsp_check *ocsp,
+					 struct x509_certificate *cert ) {
+	struct ocsp_responder *responder = &ocsp->response.responder;
+
+	/* Compare responder ID with certificate's subject */
+	return asn1_compare ( &responder->id, &cert->subject.raw );
+}
+
+/**
+ * Compare responder's certificate public key hash
+ *
+ * @v ocsp		OCSP check
+ * @v cert		Certificate
+ * @ret difference	Difference as returned by memcmp()
+ */
+static int ocsp_compare_responder_key_hash ( struct ocsp_check *ocsp,
+					     struct x509_certificate *cert ) {
+	struct ocsp_responder *responder = &ocsp->response.responder;
+	uint8_t ctx[SHA1_CTX_SIZE];
+	uint8_t digest[SHA1_DIGEST_SIZE];
+	int difference;
+
+	/* Sanity check */
+	difference = ( sizeof ( digest ) - responder->id.len );
+	if ( difference )
+		return difference;
+
+	/* Generate SHA1 hash of certificate's public key */
+	digest_init ( &sha1_algorithm, ctx );
+	digest_update ( &sha1_algorithm, ctx,
+			cert->subject.public_key.raw_bits.data,
+			cert->subject.public_key.raw_bits.len );
+	digest_final ( &sha1_algorithm, ctx, digest );
+
+	/* Compare responder ID with SHA1 hash of certificate's public key */
+	return memcmp ( digest, responder->id.data, sizeof ( digest ) );
+}
+
+/**
+ * Parse OCSP responder ID
+ *
+ * @v ocsp		OCSP check
+ * @v raw		ASN.1 cursor
+ * @ret rc		Return status code
+ */
+static int ocsp_parse_responder_id ( struct ocsp_check *ocsp,
+				     const struct asn1_cursor *raw ) {
+	struct ocsp_responder *responder = &ocsp->response.responder;
+	struct asn1_cursor *responder_id = &responder->id;
+	unsigned int type;
+
+	/* Enter responder ID */
+	memcpy ( responder_id, raw, sizeof ( *responder_id ) );
+	type = asn1_type ( responder_id );
+	asn1_enter_any ( responder_id );
+
+	/* Identify responder ID type */
+	switch ( type ) {
+	case ASN1_EXPLICIT_TAG ( 1 ) :
+		DBGC2 ( ocsp, "OCSP %p \"%s\" responder identified by name\n",
+			ocsp, ocsp->cert->subject.name );
+		responder->compare = ocsp_compare_responder_name;
+		return 0;
+	case ASN1_EXPLICIT_TAG ( 2 ) :
+		DBGC2 ( ocsp, "OCSP %p \"%s\" responder identified by key "
+			"hash\n", ocsp, ocsp->cert->subject.name );
+		responder->compare = ocsp_compare_responder_key_hash;
+		return 0;
+	default:
+		DBGC ( ocsp, "OCSP %p \"%s\" unsupported responder ID type "
+		       "%d\n", ocsp, ocsp->cert->subject.name, type );
+		return -ENOTSUP_RESPONDER_ID;
+	}
+}
+
 /**
  * Parse OCSP certificate ID
  *
 	/* Skip version, if present */
 	asn1_skip_if_exists ( &cursor, ASN1_EXPLICIT_TAG ( 0 ) );
 
-	/* Skip responderID */
+	/* Parse responderID */
+	if ( ( rc = ocsp_parse_responder_id ( ocsp, &cursor ) ) != 0 )
+		return rc;
 	asn1_skip_any ( &cursor );
 
 	/* Skip producedAt */
 			      const struct asn1_cursor *raw ) {
 	struct ocsp_response *response = &ocsp->response;
 	struct asn1_cursor cursor;
+	struct x509_certificate *cert;
 	int rc;
 
 	/* Enter certs */
 	 * multiple certificates, but the protocol requires that the
 	 * OCSP signing certificate must either be the issuer itself,
 	 * or must be directly issued by the issuer (see RFC2560
-	 * section 4.2.2.2 "Authorized Responders").
+	 * section 4.2.2.2 "Authorized Responders").  We therefore
+	 * need to identify only the single certificate matching the
+	 * Responder ID.
 	 */
-	if ( ( cursor.len != 0 ) &&
-	     ( ( rc = x509_certificate ( cursor.data, cursor.len,
-					 &response->signer ) ) != 0 ) ) {
-		DBGC ( ocsp, "OCSP %p \"%s\" could not parse certificate: "
-		       "%s\n", ocsp, ocsp->cert->subject.name, strerror ( rc ));
-		DBGC_HDA ( ocsp, 0, cursor.data, cursor.len );
-		return rc;
+	while ( cursor.len ) {
+
+		/* Parse certificate */
+		if ( ( rc = x509_certificate ( cursor.data, cursor.len,
+					       &cert ) ) != 0 ) {
+			DBGC ( ocsp, "OCSP %p \"%s\" could not parse "
+			       "certificate: %s\n", ocsp,
+			       ocsp->cert->subject.name, strerror ( rc ) );
+			DBGC_HDA ( ocsp, 0, cursor.data, cursor.len );
+			return rc;
+		}
+
+		/* Use if this certificate matches the responder ID */
+		if ( response->responder.compare ( ocsp, cert ) == 0 ) {
+			response->signer = cert;
+			DBGC2 ( ocsp, "OCSP %p \"%s\" response is signed by "
+				"\"%s\"\n", ocsp, ocsp->cert->subject.name,
+				response->signer->subject.name );
+			return 0;
+		}
+
+		/* Otherwise, discard this certificate */
+		x509_put ( cert );
+		asn1_skip_any ( &cursor );
 	}
-	DBGC2 ( ocsp, "OCSP %p \"%s\" response is signed by \"%s\"\n", ocsp,
-		ocsp->cert->subject.name, response->signer->subject.name );
 
-	return 0;
+	DBGC ( ocsp, "OCSP %p \"%s\" missing responder certificate\n",
+	       ocsp, ocsp->cert->subject.name );
+	return -EACCES_NO_RESPONDER;
 }
 
 /**

src/include/ipxe/ocsp.h

 #define OCSP_STATUS_SIG_REQUIRED	0x05
 #define OCSP_STATUS_UNAUTHORIZED	0x06
 
+struct ocsp_check;
+
 /** An OCSP request */
 struct ocsp_request {
 	/** Request builder */
 	struct asn1_cursor cert_id;
 };
 
+/** An OCSP responder */
+struct ocsp_responder {
+	/**
+	 * Check if certificate is the responder's certificate
+	 *
+	 * @v ocsp		OCSP check
+	 * @v cert		Certificate
+	 * @ret difference	Difference as returned by memcmp()
+	 */
+	int ( * compare ) ( struct ocsp_check *ocsp,
+			    struct x509_certificate *cert );
+	/** Responder ID */
+	struct asn1_cursor id;
+};
+
 /** An OCSP response */
 struct ocsp_response {
 	/** Raw response */
 	void *data;
 	/** Raw tbsResponseData */
 	struct asn1_cursor tbs;
+	/** Responder */
+	struct ocsp_responder responder;
 	/** Time at which status is known to be correct */
 	time_t this_update;
 	/** Time at which newer status information will be available */