[crypto] Add DER image format

Add DER-encoded ASN.1 as an image format.  There is no fixed signature
for DER files.  We treat an image as DER if it comprises a single
valid SEQUENCE object covering the entire length of the image.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/image/der.c

@@ -0,0 +1,120 @@
+/*
+ * Copyright (C) 2016 Michael Brown <mbrown@fensystems.co.uk>.
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+ * 02110-1301, USA.
+ *
+ * You can also choose to distribute this program under the terms of
+ * the Unmodified Binary Distribution Licence (as given in the file
+ * COPYING.UBDL), provided that you have satisfied its requirements.
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER_OR_UBDL );
+
+#include <stdlib.h>
+#include <errno.h>
+#include <assert.h>
+#include <ipxe/asn1.h>
+#include <ipxe/der.h>
+#include <ipxe/uaccess.h>
+#include <ipxe/image.h>
+
+/** @file
+ *
+ * DER-encoded ASN.1 data
+ *
+ */
+
+/**
+ * Extract ASN.1 object from image
+ *
+ * @v image		DER image
+ * @v offset		Offset within image
+ * @v cursor		ASN.1 cursor to fill in
+ * @ret next		Offset to next image, or negative error
+ *
+ * The caller is responsible for eventually calling free() on the
+ * allocated ASN.1 cursor.
+ */
+static int der_asn1 ( struct image *image, size_t offset __unused,
+		      struct asn1_cursor **cursor ) {
+	void *data;
+
+	/* Allocate cursor and data buffer */
+	*cursor = malloc ( sizeof ( **cursor ) + image->len );
+	if ( ! *cursor )
+		return -ENOMEM;
+	data = ( ( ( void * ) *cursor ) + sizeof ( **cursor ) );
+
+	/* Populate cursor and data buffer */
+	(*cursor)->data = data;
+	(*cursor)->len = image->len;
+	copy_from_user ( data, image->data, 0, image->len );
+
+	return image->len;
+}
+
+/**
+ * Probe DER image
+ *
+ * @v image		DER image
+ * @ret rc		Return status code
+ */
+static int der_probe ( struct image *image ) {
+	struct asn1_cursor cursor;
+	uint8_t buf[8];
+	size_t extra;
+	size_t total;
+	int len;
+	int rc;
+
+	/* Sanity check: no realistic DER image can be smaller than this */
+	if ( image->len < sizeof ( buf ) )
+		return -ENOEXEC;
+
+	/* Prepare partial cursor */
+	cursor.data = buf;
+	cursor.len = sizeof ( buf );
+	copy_from_user ( buf, image->data, 0, sizeof ( buf ) );
+	extra = ( image->len - sizeof ( buf ) );
+
+	/* Get length of ASN.1 sequence */
+	len = asn1_start ( &cursor, ASN1_SEQUENCE, extra );
+	if ( len < 0 ) {
+		rc = len;
+		DBGC ( image, "DER %s is not valid ASN.1: %s\n",
+		       image->name, strerror ( rc ) );
+		return rc;
+	}
+
+	/* Add length of tag and length bytes consumed by asn1_start() */
+	total = ( len + ( cursor.data - ( ( void * ) buf ) ) );
+	assert ( total <= image->len );
+
+	/* Check that image comprises a single well-formed ASN.1 object */
+	if ( total != image->len ) {
+		DBGC ( image, "DER %s is not single ASN.1\n", image->name );
+		return -ENOEXEC;
+	}
+
+	return 0;
+}
+
+/** DER image type */
+struct image_type der_image_type __image_type ( PROBE_NORMAL ) = {
+	.name = "DER",
+	.probe = der_probe,
+	.asn1 = der_asn1,
+};