fix(auth): properly validate X.509 extensions

get_ext() does not check the return value of the various mbedtls_*
functions, as cert_parse() is assumed to have guaranteed that they will
always succeed.  However, it passes the end of an extension as the end
pointer to these functions, whereas cert_parse() passes the end of the
TBSCertificate.  Furthermore, cert_parse() does *not* check that the
contents of the extension have the same length as the extension itself.
Before fd37982 ("fix(auth): forbid junk after extensions"),
cert_parse() also does not check that the extension block extends to the
end of the TBSCertificate.

This is a problem, as mbedtls_asn1_get_tag() leaves *p and *len
undefined on failure.  In practice, this results in get_ext() continuing
to parse at different offsets than were used (and validated) by
cert_parse(), which means that the in-bounds guarantee provided by
cert_parse() no longer holds.

This patch fixes the remaining flaw by enforcing that the contents of an
extension are the same length as the extension itself.

Change-Id: Id4570f911402e34d5d6c799ae01a01f184c68d7c
Signed-off-by: Demi Marie Obenour <demiobenour@gmail.com>
Signed-off-by: Sandrine Bailleux <sandrine.bailleux@arm.com>

Author: DemiMarie

drivers/auth/mbedtls/mbedtls_x509_parser.c

@@ -329,33 +329,39 @@ static int cert_parse(void *img, unsigned int img_len)
 	 * in the boot chain.
 	 */
 	do {
+		unsigned char *end_ext_data;
+
 		ret = mbedtls_asn1_get_tag(&p, end, &len,
 					   MBEDTLS_ASN1_CONSTRUCTED |
 					   MBEDTLS_ASN1_SEQUENCE);
 		if (ret != 0) {
 			return IMG_PARSER_ERR_FORMAT;
 		}
+		end_ext_data = p + len;
 
 		/* Get extension ID */
-		ret = mbedtls_asn1_get_tag(&p, end, &len, MBEDTLS_ASN1_OID);
+		ret = mbedtls_asn1_get_tag(&p, end_ext_data, &len, MBEDTLS_ASN1_OID);
 		if (ret != 0) {
 			return IMG_PARSER_ERR_FORMAT;
 		}
 		p += len;
 
 		/* Get optional critical */
-		ret = mbedtls_asn1_get_bool(&p, end, &is_critical);
+		ret = mbedtls_asn1_get_bool(&p, end_ext_data, &is_critical);
 		if ((ret != 0) && (ret != MBEDTLS_ERR_ASN1_UNEXPECTED_TAG)) {
 			return IMG_PARSER_ERR_FORMAT;
 		}
 
-		/* Data should be octet string type */
-		ret = mbedtls_asn1_get_tag(&p, end, &len,
+		/*
+		 * Data should be octet string type and must use all bytes in
+		 * the Extension.
+		 */
+		ret = mbedtls_asn1_get_tag(&p, end_ext_data, &len,
 					   MBEDTLS_ASN1_OCTET_STRING);
-		if (ret != 0) {
+		if ((ret != 0) || ((p + len) != end_ext_data)) {
 			return IMG_PARSER_ERR_FORMAT;
 		}
-		p += len;
+		p = end_ext_data;
 	} while (p < end);
 
 	if (p != end) {