[crypto] Treat high tag numbers as invalid

ASN.1 allows for multi-byte tag numbers by setting the low five bits
of the first tag byte to 0x1f.  No tag that we need to handle has this
format, and the existing checks for specific tag numbers will already
fail to match against such a tag (treating it as a normal single-byte
tag number).

Refuse to parse any tag with a high tag number format, to guard
against future bugs that could arise because the tag length would be
calculated incorrectly.

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

src/crypto/asn1.c

@@ -151,6 +151,10 @@ FILE_SECBOOT ( PERMITTED );
 	__einfo_error ( EINFO_ENOTSUP_ALGORITHM )
 #define EINFO_ENOTSUP_ALGORITHM \
 	__einfo_uniqify ( EINFO_ENOTSUP, 0x01, "Unsupported algorithm" )
+#define ENOTSUP_HIGH \
+	__einfo_error ( EINFO_ENOTSUP_HIGH )
+#define EINFO_ENOTSUP_HIGH \
+	__einfo_uniqify ( EINFO_ENOTSUP, 0x02, "Unsupported high tag number" )
 #define ENOTTY_ALGORITHM \
 	__einfo_error ( EINFO_ENOTTY_ALGORITHM )
 #define EINFO_ENOTTY_ALGORITHM \
@@ -185,6 +189,14 @@ static int asn1_start ( struct asn1_cursor *cursor, unsigned int type ) {
 		return -EINVAL_ASN1_EMPTY;
 	}
 
+	/* Refuse to handle multi-byte tags */
+	if ( ( asn1_type ( cursor ) & ASN1_HIGH ) == ASN1_HIGH ) {
+		DBGC ( cursor, "ASN1 %p has unsupported high tag number\n",
+		       cursor );
+		asn1_invalidate_cursor ( cursor );
+		return -ENOTSUP_HIGH;
+	}
+
 	/* Check the tag byte */
 	if ( ( type != ASN1_ANY ) && ( type != asn1_type ( cursor ) ) ) {
 		DBGC ( cursor, "ASN1 %p type mismatch (expected %d, got %d)\n",

src/include/ipxe/asn1.h

@@ -86,6 +86,9 @@ struct asn1_builder_header {
 /** ASN.1 generalized time */
 #define ASN1_GENERALIZED_TIME 0x18
 
+/** ASN.1 high tag number */
+#define ASN1_HIGH 0x1f
+
 /** ASN.1 sequence */
 #define ASN1_SEQUENCE 0x30