KEYS: X.509: Fix Basic Constraints CA flag parsing

Fix the X.509 Basic Constraints CA flag parsing to correctly handle
the ASN.1 DER encoded structure. The parser was incorrectly treating
the length field as the boolean value.

Per RFC 5280 section 4.1, X.509 certificates must use ASN.1 DER encoding.
According to ITU-T X.690, a DER-encoded BOOLEAN is represented as:

Tag (0x01), Length (0x01), Value (0x00 for FALSE, 0xFF for TRUE)

The basicConstraints extension with CA:TRUE is encoded as:

  SEQUENCE (0x30) | Length | BOOLEAN (0x01) | Length (0x01) | Value (0xFF)
                             ^-- v[2]         ^-- v[3]        ^-- v[4]

The parser was checking v[3] (the length field, always 0x01) instead
of v[4] (the actual boolean value, 0xFF for TRUE in DER encoding).

Also handle the case where the extension is an empty SEQUENCE (30 00),
which is valid for CA:FALSE when the default value is omitted as
required by DER encoding rules (X.690 section 11.5).

Per ITU-T X.690-0207:
- Section 11.5: Default values must be omitted in DER
- Section 11.1: DER requires TRUE to be encoded as 0xFF

Link: https://datatracker.ietf.org/doc/html/rfc5280
Link: https://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf
Fixes: 30eae2b ("KEYS: X.509: Parse Basic Constraints for CA")
Signed-off-by: Fan Wu <wufan@kernel.org>
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

Author: jxwufan

crypto/asymmetric_keys/x509_cert_parser.c

@@ -610,11 +610,14 @@ int x509_process_extension(void *context, size_t hdrlen,
 		/*
 		 * Get hold of the basicConstraints
 		 * v[1] is the encoding size
-		 *	(Expect 0x2 or greater, making it 1 or more bytes)
+		 *	(Expect 0x00 for empty SEQUENCE with CA:FALSE, or
+		 *	0x03 or greater for non-empty SEQUENCE)
 		 * 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)
+		 * v[3] is the length of the ASN1_BOOL
+		 *	(Expect 1 for a single byte boolean)
+		 * v[4] is the contents of the ASN1_BOOL
+		 *	(Expect 0xFF if the CA is TRUE)
 		 * vlen should match the entire extension size
 		 */
 		if (v[0] != (ASN1_CONS_BIT | ASN1_SEQ))
@@ -623,8 +626,13 @@ int x509_process_extension(void *context, size_t hdrlen,
 			return -EBADMSG;
 		if (v[1] != vlen - 2)
 			return -EBADMSG;
-		if (vlen >= 4 && v[1] != 0 && v[2] == ASN1_BOOL && v[3] == 1)
+		/* Empty SEQUENCE means CA:FALSE (default value omitted per DER) */
+		if (v[1] == 0)
+			return 0;
+		if (vlen >= 5 && v[2] == ASN1_BOOL && v[3] == 1 && v[4] == 0xFF)
 			ctx->cert->pub->key_eflags |= 1 << KEY_EFLAG_CA;
+		else
+			return -EBADMSG;
 		return 0;
 	}