Re: [PATCH v3 04/10] KEYS: X.509: Parse Key Usage

Jarkko Sakkinen
2023-01-04T11:44:08Z

On Tue, Dec 13, 2022 at 07:33:55PM -0500, Eric Snowberg wrote:
> Parse the X.509 Key Usage. The key usage extension defines the purpose of
> the key contained in the certificate.
>
> id-ce-keyUsage OBJECT IDENTIFIER ::= { id-ce 15 }
>
> KeyUsage ::= BIT STRING {
> digitalSignature (0),
> contentCommitment (1),
> keyEncipherment (2),
> dataEncipherment (3),
> keyAgreement (4),
> keyCertSign (5),
> cRLSign (6),
> encipherOnly (7),
> decipherOnly (8) }
>
> If the keyCertSign is set, store it in the x509_certificate structure.
> This will be used in a follow on patch that requires knowing the
> certificate key usage type.
>
> Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
> ---
> crypto/asymmetric_keys/x509_cert_parser.c | 22 ++++++++++++++++++++++
> crypto/asymmetric_keys/x509_parser.h | 1 +
> 2 files changed, 23 insertions(+)
>
> diff --git a/crypto/asymmetric_keys/x509_cert_parser.c b/crypto/asymmetric_keys/x509_cert_parser.c
> index b4443e507153..edb22cf04eed 100644
> --- a/crypto/asymmetric_keys/x509_cert_parser.c
> +++ b/crypto/asymmetric_keys/x509_cert_parser.c
> @@ -579,6 +579,28 @@ int x509_process_extension(void *context, size_t hdrlen,
> return 0;
> }
>
> + if (ctx->last_oid == OID_keyUsage) {
> + /*
> + * Get hold of the keyUsage bit string to validate keyCertSign
> + * v[1] is the encoding size
> + * (Expect either 0x02 or 0x03, making it 1 or 2 bytes)
> + * v[2] is the number of unused bits in the bit string
> + * (If >= 3 keyCertSign is missing)
> + * v[3] and possibly v[4] contain the bit string
> + * 0x04 is where KeyCertSign lands in this bit string (from
> + * RFC 5280 4.2.1.3)
> + */
> + if (v[0] != ASN1_BTS)
> + return -EBADMSG;
> + if (vlen < 4)
> + return -EBADMSG;
> + if (v[1] == 0x02 && v[2] <= 2 && (v[3] & 0x04))
> + ctx->cert->kcs_set = true;
> + else if (vlen > 4 && v[1] == 0x03 && (v[3] & 0x04))
> + ctx->cert->kcs_set = true;
> + return 0;

This is much more easier to follow thanks to explanation.

> + }
> +
> if (ctx->last_oid == OID_authorityKeyIdentifier) {
> /* Get hold of the CA key fingerprint */
> ctx->raw_akid = v;
> diff --git a/crypto/asymmetric_keys/x509_parser.h b/crypto/asymmetric_keys/x509_parser.h
> index 7c5c0ad1c22e..74a9f929e400 100644
> --- a/crypto/asymmetric_keys/x509_parser.h
> +++ b/crypto/asymmetric_keys/x509_parser.h
> @@ -39,6 +39,7 @@ struct x509_certificate {
> bool unsupported_sig; /* T if signature uses unsupported crypto */
> bool blacklisted;
> bool root_ca; /* T if basic constraints CA is set */
> + bool kcs_set; /* T if keyCertSign is set */
> };
>
> /*
> --
> 2.27.0
>

LGTM but I'll hold with reviewed-by's up until the patch set overally
looks good to me and I have actually tested it.

BR, Jarkko