Re: [PATCH v5 4/6] KEYS: X.509: Parse Key Usage

[Jarkko Sakkinen <jarkko@kernel.org>]

On Thu, Mar 02, 2023 at 11:46:50AM -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 or digitalSignature is set, store it in the
> public_key structure. Having the purpose of the key being stored
> during parsing, allows enforcement on the usage field in the future.
> This will be used in a follow on patch that requires knowing the
> certificate key usage type.
> 
> Link: https://www.rfc-editor.org/rfc/rfc5280#section-4.2.1.3
> Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
> Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
> ---
>  crypto/asymmetric_keys/x509_cert_parser.c | 28 +++++++++++++++++++++++
>  include/crypto/public_key.h               |  2 ++
>  2 files changed, 30 insertions(+)
> 
> diff --git a/crypto/asymmetric_keys/x509_cert_parser.c b/crypto/asymmetric_keys/x509_cert_parser.c
> index 77547d4bd94d..0a7049b470c1 100644
> --- a/crypto/asymmetric_keys/x509_cert_parser.c
> +++ b/crypto/asymmetric_keys/x509_cert_parser.c
> @@ -579,6 +579,34 @@ int x509_process_extension(void *context, size_t hdrlen,
>  		return 0;
>  	}
>  
> +	if (ctx->last_oid == OID_keyUsage) {
> +		/*
> +		 * Get hold of the keyUsage bit string
> +		 * 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 when v[1] = 0x02)
> +		 * v[3] and possibly v[4] contain the bit string
> +		 *
> +		 * From RFC 5280 4.2.1.3:
> +		 *   0x04 is where keyCertSign lands in this bit string
> +		 *   0x80 is where digitalSignature lands in this bit string
> +		 */
> +		if (v[0] != ASN1_BTS)
> +			return -EBADMSG;
> +		if (vlen < 4)
> +			return -EBADMSG;
> +		if (v[2] >= 8)
> +			return -EBADMSG;
> +		if (v[3] & 0x80)
> +			ctx->cert->pub->key_eflags |= 1 << KEY_EFLAG_DIGITALSIG;
> +		if (v[1] == 0x02 && v[2] <= 2 && (v[3] & 0x04))
> +			ctx->cert->pub->key_eflags |= 1 << KEY_EFLAG_KEYCERTSIGN;
> +		else if (vlen > 4 && v[1] == 0x03 && (v[3] & 0x04))
> +			ctx->cert->pub->key_eflags |= 1 << KEY_EFLAG_KEYCERTSIGN;
> +		return 0;
> +	}
> +
>  	if (ctx->last_oid == OID_authorityKeyIdentifier) {
>  		/* Get hold of the CA key fingerprint */
>  		ctx->raw_akid = v;
> diff --git a/include/crypto/public_key.h b/include/crypto/public_key.h
> index c401762850f2..03c3fb990d59 100644
> --- a/include/crypto/public_key.h
> +++ b/include/crypto/public_key.h
> @@ -30,6 +30,8 @@ struct public_key {
>  	const char *pkey_algo;
>  	unsigned long key_eflags;	/* key extension flags */
>  #define KEY_EFLAG_CA		0	/* set if the CA basic constraints is set */
> +#define KEY_EFLAG_DIGITALSIG	1	/* set if the digitalSignature usage is set */
> +#define KEY_EFLAG_KEYCERTSIGN	2	/* set if the keyCertSign usage is set */
>  };
>  
>  extern void public_key_free(struct public_key *key);
> -- 
> 2.27.0
> 

Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>

BR, Jarkko