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Subject: Re: [PATCH 3/7] KEYS: X.509: Parse Key Usage
From:   Mimi Zohar <zohar@linux.ibm.com>
To:     Eric Snowberg <eric.snowberg@oracle.com>, dhowells@redhat.com,
        dwmw2@infradead.org, jarkko@kernel.org,
        linux-integrity@vger.kernel.org
Cc:     herbert@gondor.apana.org.au, davem@davemloft.net,
        dmitry.kasatkin@gmail.com, jmorris@namei.org, serge@hallyn.com,
        roberto.sassu@huawei.com, nramas@linux.microsoft.com,
        pvorel@suse.cz, tiwai@suse.de, keyrings@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org,
        linux-security-module@vger.kernel.org
Date:   Fri, 08 Apr 2022 10:39:45 -0400
In-Reply-To: <20220406015337.4000739-4-eric.snowberg@oracle.com>
References: <20220406015337.4000739-1-eric.snowberg@oracle.com>
         <20220406015337.4000739-4-eric.snowberg@oracle.com>
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On Tue, 2022-04-05 at 21:53 -0400, 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 | 20 ++++++++++++++++++++
>  crypto/asymmetric_keys/x509_parser.h      |  1 +
>  2 files changed, 21 insertions(+)
> 
> diff --git a/crypto/asymmetric_keys/x509_cert_parser.c b/crypto/asymmetric_keys/x509_cert_parser.c
> index 30f7374ea9c0..a89f1e0c8a0f 100644
> --- a/crypto/asymmetric_keys/x509_cert_parser.c
> +++ b/crypto/asymmetric_keys/x509_cert_parser.c
> @@ -576,6 +576,26 @@ 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 || vlen < 4)
> +			return -EBADMSG;
> +		if (v[1] == 0x02 && v[2] <= 2 && (v[3] & 0x04))
> +			ctx->cert->is_kcs_set = true;
> +		else if (vlen > 4 && v[1] == 0x03 && (v[3] & 0x04))
> +			ctx->cert->is_kcs_set = true;
> +		return 0;
> +	}
> +
>  	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 dc45df9f6594..d6ac0985d8a5 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		is_root_ca;		/* T if basic constraints CA is set */
> +	bool		is_kcs_set;		/* T if keyCertSign is set */
>  };

Again there is no need to prefix the variable with "is_" or suffix it
with "set".  Simply naming the variable "cert_signing" or
"keycertsign", like "self_signed", will improve code readability.

thanks,

Mimi