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

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Date:   Thu, 15 Dec 2022 06:25:24 -0500
From:   Mimi Zohar <zohar@...ux.ibm.com>
To:     Eric Snowberg <eric.snowberg@...cle.com>, jarkko@...nel.org
Cc:     dhowells@...hat.com, dwmw2@...radead.org,
        herbert@...dor.apana.org.au, davem@...emloft.net,
        dmitry.kasatkin@...il.com, paul@...l-moore.com, jmorris@...ei.org,
        serge@...lyn.com, pvorel@...e.cz, noodles@...com, tiwai@...e.de,
        kanth.ghatraju@...cle.com, konrad.wilk@...cle.com,
        erpalmer@...ux.vnet.ibm.com, coxu@...hat.com,
        keyrings@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-integrity@...r.kernel.org,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH v3 04/10] KEYS: X.509: Parse Key Usage

On Tue, 2022-12-13 at 19:33 -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.

Either in this patch or separately, the "digitalSignature" key usage
flag needs to be saved.

> 
> Signed-off-by: Eric Snowberg <eric.snowberg@...cle.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;
> +	}
> +
>  	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 */

Using acronyms as variable names makes reviewing code more difficult. 
Perhaps rename "kcs_set" to either "key_cert_sign" or "keycertsign".

>  };
>  
>  /*

-- 
thanks,

Mimi