linux-kernel - Re: [PATCH 1/4] X.509: Add CodeSigning extended key usage parsing

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Message-ID: <5eb3e055-8e41-80b5-8caf-c80bbf2f4068@suse.com>
Date:   Tue, 13 Apr 2021 16:28:11 +0200
From:   Varad Gautam <varad.gautam@...e.com>
To:     "Lee, Chun-Yi" <joeyli.kernel@...il.com>,
        David Howells <dhowells@...hat.com>
CC:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S . Miller" <davem@...emloft.net>,
        Ben Boeckel <me@...boeckel.net>,
        Randy Dunlap <rdunlap@...radead.org>,
        Malte Gell <malte.gell@....de>, keyrings@...r.kernel.org,
        linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org,
        "Lee, Chun-Yi" <jlee@...e.com>
Subject: Re: [PATCH 1/4] X.509: Add CodeSigning extended key usage parsing

Hi,

On 3/9/21 10:10 AM, Lee, Chun-Yi wrote:
> This patch adds the logic for parsing the CodeSign extended key usage
> extension in X.509. The parsing result will be set to the eku flag
> which is carried by public key. It can be used in the PKCS#7
> verification.
> 
> Signed-off-by: "Lee, Chun-Yi" <jlee@...e.com>
> ---
>  crypto/asymmetric_keys/x509_cert_parser.c | 24 ++++++++++++++++++++++++
>  include/crypto/public_key.h               |  1 +
>  include/linux/oid_registry.h              |  5 +++++
>  3 files changed, 30 insertions(+)
> 
> diff --git a/crypto/asymmetric_keys/x509_cert_parser.c b/crypto/asymmetric_keys/x509_cert_parser.c
> index 52c9b455fc7d..65721313b265 100644
> --- a/crypto/asymmetric_keys/x509_cert_parser.c
> +++ b/crypto/asymmetric_keys/x509_cert_parser.c
> @@ -497,6 +497,8 @@ int x509_process_extension(void *context, size_t hdrlen,
>  	struct x509_parse_context *ctx = context;
>  	struct asymmetric_key_id *kid;
>  	const unsigned char *v = value;
> +	int i = 0;
> +	enum OID oid;
>  
>  	pr_debug("Extension: %u\n", ctx->last_oid);
>  
> @@ -526,6 +528,28 @@ int x509_process_extension(void *context, size_t hdrlen,
>  		return 0;
>  	}
>  
> +	if (ctx->last_oid == OID_extKeyUsage) {
> +		if (v[0] != ((ASN1_UNIV << 6) | ASN1_CONS_BIT | ASN1_SEQ) ||
> +		    v[1] != vlen - 2)

A bad cert might get here with vlen < 2, which would cause indexing into v to break.
Please add a check for vlen >= 2 before this.

> +			return -EBADMSG;
> +		i += 2;
> +
> +		while (i < vlen) {
> +			/* A 10 bytes EKU OID Octet blob =
> +			 * ASN1_OID + size byte + 8 bytes OID */
> +			if (v[i] != ASN1_OID || v[i + 1] != 8 || (i + 10) > vlen)

Same here, for i == (vlen - 1), v[i + 1] would fetch outside of v. Or, does the
ASN.1 layout protect against this?

> +				return -EBADMSG;
> +
> +			oid = look_up_OID(v + i + 2, v[i + 1]);
> +			if (oid == OID_codeSigning) {
> +				ctx->cert->pub->eku |= EKU_codeSigning;
> +			}
> +			i += 10;
> +		}
> +		pr_debug("extKeyUsage: %d\n", ctx->cert->pub->eku);
> +		return 0;
> +	}
> +
>  	return 0;
>  }
>  
> diff --git a/include/crypto/public_key.h b/include/crypto/public_key.h
> index 47accec68cb0..1ccaebe2a28b 100644
> --- a/include/crypto/public_key.h
> +++ b/include/crypto/public_key.h
> @@ -28,6 +28,7 @@ struct public_key {
>  	bool key_is_private;
>  	const char *id_type;
>  	const char *pkey_algo;
> +	unsigned int eku : 9;      /* Extended Key Usage (9-bit) */
>  };
>  
>  extern void public_key_free(struct public_key *key);
> diff --git a/include/linux/oid_registry.h b/include/linux/oid_registry.h
> index 4462ed2c18cd..e20e8eb53b21 100644
> --- a/include/linux/oid_registry.h
> +++ b/include/linux/oid_registry.h
> @@ -113,9 +113,14 @@ enum OID {
>  	OID_SM2_with_SM3,		/* 1.2.156.10197.1.501 */
>  	OID_sm3WithRSAEncryption,	/* 1.2.156.10197.1.504 */
>  
> +	/* Extended key purpose OIDs [RFC 5280] */
> +	OID_codeSigning,		/* 1.3.6.1.5.5.7.3.3 */
> +
>  	OID__NR
>  };
>  
> +#define EKU_codeSigning	(1 << 2)
> +
>  extern enum OID look_up_OID(const void *data, size_t datasize);
>  extern int sprint_oid(const void *, size_t, char *, size_t);
>  extern int sprint_OID(enum OID, char *, size_t);
> 

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