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

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Date:   Thu, 15 Apr 2021 00:01:15 +0800
From:   joeyli <jlee@...e.com>
To:     Varad Gautam <varad.gautam@...e.com>
CC:     "Lee, Chun-Yi" <joeyli.kernel@...il.com>,
        David Howells <dhowells@...hat.com>,
        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
Subject: Re: [PATCH 1/4] X.509: Add CodeSigning extended key usage parsing

Hi Varad, 

Thanks for your review!

On Tue, Apr 13, 2021 at 04:28:11PM +0200, Varad Gautam wrote:
> 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.
>

I will add the check, thanks for your suggestion!
 
> > +			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?
>

I will move the "(i + 10) > vlen" to the front of "v[i + 1] != 8". It can avoid
that the last octet blob is less than 10 bytes.

Thanks!
Joey Lee
 
> > +				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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