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

Varad Gautam
2021-04-13T14:28:28Z

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@suse.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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