Re: [PATCH v3 03/10] KEYS: X.509: Parse Basic Constraints for CA

Jarkko Sakkinen
2023-01-04T11:40:40Z

On Tue, Dec 13, 2022 at 07:33:54PM -0500, Eric Snowberg wrote:
> Parse the X.509 Basic Constraints. The basic constraints extension
> identifies whether the subject of the certificate is a CA.
>
> BasicConstraints ::= SEQUENCE {
> cA BOOLEAN DEFAULT FALSE,
> pathLenConstraint INTEGER (0..MAX) OPTIONAL }
>
> If the CA is true, store it in the x509_certificate. This will be used
> in a follow on patch that requires knowing if the public key is a CA.

Please add:

Link: https://www.rfc-editor.org/rfc/rfc5280 # 4.2.1.9. Basic Constraints

> Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
> ---
> crypto/asymmetric_keys/x509_cert_parser.c | 9 +++++++++
> crypto/asymmetric_keys/x509_parser.h | 1 +
> 2 files changed, 10 insertions(+)
>
> diff --git a/crypto/asymmetric_keys/x509_cert_parser.c b/crypto/asymmetric_keys/x509_cert_parser.c
> index 7a9b084e2043..b4443e507153 100644
> --- a/crypto/asymmetric_keys/x509_cert_parser.c
> +++ b/crypto/asymmetric_keys/x509_cert_parser.c
> @@ -586,6 +586,15 @@ int x509_process_extension(void *context, size_t hdrlen,
> return 0;
> }
>
> + if (ctx->last_oid == OID_basicConstraints) {
> + if (vlen < 2 || v[0] != (ASN1_CONS_BIT | ASN1_SEQ))
> + return -EBADMSG;
> + if (v[1] != vlen - 2)
> + return -EBADMSG;

Why this instead of either:

1. Each check in separate if-statement.
2. All in a single statement:
vlen < 2 || v[0] != (ASN1_CONS_BIT | ASN1_SEQ) || v[1] != vlen - 2

It would be also nice to have some sort of explanation in a comment, given
the cryptic statement and the amount of magic numbers in it. I.e. in plain
English what does the check actually means.

> + if (vlen >= 4 && v[1] != 0 && v[2] == ASN1_BOOL && v[3] == 1)
> + ctx->cert->root_ca = true;

Ditto for the explanation part. I have really hard time deciphering this.

> + }
> +
> return 0;
> }
>
> diff --git a/crypto/asymmetric_keys/x509_parser.h b/crypto/asymmetric_keys/x509_parser.h
> index a299c9c56f40..7c5c0ad1c22e 100644
> --- a/crypto/asymmetric_keys/x509_parser.h
> +++ b/crypto/asymmetric_keys/x509_parser.h
> @@ -38,6 +38,7 @@ struct x509_certificate {
> bool self_signed; /* T if self-signed (check unsupported_sig too) */
> bool unsupported_sig; /* T if signature uses unsupported crypto */
> bool blacklisted;
> + bool root_ca; /* T if basic constraints CA is set */
> };
>
> /*
> --
> 2.27.0
>

BR, Jarkko