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

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Date:   Wed, 4 Jan 2023 11:40:26 +0000
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Eric Snowberg <eric.snowberg@...cle.com>
Cc:     zohar@...ux.ibm.com, 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 03/10] KEYS: X.509: Parse Basic Constraints for CA

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@...cle.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