Re: [PATCH v2] KEYS: X.509: Fix Basic Constraints CA flag parsing

[Lukas Wunner]

On Mon, Sep 15, 2025 at 09:15:50PM +0000, wufan@xxxxxxxxxx wrote:
> Fix the X.509 Basic Constraints CA flag parsing to correctly handle
> the ASN.1 DER encoded structure. The parser was incorrectly treating
> the length field as the boolean value.
[...]
> Link: https://datatracker.ietf.org/doc/html/rfc5280
> Link: https://www.itu.int/ITU-T/studygroups/com17/languages/X.690-0207.pdf
> Fixes: 30eae2b037af ("KEYS: X.509: Parse Basic Constraints for CA")
> Signed-off-by: Fan Wu <wufan@xxxxxxxxxx>

Reviewed-by: Lukas Wunner <lukas@xxxxxxxxx>

> @@ -623,8 +626,13 @@ int x509_process_extension(void *context, size_t hdrlen,
>  			return -EBADMSG;
>  		if (v[1] != vlen - 2)
>  			return -EBADMSG;
> -		if (vlen >= 4 && v[1] != 0 && v[2] == ASN1_BOOL && v[3] == 1)
> +		/* Empty SEQUENCE means CA:FALSE (default value omitted per DER) */
> +		if (v[1] == 0)
> +			return 0;
> +		if (vlen >= 5 && v[2] == ASN1_BOOL && v[3] == 1 && v[4] == 0xFF)
>  			ctx->cert->pub->key_eflags |= 1 << KEY_EFLAG_CA;
> +		else
> +			return -EBADMSG;
>  		return 0;
>  	}

For anyone who didn't follow the discussion and/or isn't familiar
with ASN.1, the patch first checks for a well-formed "CA:FALSE",
then checks for a well-formed "CA:TRUE", finally rejects anything
else as malformed.

Thanks,

Lukas