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

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Message-ID: <aNix2dfs0FC74Zi2@gondor.apana.org.au>
Date: Sun, 28 Sep 2025 11:56:09 +0800
From: Herbert Xu <herbert@...dor.apana.org.au>
To: wufan@...nel.org
Cc: keyrings@...r.kernel.org, linux-crypto@...r.kernel.org,
	linux-kernel@...r.kernel.org, dhowells@...hat.com, lukas@...ner.de,
	ignat@...udflare.com, davem@...emloft.net, jarkko@...nel.org,
	zohar@...ux.ibm.com, eric.snowberg@...cle.com
Subject: Re: [PATCH v2] KEYS: X.509: Fix Basic Constraints CA flag parsing

On Mon, Sep 15, 2025 at 09:15:50PM +0000, wufan@...nel.org wrote:
> From: Fan Wu <wufan@...nel.org>
> 
> 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.
> 
> Per RFC 5280 section 4.1, X.509 certificates must use ASN.1 DER encoding.
> According to ITU-T X.690, a DER-encoded BOOLEAN is represented as:
> 
> Tag (0x01), Length (0x01), Value (0x00 for FALSE, 0xFF for TRUE)
> 
> The basicConstraints extension with CA:TRUE is encoded as:
> 
>   SEQUENCE (0x30) | Length | BOOLEAN (0x01) | Length (0x01) | Value (0xFF)
>                              ^-- v[2]         ^-- v[3]        ^-- v[4]
> 
> The parser was checking v[3] (the length field, always 0x01) instead
> of v[4] (the actual boolean value, 0xFF for TRUE in DER encoding).
> 
> Also handle the case where the extension is an empty SEQUENCE (30 00),
> which is valid for CA:FALSE when the default value is omitted as
> required by DER encoding rules (X.690 section 11.5).
> 
> Per ITU-T X.690-0207:
> - Section 11.5: Default values must be omitted in DER
> - Section 11.1: DER requires TRUE to be encoded as 0xFF
> 
> 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@...nel.org>
> ---
>  crypto/asymmetric_keys/x509_cert_parser.c | 16 ++++++++++++----
>  1 file changed, 12 insertions(+), 4 deletions(-)

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt