linux-kernel - Re: [PATCH v11 2/8] pkcs7: Allow the signing algo to calculate the digest itself

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Message-ID: <2727729.1767877161@warthog.procyon.org.uk>
Date: Thu, 08 Jan 2026 12:59:21 +0000
From: David Howells <dhowells@...hat.com>
To: Ignat Korchagin <ignat@...udflare.com>
Cc: dhowells@...hat.com, Lukas Wunner <lukas@...ner.de>,
    Jarkko Sakkinen <jarkko@...nel.org>,
    Herbert Xu <herbert@...dor.apana.org.au>,
    Eric Biggers <ebiggers@...nel.org>,
    Luis Chamberlain <mcgrof@...nel.org>,
    Petr Pavlu <petr.pavlu@...e.com>, Daniel Gomez <da.gomez@...nel.org>,
    Sami Tolvanen <samitolvanen@...gle.com>,
    "Jason A . Donenfeld" <Jason@...c4.com>,
    Ard Biesheuvel <ardb@...nel.org>,
    Stephan Mueller <smueller@...onox.de>, linux-crypto@...r.kernel.org,
    keyrings@...r.kernel.org, linux-modules@...r.kernel.org,
    linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 2/8] pkcs7: Allow the signing algo to calculate the digest itself

David Howells <dhowells@...hat.com> wrote:

> Also, we probably don't actually need to copy the authattrs, just retain a
> pointer into the source buffer and the length since we don't intend to keep
> the digest around beyond the verification procedure.  So I might be able to
> get away with just a flag saying I don't need to free it.

Actually, we probably do need to copy it.  The problem is that we have to
modify the tag on the authenticatedAttributes (PKCS#7)/signedAttrs (CMS) blob
before we digest it, e.g. in pkcs7_digest():

	memcpy(sig->digest, sinfo->authattrs, sinfo->authattrs_len);
	((u8 *)sig->digest)[0] = ASN1_CONS_BIT | ASN1_SET;

as specified in RFC9882 and other places:

	3.2.  Signature Generation and Verification
	...
	When signed attributes are included, ML-DSA (pure mode) signatures are
	computed over the complete DER encoding of the SignedAttrs value
	contained in the SignerInfo's signedAttrs field.  As described in
	Section 5.4 of [RFC5652], this encoding includes the tag and length
	octets, but an EXPLICIT SET OF tag is used rather than the IMPLICIT
	[0] tag that appears in the final message. ...

We might be able to get away with modifying it in place - but I don't know
that that's true for all users.

David