From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 74B2DC4332F for ; Tue, 9 Nov 2021 01:48:43 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5C4EF6115B for ; Tue, 9 Nov 2021 01:48:43 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238000AbhKIBv1 (ORCPT ); Mon, 8 Nov 2021 20:51:27 -0500 Received: from mail.kernel.org ([198.145.29.99]:39724 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S237655AbhKIBv1 (ORCPT ); Mon, 8 Nov 2021 20:51:27 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 8EA4E61105; Tue, 9 Nov 2021 01:48:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1636422521; bh=kIA6Eokj76BQmMANJNXcdbCP8CCrVGLB+Dzn7QmgJik=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=LAwQrXi2J7WKAd6EfjqVxJOHxSdCvCDLtjxf5hTjB11OEHaCuhr/EpbffbgWd/ATL OH01or7BPZ/iyPfuX9kMceDA7SVc86ppadMmQMwTNvCmw5IZYsEQV4143cX8DcdGqd OniH/BJzoPy5D6rXTmU6PUGGhAqQXSQSnTUDU6rom54gncY1OQL7mkFU6obleQaSG5 HW+v0KomxGE0pqJHUZnPFxLwFBLzE2Mjqmy0yyR82KWCRujBVZueXkfldNlbaeu5zQ ShSg8vi6OBsM3sicwax1erEOodN0A8RSiAprDBv1DuIixy5iRLncAncsGUYtRVPP+v tQjSllh56QEUQ== Date: Mon, 8 Nov 2021 17:48:40 -0800 From: Eric Biggers To: Sasha Levin Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org, Paul Crowley , tytso@mit.edu, jaegeuk@kernel.org, corbet@lwn.net, linux-fscrypt@vger.kernel.org, linux-doc@vger.kernel.org Subject: Re: [PATCH AUTOSEL 5.10 021/101] fscrypt: allow 256-bit master keys with AES-256-XTS Message-ID: References: <20211108174832.1189312-1-sashal@kernel.org> <20211108174832.1189312-21-sashal@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20211108174832.1189312-21-sashal@kernel.org> Precedence: bulk List-ID: X-Mailing-List: linux-doc@vger.kernel.org On Mon, Nov 08, 2021 at 12:47:11PM -0500, Sasha Levin wrote: > From: Eric Biggers > > [ Upstream commit 7f595d6a6cdc336834552069a2e0a4f6d4756ddf ] > > fscrypt currently requires a 512-bit master key when AES-256-XTS is > used, since AES-256-XTS keys are 512-bit and fscrypt requires that the > master key be at least as long any key that will be derived from it. > > However, this is overly strict because AES-256-XTS doesn't actually have > a 512-bit security strength, but rather 256-bit. The fact that XTS > takes twice the expected key size is a quirk of the XTS mode. It is > sufficient to use 256 bits of entropy for AES-256-XTS, provided that it > is first properly expanded into a 512-bit key, which HKDF-SHA512 does. > > Therefore, relax the check of the master key size to use the security > strength of the derived key rather than the size of the derived key > (except for v1 encryption policies, which don't use HKDF). > > Besides making things more flexible for userspace, this is needed in > order for the use of a KDF which only takes a 256-bit key to be > introduced into the fscrypt key hierarchy. This will happen with > hardware-wrapped keys support, as all known hardware which supports that > feature uses an SP800-108 KDF using AES-256-CMAC, so the wrapped keys > are wrapped 256-bit AES keys. Moreover, there is interest in fscrypt > supporting the same type of AES-256-CMAC based KDF in software as an > alternative to HKDF-SHA512. There is no security problem with such > features, so fix the key length check to work properly with them. > > Reviewed-by: Paul Crowley > Link: https://lore.kernel.org/r/20210921030303.5598-1-ebiggers@kernel.org > Signed-off-by: Eric Biggers > Signed-off-by: Sasha Levin I don't expect any problem with backporting this, but I don't see how this follows the stable kernel rules (Documentation/process/stable-kernel-rules.rst). I don't see what distinguishes this patch from ones that don't get picked up by AUTOSEL; it seems pretty arbitrary to me. - Eric