linux-kernel - Re: [PATCH v7 06/13] x86: Add early SHA support for Secure Launch early measurements

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Message-ID: <8703f1a9-5d4d-4c5a-b9aa-3b31ad2dcd41@citrix.com>
Date:   Mon, 13 Nov 2023 23:21:36 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>,
        Eric Biggers <ebiggers@...nel.org>,
        Ross Philipson <ross.philipson@...cle.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        linux-integrity@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-crypto@...r.kernel.org, iommu@...ts.linux-foundation.org,
        kexec@...ts.infradead.org, linux-efi@...r.kernel.org,
        dpsmith@...rtussolutions.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, hpa@...or.com, ardb@...nel.org, mjg59@...f.ucam.org,
        luto@...capital.net, nivedita@...m.mit.edu,
        kanth.ghatraju@...cle.com, trenchboot-devel@...glegroups.com
Subject: Re: [PATCH v7 06/13] x86: Add early SHA support for Secure Launch
 early measurements

On 11/11/2023 8:36 pm, James Bottomley wrote:
> On Sat, 2023-11-11 at 18:19 +0000, Andrew Cooper wrote:
>> On 11/11/2023 5:44 pm, Eric Biggers wrote:
>>> On Fri, Nov 10, 2023 at 05:27:44PM -0500, Ross Philipson wrote:
>>>>  arch/x86/boot/compressed/early_sha1.c   | 12 ++++
>>>>  lib/crypto/sha1.c                       | 81
>>>> +++++++++++++++++++++++++
>>> It's surprising to still see this new use of SHA-1 after so many
>>> people objected to it in the v6 patchset.  It's also frustrating
>>> that the SHA-1 support is still being obfuscated by being combined
>>> in one patch with SHA-2 support, perhaps in an attempt to conflate
>>> the two algorithms and avoid having to give a rationale for the
>>> inclusion of SHA-1.  Finally, new functions should not be added to
>>> lib/crypto/sha1.c unless those functions have multiple users.
>> The rational was given.  Let me reiterate it.
>>
>> There are real TPMs in the world that can't use SHA-2.  The use of
>> SHA-1 is necessary to support DRTM on such systems, and there are
>> real users of such configurations.
> Given that TPM 2.0 has been shipping in bulk since Windows 10 (2015)
> and is required for Windows 11 (2021), are there really such huge
> numbers of TPM 1.2 systems involved in security functions?

Yes.

As ever, it's not as simple as a straight TPM version issue.

AMD's firmware-TPM2 isn't compatible with their DRTM implementation. 
Users are limited to whatever headers are available on the motherboard.

Furthermore, even with a TPM2, it is the firmware (Intel TXT ACM) or
hardware (AMD SKINIT) which chooses the hash algorithms to use, and in a
lot of cases the end user doesn't get a choice.

So yes - there really are modern systems which you can't use SHA-2-only
with.

>> DRTM with SHA-1-only is a damnsight better than no DTRM, even if SHA-
>> 1 is getting a little long in the tooth.
> That's not the problem.  The problem is that sha1 is seen as a
> compromised algorithm by NIST which began deprecating it in 2011 and is
> now requiring it to be removed from all systems supplied to the US
> government by 2030

This is a non-issue.

People who care about having no SHA-1 can not compile in DRTM support.

But there are people who will tolerate SHA-1 code to get DTRM support.

~Andrew