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Date: Wed, 03 Apr 2024 09:32:02 -0700
From: "Andy Lutomirski" <luto@kernel.org>
To: "Eric Biggers" <ebiggers@kernel.org>,
 "Andrew Cooper" <andrew.cooper3@citrix.com>
Cc: "Ard Biesheuvel" <ardb@kernel.org>,
 "Ross Philipson" <ross.philipson@oracle.com>,
 "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
 "the arch/x86 maintainers" <x86@kernel.org>, linux-integrity@vger.kernel.org,
 linux-doc@vger.kernel.org,
 "Linux Crypto Mailing List" <linux-crypto@vger.kernel.org>,
 kexec@lists.infradead.org, linux-efi@vger.kernel.org,
 dpsmith@apertussolutions.com, "Thomas Gleixner" <tglx@linutronix.de>,
 "Ingo Molnar" <mingo@redhat.com>, "Borislav Petkov" <bp@alien8.de>,
 "H. Peter Anvin" <hpa@zytor.com>,
 "Dave Hansen" <dave.hansen@linux.intel.com>,
 "Matthew Garrett" <mjg59@srcf.ucam.org>,
 James.Bottomley@hansenpartnership.com, peterhuewe@gmx.de, jarkko@kernel.org,
 "Jason Gunthorpe" <jgg@ziepe.ca>,
 "luto@amacapital.net" <luto@amacapital.net>,
 "Arvind Sankar" <nivedita@alum.mit.edu>,
 "Herbert Xu" <herbert@gondor.apana.org.au>, davem@davemloft.net,
 kanth.ghatraju@oracle.com, trenchboot-devel@googlegroups.com
Subject: Re: [PATCH v8 06/15] x86: Add early SHA support for Secure Launch early
 measurements
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On Fri, Feb 23, 2024, at 10:30 AM, Eric Biggers wrote:
> On Fri, Feb 23, 2024 at 06:20:27PM +0000, Andrew Cooper wrote:
>> On 23/02/2024 5:54 pm, Eric Biggers wrote:
>> > On Fri, Feb 23, 2024 at 04:42:11PM +0000, Andrew Cooper wrote:
>> >> Yes, and I agree.=C2=A0 We're not looking to try and force this in=
 with
>> >> underhand tactics.
>> >>
>> >> But a blind "nack to any SHA-1" is similarly damaging in the oppos=
ite
>> >> direction.
>> >>
>> > Well, reviewers have said they'd prefer that SHA-1 not be included =
and given
>> > some thoughtful reasons for that.  But also they've given suggestio=
ns on how to
>> > make the SHA-1 support more palatable, such as splitting it into a =
separate
>> > patch and giving it a proper justification.
>> >
>> > All suggestions have been ignored.
>>=20
>> The public record demonstrates otherwise.
>>=20
>> But are you saying that you'd be happy if the commit message read
>> something more like:
>>=20
>> ---8<---
>> For better or worse, Secure Launch needs SHA-1 and SHA-256.
>>=20
>> The choice of hashes used lie with the platform firmware, not with
>> software, and is often outside of the users control.
>>=20
>> Even if we'd prefer to use SHA-256-only, if firmware elected to start=
 us
>> with the SHA-1 and SHA-256 backs active, we still need SHA-1 to parse
>> the TPM event log thus far, and deliberately cap the SHA-1 PCRs in or=
der
>> to safely use SHA-256 for everything else.
>> ---
>
> Please take some time to read through the comments that reviewers have=
 left on
> previous versions of the patchset.

So I went and read through the old comments, and I'm lost.  In brief sum=
mary:

If the hardware+firmware only supports SHA-1, then some reviewers would =
prefer Linux not to support DRTM.  I personally think this is a bit sill=
y, but it's not entirely unreasonable.  Maybe it should be a config opti=
on?

If the hardware+firmware does support SHA-256, then it sounds (to me, re=
ading this -- I haven't dug into the right spec pages) that, for optimal=
 security, something still needs to effectively turn SHA-1 *off* at runt=
ime by capping the event log properly.  And that requires computing a SH=
A-1 hash.  And, to be clear, (a) this is only on systems that already su=
pport SHA-256 and that we should support and (b) *not* doing so leaves u=
s potentially more vulnerable to SHA-1 attacks than doing so.  And no SH=
A-256-supporting tooling will actually be compromised by a SHA-1 comprom=
ise if we cap the event log.

So is there a way forward?  Just saying "read through the comments" seem=
s like a dead end.

Thanks,
Andy