Re: [PATCH v6 06/14] x86: Add early SHA support for Secure Launch early measurements

[Thomas Gleixner <tglx@linutronix.de>]

On Fri, May 12 2023 at 12:28, Matthew Garrett wrote:
> On Fri, May 12, 2023 at 01:18:45PM +0200, Ard Biesheuvel wrote:
>> On Fri, 12 May 2023 at 13:04, Matthew Garrett <mjg59@srcf.ucam.org> wrote:
>> >
>> > On Tue, May 09, 2023 at 06:21:44PM -0700, Eric Biggers wrote:
>> >
>> > > SHA-1 is insecure.  Why are you still using SHA-1?  Don't TPMs support SHA-2
>> > > now?
>> >
>> > TXT is supported on some TPM 1.2 systems as well. TPM 2 systems are also
>> > at the whim of the firmware in terms of whether the SHA-2 banks are
>> > enabled. But even if the SHA-2 banks are enabled, if you suddenly stop
>> > extending the SHA-1 banks, a malicious actor can later turn up and
>> > extend whatever they want into them and present a SHA-1-only
>> > attestation. Ideally whatever is handling that attestation should know
>> > whether or not to expect an attestation with SHA-2, but the easiest way
>> > to maintain security is to always extend all banks.
>> >
>> 
>> Wouldn't it make more sense to measure some terminating event into the
>> SHA-1 banks instead?
>
> Unless we assert that SHA-1 events are unsupported, it seems a bit odd 
> to force a policy on people who have both banks enabled. People with 
> mixed fleets are potentially going to be dealing with SHA-1 measurements 
> for a while yet, and while there's obviously a security benefit in using 
> SHA-2 instead it'd be irritating to have to maintain two attestation 
> policies.

Why?

If you have a mixed fleet then it's not too much asked to provide two
data sets. On a TPM2 system you can enforce SHA-2 and only fallback to
SHA-1 on TPM 1.2 hardware. No?

Thanks,

        tglx

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