Re: [PATCH v9 06/19] x86: Add early SHA-1 support for Secure Launch early measurements

Andy Lutomirski
2024-08-29T03:17:23Z

On Thu, Aug 15, 2024 at 12:10 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Thu, Aug 15 2024 at 13:38, Daniel P. Smith wrote:
> > On 5/31/24 09:54, Eric W. Biederman wrote:
> >> Eric Biggers <ebiggers@kernel.org> writes:
> >>> That paragraph is also phrased as a hypothetical, "Even if we'd prefer to use
> >>> SHA-256-only". That implies that you do not, in fact, prefer SHA-256 only. Is
> >>> that the case? Sure, maybe there are situations where you *have* to use SHA-1,
> >>> but why would you not at least *prefer* SHA-256?
> >>
> >> Yes. Please prefer to use SHA-256.
> >>
> >> Have you considered implementing I think it is SHA1-DC (as git has) that
> >> is compatible with SHA1 but blocks the known class of attacks where
> >> sha1 is actively broken at this point?
> >
> > We are using the kernel's implementation, addressing what the kernel
> > provides is beyond our efforts. Perhaps someone who is interested in
> > improving the kernel's SHA1 could submit a patch implementing/replacing
> > it with SHA1-DC, as I am sure the maintainers would welcome the help.
>
> Well, someone who is interested to get his "secure" code merged should
> have a vested interested to have a non-broken SHA1 implementation if
> there is a sensible requirement to use SHA1 in that new "secure" code,
> no?
>
> Just for the record. The related maintainers can rightfully decide to
> reject known broken "secure" code on a purely technical argument.
>

Wait, hold on a second.

SHA1-DC isn't SHA1. It's a different hash function that is mostly
compatible with SHA1, is different on some inputs, and is maybe more
secure. But the _whole point_ of using SHA1 in the TPM code (well,
this really should be the whole point for new applications) is to
correctly cap the SHA1 PCRs so we can correctly _turn them off_ in the
best way without breaking compatibility with everything that might
read the event log. I think that anyone suggesting using SHA1-DC for
this purpose should give some actual analysis as to why they think
it's an improvement, let alone even valid.

Ross et al, can you confirm that your code actually, at least by
default and with a monstrous warning to anyone who tries to change the
default, caps SHA1 PCRs if SHA256 is available? And then can we maybe
all stop hassling the people trying to develop this series about the
fact that they're doing their best with the obnoxious system that the
TPM designers gave them?

Thanks,
Andy