Re: [PATCH v8 06/15] x86: Add early SHA support for Secure Launch early measurements

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On 4/3/24 4:56 PM, Eric Biggers wrote:
On Wed, Apr 03, 2024 at 09:32:02AM -0700, Andy Lutomirski wrote:
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.  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 opposite
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 suggestions 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.

The public record demonstrates otherwise.

But are you saying that you'd be happy if the commit message read
something more like:

---8<---
For better or worse, Secure Launch needs SHA-1 and SHA-256.

The choice of hashes used lie with the platform firmware, not with
software, and is often outside of the users control.

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 order
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 summary:

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 silly, but it's
not entirely unreasonable.  Maybe it should be a config option?

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

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


It seems there may be a justification for some form of SHA-1 support in this
feature.  As I've said, the problem is that it's not explained in the patchset
itself.  Rather, it just talks about "SHA" and pretends like SHA-1 and SHA-2 are
basically the same.  In fact, SHA-1 differs drastically from SHA-2 in terms of
security.  SHA-1 support should be added in a separate patch, with a clearly
explained rationale *in the patch itself* for the SHA-1 support *specifically*.

For the record, we were never trying to "pretend" or obfuscate the use of SHA-1. It was simply expedient to put the hash algorithm changes in one patch. We have now separated the patches for clarity and will add any text that explains our use and/or explain the issues with its use.

We went back through the comments and tried to address everything that came up about the use of SHA-1. We will review it all again before posting another patch set.

Thank you for your feedback.
Ross


- Eric


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