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
