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

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Date: Wed, 3 Apr 2024 21:55:41 -0700
From: ross.philipson@...cle.com
To: Eric Biggers <ebiggers@...nel.org>, Andy Lutomirski <luto@...nel.org>
Cc: Andrew Cooper <andrew.cooper3@...rix.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        linux-integrity@...r.kernel.org, linux-doc@...r.kernel.org,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        kexec@...ts.infradead.org, linux-efi@...r.kernel.org,
        dpsmith@...rtussolutions.com, Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Matthew Garrett <mjg59@...f.ucam.org>,
        James.Bottomley@...senpartnership.com, peterhuewe@....de,
        jarkko@...nel.org, Jason Gunthorpe <jgg@...pe.ca>,
        "luto@...capital.net" <luto@...capital.net>,
        Arvind Sankar <nivedita@...m.mit.edu>,
        Herbert Xu <herbert@...dor.apana.org.au>, davem@...emloft.net,
        kanth.ghatraju@...cle.com, trenchboot-devel@...glegroups.com
Subject: Re: [PATCH v8 06/15] x86: Add early SHA support for Secure Launch
 early measurements

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