linux-kernel - Re: [PATCH 0/4] keys: Introduce a keys frontend for attestation reports

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Date:   Fri, 4 Aug 2023 17:19:30 +0100
From:   Daniel P. Berrangé <berrange@...hat.com>
To:     "Huang, Kai" <kai.huang@...el.com>
Cc:     "Williams, Dan J" <dan.j.williams@...el.com>,
        "James.Bottomley@...senPartnership.com" 
        <James.Bottomley@...senpartnership.com>,
        "dhowells@...hat.com" <dhowells@...hat.com>,
        "sameo@...osinc.com" <sameo@...osinc.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jarkko@...nel.org" <jarkko@...nel.org>,
        "bp@...en8.de" <bp@...en8.de>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "dionnaglaze@...gle.com" <dionnaglaze@...gle.com>,
        "keyrings@...r.kernel.org" <keyrings@...r.kernel.org>,
        "brijesh.singh@....com" <brijesh.singh@....com>,
        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH 0/4] keys: Introduce a keys frontend for attestation
 reports

On Tue, Aug 01, 2023 at 11:45:12AM +0000, Huang, Kai wrote:
> The IOCTL vs /sysfs isn't discussed.
> 
> For instance, after rough thinking, why is the IOCTL better than below approach
> using /sysfs?
> 
> echo <REPORTDATA> > /sys/kernel/coco/tdx/attest/reportdata
> cat /sys/kernel/coco/tdx/attest/tdreport
> 
> Each "echo <REPORTDATA>" to '/sys/.../reportdata' triggers the driver to call
> TDCALL to get the TDREPORT, which is available at '/sys/.../tdreport'.

What would you suggest as behaviour with multiple processes writing
into 'reportdata' and trying to read from 'tdreport' in parallel ?
Splitting input and output across separate files removes any
transactional relationship between input and output. This approach
feels like it could easily result in buggy behaviour from concurrent
application usage, which would not be an issue with ioctl()

Also note, there needs to be scope for more than 1 input and 1 output
data items. For SNP guests, the VMPL is a input, and if fetching a
VMPL 0 report from under SVSM [1], an optional service GUID is needed.
With SVSM, there are three distinct output data blobs - attestation
report, services manifest and certificate data.

With regards,
Daniel

[1] https://www.amd.com/system/files/TechDocs/58019_1.00.pdf
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