linux-kernel - Re: [discuss] Improve and merge a driver proposed in 2013: sysfs interfaces to access TXT config space

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Date:   Tue, 22 Feb 2022 10:31:01 +0100
From:   Pavel Machek <pavel@...x.de>
To:     Jonathan McDowell <noodles@...com>
Cc:     "greg@...ah.com" <greg@...ah.com>,
        Dmitrii Okunev <xaionaro@...com>,
        "qiaowei.ren@...el.com" <qiaowei.ren@...el.com>,
        "mjg59@...f.ucam.org" <mjg59@...f.ucam.org>,
        "xiaoyan.zhang@...el.com" <xiaoyan.zhang@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "gang.wei@...el.com" <gang.wei@...el.com>,
        "platform-driver-x86@...r.kernel.org" 
        <platform-driver-x86@...r.kernel.org>
Subject: Re: [discuss] Improve and merge a driver proposed in 2013: sysfs
 interfaces to access TXT config space

On Fri 2022-02-18 18:05:47, Jonathan McDowell wrote:
> On Thu, 2022-02-17 at 13:37 +0100, Pavel Machek wrote:
> > On Thu 2022-02-17 13:34:40, greg@...ah.com wrote:
> > > On Thu, Feb 17, 2022 at 11:47:21AM +0000, Dmitrii Okunev wrote:
> > > > Hello!
> > > > 
> > > > As far as I see the patch wasn't merged. And I see that this is
> > > > the only unsolved thread in the discussion:
> > > > 
> > > > On Thu, 2013-05-16 at 18:03 +0200, Pavel Machek wrote:
> > > > > On Tue 2013-05-14 01:24:43, Qiaowei Ren wrote:
> > > > > > These interfaces are located in
> > > > > > /sys/devices/platform/intel_txt/config,
> > > > > > and including totally 37 files, providing access to Intel TXT
> > > > > > configuration registers.
> > > > > 
> > > > > This looks like very wrong interface... equivalent of /dev/mem.
> > > > 
> > > > As an active user of these registers I hope it will be merged, so
> > > > I would like to improve this patch (or rewrite it from scratch)
> > > > to make that happen. Otherwise one have to do hackery around
> > > > `/dev/mem`, which also creates problems with proper access
> > > > control.
> > > > 
> > > > To be able to improve the patch, could somebody clarify why
> > > > exactly this is a "very wrong interface"?
> > > > 
> > > > > > +What:          /sys/devices/platform/intel_txt/config/STS_ra
> > > > > > w
> > > > > > +Date:          May 2013
> > > > > > +KernelVersion: 3.9
> > > > > > +Contact:       "Qiaowei Ren" <qiaowei.ren@...el.com>
> > > > > > +Description:   TXT.STS is the general status register. This
> > > > > > read-
> > > > > > only register
> > > > > > +               is used by AC modules and the MLE to get the
> > > > > > status
> > > > > > of various
> > > > > > +               Intel TXT features.
> > > > > 
> > > > > This is not enough to allow people to understand what this
> > > > > does/should do, nor does it allow (for example) ARM people to
> > > > > implement something compatible.
> > > > > 
> > > > > Is there specific reason why "better" interface is impossible?
> > > > 
> > > > I would love to reuse Intel's public documentation [1] to provide
> > > > a proper description (with bit layout of the value).
> > > > 
> > > > [1] https://cdrdv2.intel.com/v1/dl/getContent/315168
> > > > 
> > > > > [...], nor does it allow (for example) ARM people to
> > > > > implement something compatible.
> > > > 
> > > > Do I understand correctly that a proper documentation of the
> > > > registers solves the problem?
> > > > 
> > > > > Is there specific reason why "better" interface is impossible?
> > > > 
> > > > What are specific problems with the current interface?
> > > 
> > > What do you mean by "current" here?  You are referring to an email
> > > from 2013, 9 years ago.
> > > 
> > > If you want to propose the change again, correctly update the patch
> > > and submit it that way.
> > 
> > I don't believe taking hardware registers and exposing them 1-to-1 in
> > sysfs is the way to go.
> > 
> > We would like same /sys interface on different hardware, and simply
> > exposing Intel's registers in /sys will not do the job.
> 
> So, for our particular use case what we want to be able to see is the
> status of the TXT device, so when attestation fails it's possible to
> diagnose where that might have happened. At a minimum details from the
> status register are folded into the first measurement, and the error
> register can provide valuable insight as to what the TXT device thinks
> failed.
> 
> At present these details are retrieved from /dev/mem, but this is less
> than ideal and prevents the use of, say, kernel lockdown. As a result
> we'd like to export the appropriate details via sysfs. These are likely
> to be extremely security block implementation specific, so I'm not
> clear that a generic agnostic interface is appropriate to retrieve
> these details.

> Do you have the same objection to a read only set of information
> (rather than the full control offered by the initial submission)?

Might be a job for debugfs?
							Pavel

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HRB 165235 Munich, Office: Kirchenstr.5, D-82194 Groebenzell, Germany

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