linux-kernel - Re: [PATCH v2] x86/mce: Include the PPIN in machine check records when it is available

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Date:   Wed, 23 Nov 2016 12:56:18 -0800
From:   Tony Luck <tony.luck@...il.com>
To:     Henrique de Moraes Holschuh <hmh@....eng.br>
Cc:     Borislav Petkov <bp@...e.de>, "Luck, Tony" <tony.luck@...el.com>,
        Andi Kleen <andi@...stfloor.org>,
        Ashok Raj <ashok.raj@...el.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86/mce: Include the PPIN in machine check records when it is available

IMHO people who really care should find the BIOS option and disable it there. 

Having Linux take responsibility seems a little weird. If we do go that route it should be early in setup_arch() before any callbacks to other subsystems to avoid and endless games of whack-a-mole.

I also wonder about the level of outrage this time around. The feature has been sitting there for three full generations: Ivybridge (tick), Haswell (tock) and another tick for Broadwell. Do privacy folks not read each new SDM from cover to cover?

Sent from my iPhone

> On Nov 23, 2016, at 09:29, Henrique de Moraes Holschuh <hmh@....eng.br> wrote:
> 
>> On Wed, 23 Nov 2016, Borislav Petkov wrote:
>>> On Wed, Nov 23, 2016 at 11:29:51AM -0200, Henrique de Moraes Holschuh wrote:
>>> 1. Assuming we can do it, always lock it when it is found to be unlocked
>>>   at kernel boot.
>> 
>> Because...?
> 
> Privacy, and the fact that /dev/cpu/msr exists and is enabled on
> almost all general-use distros.
> 
>>> 2. Not attempt to change its state from disabled to enabled *unless*
>>>   given a command line parameter authorizing it.  A kconfig-based
>>>   solution for default+command line override would also work well IMHO,
>>>   if it makes more sense.
>> 
>> You can't reenable it:
> 
> Yeah, I just found the description for that thing in the IA32 manual. 
> 
> It can be disabled + unlocked, disabled + locked, or enabled + unlocked.
> Once locked, it will stay disabled until the next reboot.
> 
> However, the manual makes it clear we are _not_ supposed to leave it
> enabled + unlocked.  Apparently, we're supposed to do our business and
> disable+lock it (i.e.  enable, read and store/process, disable+lock).
> 
> Looks like it is supposed to be used in a way that protects privacy by
> making it very hard for general use software to depend on it existing
> and being enabled.
> 
> -- 
>  Henrique Holschuh