Date: Wed, 23 Nov 2016 15:29:17 -0200
From: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
To: Borislav Petkov <bp@suse.de>
Cc: "Luck, Tony" <tony.luck@intel.com>, Andi Kleen <andi@firstfloor.org>,
        Ashok Raj <ashok.raj@intel.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2] x86/mce: Include the PPIN in machine check records
 when it is available
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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