linux-kernel - Re: [PATCH RFC 2/4] x86/arch_prctl: add ARCH_GET_NOPTI and ARCH_SET

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Date:   Mon, 8 Jan 2018 18:26:48 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Willy Tarreau <w@....eu>, linux-kernel@...r.kernel.org,
        x86@...nel.org, tglx@...utronix.de, gnomes@...rguk.ukuu.org.uk,
        torvalds@...ux-foundation.org, Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH RFC 2/4] x86/arch_prctl: add ARCH_GET_NOPTI and
 ARCH_SET_NOPTI to enable/disable PTI


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, Jan 08, 2018 at 06:05:31PM +0100, Ingo Molnar wrote:
> > Note that there is somewhat of a fuzzy detail regarding AMD CPUs which are marked 
> > as 'Meltdown safe': should an explicit request to turn on PTI be honored by the 
> > kernel? Should that be some sort of separate 'force PTI on' attribute?
> 
> AMD should not have FEATURE_PTI enabled, and thus not end up in any code
> that cares about TIF_NOPTI.

I know, this is the status quo.

Nevertheless:

 - if someone disbelieves AMD's claims and wants to force-enable it, should it be 
   possible without patching the kernel?

 - or if someone wants to test it on AMD to increase test coverage. pti=on will 
   already be force-enable it on AMD CPUs.

Likewise, there's the counter part on the app level PTI disabling/enabling 
ABI functionality as well:

 - should there be a way for sysadmins to force PTI enabled, even on apps that 
   want to turn it off?

 - should there be a way for sysadmins to force PTI disabled, even for apps that 
   want to turn it on?

If we decide that we want to allow fine-grained, per app control of PTI, then all 
of these look valid scenarios to me.

Thanks,

	Ingo