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 18:26:46 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Ingo Molnar <mingo@...nel.org>
cc:     Willy Tarreau <w@....eu>, linux-kernel@...r.kernel.org,
        x86@...nel.org, gnomes@...rguk.ukuu.org.uk,
        torvalds@...ux-foundation.org, Dave Hansen <dave@...1.net>,
        Andy Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH RFC 2/4] x86/arch_prctl: add ARCH_GET_NOPTI and ARCH_SET_NOPTI
 to enable/disable PTI

On Mon, 8 Jan 2018, Ingo Molnar wrote:
> * Thomas Gleixner <tglx@...utronix.de> wrote:
> > Per task is really an odd choice. That should be per process I think, but
> > that of course needs synchronization of some form. Aside of that we need to
> > think about fork().
> 
> So per task (thread) is the most natural approach to low level asm flaggery.

Well, yes and no. PTI is a property of the mm/pgdir and that's process
wide.
 
> Making it per thread also makes some sense conceptually: in a complex 
> multi-threaded runtime implementation some threads might never execute
> 'untrusted' code, some might. No need to penalize the 'server' threads.

If one thread runs untrusted code then your 'trusted' thread is not longer
trusted either as they share everything.

Thanks,

	tglx