linux-kernel - Re: [PATCH 3/7] x86/enter: Use IBRS on syscall and interrupts

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Date:   Thu, 4 Jan 2018 21:11:23 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/7] x86/enter: Use IBRS on syscall and interrupts

On 01/04/2018 08:51 PM, Andy Lutomirski wrote:
> Do we need an arch_prctl() to enable IBRS for user mode?

Eventually, once the dust settles.  I think there's a spectrum of
paranoia here, that is roughly (with increasing paranoia):

1. do nothing
2. do retpoline
3. do IBRS in kernel
4. do IBRS always

I think you're asking for ~3.5.

Patches for 1-3 are out there and 4 is pretty straightforward.  Doing a
arch_prctl() is still straightforward, but will be a much more niche
thing than any of the other choices.  Plus, with a user interface, we
have to argue over the ABI for at least a month or two. ;)