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

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Date:   Fri, 5 Jan 2018 12:27:28 +0000
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>
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>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 3/7] x86/enter: Use IBRS on syscall and interrupts

> Dave Hansen <dave.hansen@...el.com> wrote:
>> 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. ;)

I was chatting to Andrea about this, and we came to the conclusion one
use might be for qemu;  I was worried about (theoretically) whether
userspace in a guest could read privileged data from the guest kernel by
attacking the qemu process rather than by attacking the kernels.

Dave

--
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK