linux-kernel - RE: [PATCH 0/7] IBRS patch series

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Date:   Thu, 4 Jan 2018 21:22:34 +0000
From:   "Van De Ven, Arjan" <arjan.van.de.ven@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>,
        David Woodhouse <dwmw2@...radead.org>
CC:     Tim Chen <tim.c.chen@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Andrea Arcangeli" <aarcange@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        "Linux Kernel Mailing List" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 0/7] IBRS patch series

> On Thu, Jan 4, 2018 at 11:19 AM, David Woodhouse <dwmw2@...radead.org>
> wrote:
> >
> > On Skylake the target for a 'ret' instruction may also come from the
> > BTB. So if you ever let the RSB (which remembers where the 'call's came
> > from get empty, you end up vulnerable.
> 
> That sounds like it could cause mispredicts, but it doesn't sound _exploitable_.
> 
> Sure, interrupts in between the call instruction and the 'ret' could
> overflow the return stack. And we could migrate to another CPU. And so
> apparently SMM clears the return stack too.
> 
> ... but again, none of them sound even remotely _exploitable_.
> 

this is about a level of paranoia you are comfortable with.

Retpoline on Skylake raises the bar for the issue enormously, but there are a set of corner cases that exist and that are not trivial to prove you dealt with them.

personally I am comfortable with retpoline on Skylake, but I would like to have IBRS as an opt in for the paranoid.