linux-kernel - Re: [patch RFC 5/5] x86/speculation: Add basic speculation control code

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Date:   Wed, 10 Jan 2018 12:29:44 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Andrea Arcangeli <aarcange@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...uxfoundation.org>, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>
Subject: Re: [patch RFC 5/5] x86/speculation: Add basic speculation control
 code

On Wed, 2018-01-10 at 13:17 +0100, Andrea Arcangeli wrote:
> On Wed, Jan 10, 2018 at 12:09:34PM +0000, David Woodhouse wrote:
> > That is not consistent with the documentation I've seen, which Intel
> > have so far utterly failed to publish AFAICT.
> > 
> > "a near indirect jump/call/return may be affected by code in a less privileged
> > prediction mode that executed AFTER IBRS mode was last written with a value of 1"
> 
> You must have misunderstood the context there, or the above text is
> wrong to begin with.

That's a quote from the Intel documentation for the IBRS feature.
Go read it, please.

> > The kernel is only protected from branch targets set in userspace
> > *BEFORE* the IBRS mode was last set to 1. If you set it to 1, then
> > leave it like that while you run userspace and then kernel code again,
> > you are not protected.
> 
> I'm sure you've got it wrong, that would be crazy if it would be the
> case.

Andrea, what part of this whole fucking mess isn't entirely batshit
insane to start with? :)

I think you are confused with the future IBRS_ATT option which will
exist on new hardware. 

Right now, IBRS works as I described it because that's the best they
could do with microcode.
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