linux-kernel - Re: [patch V2 27/28] x86/speculation: Add seccomp Spectre v2 user space protection mode

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Date:   Sun, 25 Nov 2018 23:28:59 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
cc:     Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Jiri Kosina <jkosina@...e.cz>, thomas.lendacky@....com,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>, dave.hansen@...el.com,
        Casey Schaufler <casey.schaufler@...el.com>,
        "Mallick, Asit K" <asit.k.mallick@...el.com>,
        "Van De Ven, Arjan" <arjan@...ux.intel.com>, jcm@...hat.com,
        longman9394@...il.com, Greg KH <gregkh@...uxfoundation.org>,
        david.c.stewart@...el.com, Kees Cook <keescook@...omium.org>
Subject: Re: [patch V2 27/28] x86/speculation: Add seccomp Spectre v2 user
 space protection mode

On Sun, 25 Nov 2018, Linus Torvalds wrote:

> [ You forgot to fix your quilt setup.. ]

Duh. Should have pinned that package.

> On Sun, 25 Nov 2018, Thomas Gleixner wrote:
> >
> > The mitigation guide documents how STIPB works:
> >
> >    Setting bit 1 (STIBP) of the IA32_SPEC_CTRL MSR on a logical processor
> >    prevents the predicted targets of indirect branches on any logical
> >    processor of that core from being controlled by software that executes
> >    (or executed previously) on another logical processor of the same core.
> 
> Can we please just fix this stupid lie?

Well, it's not a lie. The above is correct, it just does not tell WHY this
works.

> Yes, Intel calls it "STIBP" and tries to make it out to be about the
> indirect branch predictor being per-SMT thread.
> 
> But the reason it is unacceptable is apparently because in reality it just
> disables indirect branch prediction entirely. So yes, *technically* it's
> true that that limits indirect branch prediction to just a single SMT
> core, but in reality it is just a "go really slow" mode.

Indeed. Just checked the documentation again, it's also not clear whether
IBPB is required if STIPB is in use.

Thanks,

	tglx