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

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Date:   Mon, 26 Nov 2018 15:48:42 -0500
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        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>,
        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

Hello,

On Sun, Nov 25, 2018 at 11:28:59PM +0100, Thomas Gleixner wrote:
> Indeed. Just checked the documentation again, it's also not clear whether
> IBPB is required if STIPB is in use.

I tried to ask this question too earlier:

https://lkml.kernel.org/r/20181119234528.GJ29258@redhat.com

If the BTB mistraining in SECCOMP context with STIBP set in SPEC_CTRL,
can still influence the hyperthreading sibling after STIBP is cleared,
IBPB is needed before clearing STIBP. Otherwise it's not. Unless told
otherwise, it'd be safe to assume IBPB is needed in such case.

The SPEC_CTRL MSR specs seems a catch-all lowest common denominator
and so intuition or measurement of the exact behavior in one CPU
model, don't necessarily give a result that can be applied to all
microcodes out there.

Thanks,
Andrea