linux-kernel - Re: [patch 15/24] x86/speculation: Add command line control for indirect branch speculation

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Message-ID: <alpine.DEB.2.21.1811221002560.1571@nanos.tec.linutronix.de>
Date:   Thu, 22 Nov 2018 10:07:33 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Borislav Petkov <bp@...en8.de>
cc:     Tom Lendacky <thomas.lendacky@....com>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [patch 15/24] x86/speculation: Add command line control for
 indirect branch speculation

On Thu, 22 Nov 2018, Thomas Gleixner wrote:
> On Thu, 22 Nov 2018, Borislav Petkov wrote:
> > > +
> > > +	/* Initialize Indirect Branch Prediction Barrier */
> > > +	if (boot_cpu_has(X86_FEATURE_IBPB)) {
> > > +		setup_force_cpu_cap(X86_FEATURE_USE_IBPB);
> > > +		pr_info("Spectre v2 mitigation: Enabling Indirect Branch Prediction Barrier\n");
> > > +	}
> > 
> > So AFAICT, if coming in here with AUTO, we won't enable IBPB and I
> > *think* AMD wants IBPB enabled. At least the whitepaper says:
> > 
> > "IBPB combined with Reptoline software support is the AMD recommended
> > setting for Linux mitigation of Google Project Zero Variant 2
> > (Spectre)."
> 
> Ok. That's indeed a step backwards, because we don't do IBPB in KVM
> anymore. I'll fix that tomorrow morning when brain is more awake.

OTOH, off means that all of it is disabled. Which was the case already
before this when spectre_v2=off is on the command line.

Now with the default to prctl/seccomp the IBPB in KVM is enabled. So no
change there.

> IBPB on context switch is controlled separately anyway now, so that's a
> nobrainer to sort out.
> 
> Though I wait for Toms answer whether we really want IBPB on context switch
> for AMD by default.

That still stands. But if we want to do that, then we need to optimize it a
bit. Isn't that hard, but ...

Thanks,

	tglx