linux-kernel - Re: Avoid speculative indirect calls in kernel

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Date:   Thu, 4 Jan 2018 19:05:06 +0100
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Andrew Cooper <andrew.cooper3@...rix.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        "pavel@....cz" <pavel@....cz>,
        "tim.c.chen@...ux.intel.com" <tim.c.chen@...ux.intel.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "torvalds@...ux-foundation.org" <torvalds@...ux-foundation.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "andi@...stfloor.org" <andi@...stfloor.org>,
        "gnomes@...rguk.ukuu.org.uk" <gnomes@...rguk.ukuu.org.uk>,
        "gregkh@...ux-foundation.org" <gregkh@...ux-foundation.org>
Subject: Re: Avoid speculative indirect calls in kernel

Hello,

On Thu, Jan 04, 2018 at 06:15:01PM +0100, Paolo Bonzini wrote:
> On 04/01/2018 18:13, Dave Hansen wrote:
> > On 01/04/2018 08:25 AM, Andrea Arcangeli wrote:
> >> It's only where SPEC_CTRL is missing and only IBPB_SUPPORT is
> >> available, that ibrs 0 ibpb 2 is the only option to fix variant#2 for
> >> good.
> > 
> > Could you help us decode what "ibrs 0 ibpb 2" means to you?
> 
> IBRS 0 = disabled
> IBRS 1 = only kernel sets IBRS=1
> IBRS 2 = indirect branch prediction fully disabled, or do the right
> thing on future processors
> 
> IBPB 0 = disabled
> IBPB 1 = on context switch
> IBPB 2 = on every kernel or hypervisor entry

Yes.

ibrs 0 ibpb 2 kernel entry and vmexit.

ibpb 2 if set, is forcing ibrs to 0 (it's sharing the same branch in
the kernel entry points and it wouldn't make sense anyway to enable
ibrs with ibpb 2).

ibrs 0 ibpb 2 is only ever activated if SPEC_CTRL is missing but
IBPB_SUPPORT is present and it does the same as stuff_RSB, imagine it
like a stuff_IBP where stuff_RSB is already called.