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

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Date:   Thu, 22 Nov 2018 11:10:07 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.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, Nov 22, 2018 at 10:18:58AM +0100, Peter Zijlstra wrote:
> Right; that retpoline + IBPB case is one that came up earlier when we
> talked about this stuff. The IBPB also helps against app2app BTB ASLR
> attacks. So even if you have userspace retpoline, you might still want
> IBPB.
> 
> But yes, this should be relatively straight forward to allow/fix with
> the proposed code.

So I got some feedback from AMD that IBPB on context switch has a
small perf impact and they wouldn't mind it being enabled by default
considering that it provides protection against a lot of attack
scenarios. Basically, what the recommendation says.

But if we go and do opt-in, then they're fine with it being off by
default if we decide to do it so in the kernel.

-- 
Regards/Gruss,
    Boris.

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