linux-kernel - Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict Indirect Branch Speculation

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Date:   Wed, 24 Jan 2018 01:22:03 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Borislav Petkov <bp@...e.de>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        Janakarajan Natarajan <Janakarajan.Natarajan@....com>,
        Joerg Roedel <joro@...tes.org>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Laura Abbott <labbott@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        kvm list <kvm@...r.kernel.org>, X86 ML <x86@...nel.org>,
        Arjan Van De Ven <arjan.van.de.ven@...el.com>
Subject: Re: [RFC 09/10] x86/enter: Create macros to restrict/unrestrict
 Indirect Branch Speculation

On Tue, 2018-01-23 at 17:00 -0800, Andy Lutomirski wrote:
> On Tue, Jan 23, 2018 at 4:47 PM, Tim Chen <tim.c.chen@...ux.intel.com> wrote:
> > 
> > On 01/23/2018 03:14 PM, Woodhouse, David wrote:
> > > 
> > > On Tue, 2018-01-23 at 14:49 -0800, Andi Kleen wrote:
> > > > 
> > > > > 
> > > > > Not sure.  Maybe to start, the answer might be to allow it to be set for
> > > > > the ultra-paranoid, but in general don't enable it by default.  Having it
> > > > > enabled would be an alternative to someone deciding to disable SMT, since
> > > > > that would have even more of a performance impact.
> > > > I agree. A reasonable strategy would be to only enable it for
> > > > processes that have dumpable disabled. This should be already set for
> > > > high value processes like GPG, and allows others to opt-in if
> > > > they need to.
> > > That seems to make sense, and I think was the solution we were
> > > approaching for IBPB on context switch too, right?
> > > 
> > > Are we generally agreed on dumpable as the criterion for both of those?
> > > 
> > It is a reasonable approach.  Let a process who needs max security
> > opt in with disabled dumpable. It can have a flush with IBPB clear before
> > starting to run, and have STIBP set while running.
> > 
> Do we maybe want a separate opt in?  I can easily imagine things like
> web browsers that *don't* want to be non-dumpable but do want this
> opt-in.
 
This is to protect you from another local process running on a HT
sibling. Not the kind of thing that web browsers are normally worrying
about.

> Also, what's the performance hit of STIBP?

Varies per CPU generation, but generally approaching that of full IBRS
I think? I don't recall looking at this specifically (since we haven't
actually used it for this yet).
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