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

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Date:   Tue, 23 Jan 2018 07:01:31 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Ingo Molnar <mingo@...nel.org>,
        David Woodhouse <dwmw2@...radead.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        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>,
        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>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        KVM list <kvm@...r.kernel.org>,
        the arch/x86 maintainers <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 01/23/2018 01:27 AM, Ingo Molnar wrote:
> 
>  - All asynchronous contexts (IRQs, NMIs, etc.) stuff the RSB before IRET. (The 
>    tracking could probably made IRQ and maybe even NMI safe, but the worst-case 
>    nesting scenarios make my head ache.)

This all sounds totally workable to me.  We talked about using ftrace
itself to track call depth, but it would be unusable in production, of
course.  This seems workable, though.  You're also totally right about
the zero overhead on most kernels with it turned off when we don't need
RSB underflow protection (basically pre-Skylake).

I also agree that the safe thing to do is to just stuff before iret.  I
bet we can get a ftrace-driven RSB tracker working precisely enough even
with NMIs, but it's way simpler to just stuff and be done with it for now.