linux-kernel - Re: [RFC 04/10] x86/mm: Only flush indirect branches when switching into non dumpable process

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Date:   Sun, 21 Jan 2018 06:07:11 -0800
From:   "H.J. Lu" <hjl.tools@...il.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        LKML <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>,
        Dave Hansen <dave.hansen@...el.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        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>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Tom Lendacky <thomas.lendacky@....com>, kvm@...r.kernel.org,
        x86@...nel.org
Subject: Re: [RFC 04/10] x86/mm: Only flush indirect branches when switching
 into non dumpable process

On Sun, Jan 21, 2018 at 4:04 AM, David Woodhouse <dwmw2@...radead.org> wrote:
>
>> On Sat, Jan 20, 2018 at 08:22:55PM +0100, KarimAllah Ahmed wrote:
>>> From: Tim Chen <tim.c.chen@...ux.intel.com>
>>>
>>> Flush indirect branches when switching into a process that marked
>>> itself non dumpable.  This protects high value processes like gpg
>>> better, without having too high performance overhead.
>>
>> So if I understand it right, this is only needed if the 'other'
>> executable itself is susceptible to spectre. If say someone audited gpg
>> for spectre-v1 and build it with retpoline, it would be safe to not
>> issue the IBPB, right?
>
>
> Spectre V2 not v1. V1 is separate.
> For V2 retpoline is enough... as long as all the libraries have it too.
>
>> So would it make sense to provide an ELF flag / personality thing such
>> that userspace can indicate its spectre-safe?
>
> Yes, Arjan and I were pondering that yesterday; it probably does make
> sense. Also for allowing a return to userspace after vmexit, if the army
> process itself is so marked.

Please take a look at how CET is handled in program property in
x86-64 psABI for CET:

https://github.com/hjl-tools/x86-psABI/wiki/x86-64-psABI-cet.pdf


-- 
H.J.