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

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Message-ID: <20180121112224.GH2269@hirez.programming.kicks-ass.net>
Date:   Sun, 21 Jan 2018 12:22:24 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     KarimAllah Ahmed <karahmed@...zon.de>
Cc:     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 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?

So would it make sense to provide an ELF flag / personality thing such
that userspace can indicate its spectre-safe?

I realize that this is all future work, because so far auditing for v1
is a lot of pain (we need better tools), but would it be something that
makes sense in the longer term?