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 12:04:03 -0000
From:   "David Woodhouse" <dwmw2@...radead.org>
To:     "Peter Zijlstra" <peterz@...radead.org>, hjl.tools@...il.com
Cc:     "KarimAllah Ahmed" <karahmed@...zon.de>,
        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?


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.

> 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?

It's *only* retpoline so it isn't actually that much. Although I'm wary of
Cc'ing HJ on such thoughts because he seems to never sleep and always
respond promptly with "OK I did that... " :)

If we did systematically do this in userspace we'd probably want to do
external thunks there too, and a flag in the auxvec to tell it not to
bother (for IBRS_ALL etc.).

-- 
dwmw2