linux-kernel - Re: [PATCH 06/18] x86, barrier: stop speculation for failed access

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Message-ID: <CAPcyv4g502B4DK9ZVT0m3T-CRAMnZ3+R6v_hPga6mQAtQAGM_A@mail.gmail.com>
Date:   Tue, 9 Jan 2018 14:35:48 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-arch@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Network Development <netdev@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH 06/18] x86, barrier: stop speculation for failed access_ok

On Tue, Jan 9, 2018 at 2:23 PM, Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> On Tue, Jan 09, 2018 at 01:59:04PM -0800, Dan Williams wrote:
>> > Right, but what's the purpose of preventing speculation past
>> > access_ok()?
>>
>> Caution. It's the same rationale for the nospec_array_ptr() patches.
>> If we, kernel community, can identify any possible speculation past a
>> bounds check we should inject a speculation mitigation. Unless there's
>> a way to be 100% certain that the first unwanted speculation can be
>> turned into a gadget later on in the instruction stream, err on the
>> side of shutting it down early.
>
> I'm all for being cautious.  The nospec_array_ptr() patches are fine,
> and they make sense in light of the variant 1 CVE.
>
> But that still doesn't answer my question.  I haven't seen *any*
> rationale for this patch.  It would be helpful to at least describe
> what's being protected against, even if it's hypothetical.  How can we
> review it if the commit log doesn't describe its purpose?

Certainly the changelog needs improvement, I'll roll these concerns
into v2 and we can go from there.