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

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Message-ID: <CAPcyv4jLbbASjcfu2dvxXbY8+Q_=wLO6zKGhCBdzRGXHBNuL1w@mail.gmail.com>
Date:   Fri, 5 Jan 2018 21:47:02 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     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 Fri, Jan 5, 2018 at 6:52 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Fri, Jan 5, 2018 at 5:10 PM, Dan Williams <dan.j.williams@...el.com> wrote:
>> From: Andi Kleen <ak@...ux.intel.com>
>>
>> When access_ok fails we should always stop speculating.
>> Add the required barriers to the x86 access_ok macro.
>
> Honestly, this seems completely bogus.
>
> The description is pure garbage afaik.
>
> The fact is, we have to stop speculating when access_ok() does *not*
> fail - because that's when we'll actually do the access. And it's that
> access that needs to be non-speculative.
>
> That actually seems to be what the code does (it stops speculation
> when __range_not_ok() returns false, but access_ok() is
> !__range_not_ok()). But the explanation is crap, and dangerous.

Oh, bother, yes, good catch. It's been a long week.  I'll take a look
at moving this to uaccess_begin().