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

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Message-ID: <CA+55aFzeCHgAtz4vCR9YaUxkuesCNEht56dKJmpytx2A-JmJkg@mail.gmail.com>
Date:   Fri, 5 Jan 2018 18:52:07 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Dan Williams <dan.j.williams@...el.com>
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 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.

             Linus