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

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Message-ID: <CAPcyv4hVX4h9j-Uf=Ja5_0p3+qiWOiPd4cUaxoJD9en5TiVGPw@mail.gmail.com>
Date:   Mon, 8 Jan 2018 13:09:37 -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 Sat, Jan 6, 2018 at 5:20 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Sat, Jan 6, 2018 at 3:31 PM, Dan Williams <dan.j.williams@...el.com> wrote:
>>
>> I assume if we put this in uaccess_begin() we also need audit for
>> paths that use access_ok but don't do on to call uaccess_begin()? A
>> quick glance shows a few places where we are open coding the stac().
>> Perhaps land the lfence in stac() directly?
>
> Yeah, we should put it in uaccess_begin(), and in the actual user
> accessor helpers that do stac. Some of them probably should be changed
> to use uaccess_begin() instead while at it.
>
> One question for the CPU people: do we actually care and need to do
> this for things that might *write* to something? The speculative write
> obviously is killed, but does it perhaps bring in a cacheline even
> when killed?

As far as I understand a write could trigger a request-for-ownership
read for the target cacheline.

> Because maybe we don't need the lfence in put_user(), only in get_user()?

Even though writes can trigger reads, as far as I can see the write
needs to be dependent on the first out-of-bounds read:

if (x < max)
    y = array1[x];
    put_user(array2 + y, z);

...in other words that first read should be annotated with
nospec_array_ptr() making an lfence in put_user() or other writes
moot.

yp = nospec_array_ptr(array1, x, max);
if (yp)
    y = *yp;
    put_user(array2 + y, z);