linux-kernel - Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in get

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Date:   Wed, 17 Jan 2018 10:12:06 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Al Viro <viro@...iv.linux.org.uk>
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>,
        Kees Cook <keescook@...omium.org>,
        kernel-hardening@...ts.openwall.com,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alan Cox <alan@...ux.intel.com>
Subject: Re: [PATCH v3 8/9] x86: use __uaccess_begin_nospec and ASM_IFENCE in
 get_user paths

On Tue, Jan 16, 2018 at 10:50 PM, Dan Williams <dan.j.williams@...el.com> wrote:
> On Tue, Jan 16, 2018 at 10:28 PM, Al Viro <viro@...iv.linux.org.uk> wrote:
[..]
>> Anything that open-codes copy_from_user() that way is *ALREADY* fucked if
>> it cares about the overhead - recent x86 boxen will have slowdown from
>> hell on stac()/clac() pairs.  Anything like that on a hot path is already
>> deep in trouble and needs to be found and fixed.  What drivers would those
>> be?
>
> So I took a closer look and the pattern is not copy_from_user it's
> more like __get_user + write-to-hardware loops. If the performance is
> already expected to be bad for those then perhaps an lfence each loop
> iteration won't be much worse. It's still a waste because the lfence
> is only needed once after the access_ok.
>
>> We don't have that many __get_user() users left outside of arch/*
>> anymore...

Given the concern of having something easy to backport first I think
we should start with lfence in __uaccess_begin(). Any deeper changes
to the access_ok() + __get_user calling convention can build on top of
that baseline.