linux-kernel - Re: [PATCH v2 07/19] x86: introduce __uaccess_begin_nospec and ASM

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Date:   Fri, 12 Jan 2018 14:41:26 -0600
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Dan Williams <dan.j.williams@...el.com>,
        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, X86 ML <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "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 v2 07/19] x86: introduce __uaccess_begin_nospec and
 ASM_IFENCE

On Fri, Jan 12, 2018 at 12:01:04PM -0800, Linus Torvalds wrote:
> On Fri, Jan 12, 2018 at 11:26 AM, Dan Williams <dan.j.williams@...el.com> wrote:
> >
> > By the time we get to de-reference uptr we know it is not pointing at
> > kernel memory, because access_ok would have failed and the cpu would
> > have waited for that failure result before doing anything else.
> 
> I'm not actually convinced that's right in the original patches,
> exactly because of the issue that Josh pointed out: even if there is a
> comparison inside access_ok() that will be properly serialized, then
> that comparison can (and sometimes does) just cause a truth value to
> be generated, and then there  might be *another* comparison of that
> return value after the lfence. And while the return value is table,
> the conditional branch on that comparison isn't.
> 
> The new model of just doing it together with the STAC should be fine, though.

Aha, that clears it up for me, thanks.  I was still thinking about the
previous version of the patch which had the barrier in access_ok().  I
didn't realize the new version moved the barrier to after the
access_ok() checks.

-- 
Josh