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Message-ID: <20180112204126.257jubn2hccgtzdt@treble>
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
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