linux-kernel - Re: [PATCH v5] x86: make sure IDT is page aligned

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Date:	Tue, 16 Jul 2013 13:47:50 -0700
From:	Kees Cook <keescook@...omium.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Yinghai Lu <yinghai@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	"the arch/x86 maintainers" <x86@...nel.org>,
	Seiji Aguchi <seiji.aguchi@....com>,
	Fenghua Yu <fenghua.yu@...el.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	PaX Team <pageexec@...il.com>
Subject: Re: [PATCH v5] x86: make sure IDT is page aligned

On Tue, Jul 16, 2013 at 1:33 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> On Tue, 2013-07-16 at 13:28 -0700, Kees Cook wrote:
>> On Tue, Jul 16, 2013 at 1:21 PM, Yinghai Lu <yinghai@...nel.org> wrote:
>> > On Tue, Jul 16, 2013 at 11:34 AM, Kees Cook <keescook@...omium.org> wrote:
>> >> Since the IDT is referenced from a fixmap, make sure it is page aligned.
>> >> Merge with 32-bit one, since it was already aligned to deal with F00F
>> >> bug. Since bss is cleared before IDT setup, it can live there. This also
>> >> moves the other *_idt_table variables into common locations.
>> >>
>
>> It seemed more correct to me to define all the IDTs the same, but
>> there was no technical reason for that, just one of regularity. I only
>> care about keeping the real IDT page aligned. :) I'm fine to do
>> whatever is deemed "correct". :)
>
> I'm actually unfamiliar with the F00F bug (heard of it, but have no idea
> what it is). What happens if the F00F bug exists and we switch to an IDT
> that's not paged aligned? Is that an issue?

Regardless of F00F, the IDT is now unconditionally being set up in a
fixmap entry (so that the unprivileged "sidt" instruction won't leak a
"real" kernel address, and so that this exposed address is read-only).
If the real IDT is not page aligned, the fixmap IDT will appear offset
and everything starts calling the wrong handlers.

The other IDTs don't need to be page aligned, but I marked them that
way in the clean up because it seemed sensible to define these tables
similarly. I can change the others to be __cacheline_aligned_bss if
that's desired.

-Kees

--
Kees Cook
Chrome OS Security
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