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

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Date:	Tue, 16 Jul 2013 15:03:08 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Kees Cook <keescook@...omium.org>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Yinghai Lu <yinghai@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.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 07/16/2013 01:47 PM, Kees Cook wrote:
> 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.
> 

I'm fine keeping them as page aligned.  They are page-sized on x86-64
anyway (half page on i386).

	-hpa


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