linux-kernel - Re: [kernel-hardening] Re: [PATCH] x86: make IDT read-only

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Date:	Tue, 9 Apr 2013 11:22:27 -0700
From:	Kees Cook <keescook@...omium.org>
To:	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Alex Shi <alex.shi@...el.com>,
	Borislav Petkov <borislav.petkov@....com>,
	Alexander Duyck <alexander.h.duyck@...el.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"virtualization@...ts.linux-foundation.org" 
	<virtualization@...ts.linux-foundation.org>,
	Dan Rosenberg <drosenberg@...curity.com>,
	Julien Tinnes <jln@...gle.com>, Will Drewry <wad@...omium.org>,
	Eric Northup <digitaleric@...gle.com>
Subject: Re: [kernel-hardening] Re: [PATCH] x86: make IDT read-only

On Tue, Apr 9, 2013 at 2:23 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Mon, 8 Apr 2013, H. Peter Anvin wrote:
>
>> On 04/08/2013 03:43 PM, Kees Cook wrote:
>> > This makes the IDT unconditionally read-only. This primarily removes
>> > the IDT from being a target for arbitrary memory write attacks. It has
>> > an added benefit of also not leaking (via the "sidt" instruction) the
>> > kernel base offset, if it has been relocated.
>> >
>> > Signed-off-by: Kees Cook <keescook@...omium.org>
>> > Cc: Eric Northup <digitaleric@...gle.com>
>>
>> Also, tglx: does this interfere with your per-cpu IDT efforts?
>
> I don't think so. And it's on the backburner at the moment.

What would be a good way to do something similar for the GDT? sgdt
leaks GDT location as well, and even though it's percpu, it should be
trivial to figure out a kernel base address, IIUC.

$ ./sgdt
ffff88001fc04000
# cat /sys/kernel/debug/kernel_page_tables
...
---[ Low Kernel Mapping ]---
...
0xffff880001e00000-0xffff88001fe00000         480M     RW         PSE GLB NX pmd

With the IDT patch, things look good for sidt:

$ ./sidt
ffffffffff579000
# cat /sys/kernel/debug/kernel_page_tables
...
---[ End Modules ]---
0xffffffffff579000-0xffffffffff57a000           4K     ro             GLB NX pte

Can we create a RO fixed per-cpu area?

-Kees

--
Kees Cook
Chrome OS Security
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