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

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Message-ID: <CAG7+5M3==rndLLak9cZBKKdgQMmJMpSP_jpQd8HOVGp6LhUAgw@mail.gmail.com>
Date:	Tue, 9 Apr 2013 11:54:15 -0700
From:	Eric Northup <digitaleric@...gle.com>
To:	Kees Cook <keescook@...omium.org>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	"kernel-hardening@...ts.openwall.com" 
	<kernel-hardening@...ts.openwall.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>
Subject: Re: [kernel-hardening] Re: [PATCH] x86: make IDT read-only

On Tue, Apr 9, 2013 at 11:46 AM, Kees Cook <keescook@...omium.org> wrote:
> On Tue, Apr 9, 2013 at 11:39 AM, H. Peter Anvin <hpa@...or.com> wrote:
>> On 04/09/2013 11:31 AM, Kees Cook wrote:
>>>>> ...
>>>>> 0xffff880001e00000-0xffff88001fe00000         480M     RW         PSE GLB NX pmd
>>>>>
>>>>
>>>> That is the 1:1 memory map area...
>>>
>>> Meaning what?
>>>
>>> -Kees
>>>
>>
>> That's the area in which we just map 1:1 to memory.  Anything allocated
>> with e.g. kmalloc() ends up with those addresses.
>
> Ah-ha! Yes, I see now when comparing the debug/kernel_page_tables
> reports. It's just the High Kernel Mapping that we care about.
> Addresses outside that range are less of a leak. Excellent, then GDT
> may not be a problem. Whew.

The GDT is a problem if the address returned by 'sgdt' is
kernel-writable - it doesn't necessarily reveal the random offset, but
I'm pretty sure that writing to the GDT could cause privilege
escalation.

>
> Does the v2 IDT patch look okay, BTW?
>
> -Kees
>
> --
> Kees Cook
> Chrome OS Security
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