linux-kernel - Re: [PATCH v2] kexec: add sysctl to disable kexec

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Message-ID: <CAGXu5j+1-X+35ks29XuG8g=ZLVSFSmQJbTETdc3QYqOonpNVTQ@mail.gmail.com>
Date:	Tue, 10 Dec 2013 11:33:16 -0800
From:	Kees Cook <keescook@...omium.org>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matthew Garrett <matthew.garrett@...ula.com>,
	Rob Landley <rob@...dley.net>,
	Eric Biederman <ebiederm@...ssion.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Mel Gorman <mgorman@...e.de>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	kexec@...ts.infradead.org
Subject: Re: [PATCH v2] kexec: add sysctl to disable kexec

On Tue, Dec 10, 2013 at 11:14 AM, Vivek Goyal <vgoyal@...hat.com> wrote:
> On Tue, Dec 10, 2013 at 10:54:00AM -0800, H. Peter Anvin wrote:
>> On 12/10/2013 10:33 AM, Vivek Goyal wrote:
>> > On Tue, Dec 10, 2013 at 08:32:38AM -0800, H. Peter Anvin wrote:
>> >> Of course it isn't.
>> >
>> > I am not sure what are you trying to say. This is too brief.
>> >
>> > Thanks
>> > Vivek
>> >
>>
>> Of course it is not sufficient.  Once you can get arbitrary code into
>> kernel space (CPL 0) you can do anything, and "disabling jump back" is
>> just a speed bump.
>
> Agreed that disabling jump back is only a speed bump.
>
> Kees, so how would be use this knob?
>
> - If I put it in some init script, then root will permission to modify
>   and remove it.

Correct. Same applies to changing the kernel itself, yes. However, all
those options require a reboot, and unexpected system reboots should
signal a significant problem to a system owner. :) This is a big step
better than a silent kernel root kit getting installed.

> - Can one specify sysctl parameters on command line? If yes, then one
>   can disable this using kernel command line and in that case kdump will
>   be disabled too.

If they can be set on the command line, I'd like to know about it.
That would let me flip the bit even earlier, since I don't use kexec
at all (and don't need to wait until boot up is done).

-Kees

-- 
Kees Cook
Chrome OS Security
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