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Subject: Re: [PATCH v2] kexec: add sysctl to disable kexec
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Tue, 10 Dec 2013 08:32:38 -0800
To: Vivek Goyal <vgoyal@redhat.com>, Kees Cook <keescook@chromium.org>
CC: LKML <linux-kernel@vger.kernel.org>, Rik van Riel <riel@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew Garrett <matthew.garrett@nebula.com>,
        Rob Landley <rob@landley.net>, Eric Biederman <ebiederm@xmission.com>,
        Ingo Molnar <mingo@kernel.org>, Peter Zijlstra <peterz@infradead.org>,
        Mel Gorman <mgorman@suse.de>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        kexec@lists.infradead.org
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Of course it isn't.

Vivek Goyal <vgoyal@redhat.com> wrote:
>On Mon, Dec 09, 2013 at 05:06:10PM -0800, Kees Cook wrote:
>> On Mon, Dec 9, 2013 at 4:34 PM, H. Peter Anvin <hpa@zytor.com> wrote:
>> > On 12/09/2013 04:16 PM, Kees Cook wrote:
>> >> For general-purpose (i.e. distro) kernel builds it makes sense to
>build with
>> >> CONFIG_KEXEC to allow end users to choose what kind of things they
>want to do
>> >> with kexec. However, in the face of trying to lock down a system
>with such
>> >> a kernel, there needs to be a way to disable kexec (much like
>module loading
>> >> can be disabled). Without this, it is too easy for the root user
>to modify
>> >> kernel memory even when CONFIG_STRICT_DEVMEM and modules_disabled
>are set.
>> >>
>> >> Signed-off-by: Kees Cook <keescook@chromium.org>
>> >> Acked-by: Rik van Riel <riel@redhat.com>
>> >
>> > So the logic is to load a crashkernel and then lock down the
>machine
>> > before services, networking etc. are enabled?
>> 
>> Right, or to just turn it off at boot time if kexec will not be used
>at all.
>
>kdump kernel is loaded with the help of kdump service. Different
>distro's
>might have different dependencies for that serivce. But recently in
>fedora
>we wait network to come up before starting that service. (So that nfs
>targets can be mounted and checked for valid dump destinations).
>
>IOW, crash kernel is loaded quite late in the game (quite a few
>services
>have run and possibly networking is up too). To me, practically one
>will
>disable kdump also if you change state of this knob early.
>
>Thanks
>Vivek

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