Re: [PATCH v2] kexec: add sysctl to disable kexec

Kees Cook
2013-12-11T23:22:21Z

On Wed, Dec 11, 2013 at 3:15 PM, Eric W. Biederman
<ebiederm@xmission.com> wrote:
> Kees Cook <keescook@chromium.org> writes:
>
>> On Wed, Dec 11, 2013 at 9:52 AM, Eric W. Biederman
>> <ebiederm@xmission.com> wrote:
>>> Kees Cook <keescook@chromium.org> writes:
>>>
>>>> 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.
>>>
>>> So let me get this straight. You object to what happens in sys_reboot
>>> so you patch sys_kexec_load?
>>
>> Yes; it's the entry point for loading the image used for crashes and
>> LINUX_REBOOT_CMD_KEXEC.
>>
>>> You give someone the privilege to boot whatever they want and yet you
>>> don't want to support them booting whatever they want?
>>>
>>> I'm sorry my brain is hurting trying to understand the logic of this
>>> patch.
>>
>> I'm not trying to claim this fixes all attack vectors from a root
>> user. That is exceedingly hard. :) However, kexec gives the root user
>> a trivial (and undetectable) way to modify the running kernel.
>> Providing an option to block sys_kexec_load for systems that will
>> never use it (or will use it once at startup) is valuable in several
>> situations. There's no reason to make an attacker's job easier, and
>> this doesn't get in any one else's way.
>
> I am simply trying to point out your patch is incomplete and silly as
> presented.
>
> LINUX_REBOOT_CMD_KEXEC should be disabled if you are calling the sysctl
> kexec_disable. If you want to it to be kexec_load_disable please call
> it that. A kexec_load_disable is a different thing than a
> kexec_disable.
>
> A kexec_disable would block both sys_kexec_load and
> sys_reboot(LINUX_REBOOT_CMD_KEXEC) and would remove any staged kexec
> images. Allowing you to stop considering kexec past that point from a
> security analysis perspective. That is what it sounds like you want.
>
> What I am asking for is an problem description and an implementation
> that are in sync, and a problem description that people can look at and
> say was this a complete implementation.
>
> Right now I can not tell what was intended so I can not truly tell if
> the patch is correct. Code with that property does not serve anyone any
> good, especially as the kernel evolves.
>
> So please decide if you want kexec_disable or kexec_load_disable, or if
> you want something a little more nuanced.
>
> Right now I think a full and complete kexec_disable makes sense. I
> probably won't use it but I think it makes sense. Your patch
> implementiong kexec_load_disable seems to be a half solution that tries
> to please everyone and does not serve anyone well. So I don't see the
> point.

Sure, it seems reasonable to clarify it's purpose. I would like block
_changes_ to the kexec image. In the case of systems not using kexec,
this freezes it to "no kexec image", for those with a crash kernel, it
freezes it to just that image, and makes sure it can't change until
reboot. Blocking LINUX_REBOOT_CMD_KEXEC doesn't make sense in the
latter sense, so it seems like changing the sysctl to
"kexec_load_disable" is the sensible direction. I can send a v3 that
changes the name and clarifies the purpose and potential uses.

-Kees

--
Kees Cook
Chrome OS Security