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SubjectRe: [PATCH V31 25/25] debugfs: Disable open() when kernel is locked down
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> On Mar 26, 2019, at 10:06 PM, Greg KH <gregkh@linuxfoundation.org> wrote:
>
>> On Tue, Mar 26, 2019 at 09:29:14PM -0700, Andy Lutomirski wrote:
>>> On Tue, Mar 26, 2019 at 5:31 PM Greg KH <gregkh@linuxfoundation.org> wrote:
>>>
>>>> On Tue, Mar 26, 2019 at 12:20:24PM -0700, Andy Lutomirski wrote:
>>>> On Tue, Mar 26, 2019 at 11:28 AM Matthew Garrett
>>>> <matthewgarrett@google.com> wrote:
>>>>>
>>>>> From: Matthew Garrett <mjg59@google.com>
>>>>>
>>>>> debugfs has not been meaningfully audited in terms of ensuring that
>>>>> userland cannot trample over the kernel. At Greg's request, disable
>>>>> access to it entirely when the kernel is locked down. This is done at
>>>>> open() time rather than init time as the kernel lockdown status may be
>>>>> made stricter at runtime.
>>>>
>>>> Ugh. Some of those files are very useful. Could this perhaps still
>>>> allow O_RDONLY if we're in INTEGRITY mode?
>>>
>>> Useful for what? Debugging, sure, but for "normal operation", no kernel
>>> functionality should ever require debugfs. If it does, that's a bug and
>>> should be fixed.
>>>
>>
>> I semi-regularly read files in debugfs to diagnose things, and I think
>> it would be good for this to work on distro kernels.
>
> Doing that for debugging is wonderful. People who want this type of
> "lock down" are trading potential security for diagnositic ability.
>

I think you may be missing the point of splitting lockdown to separate integrity and confidentiality. Can you actually think of a case where *reading* a debugfs file can take over a kernel?
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