linux-kernel - Re: [PATCH V31 25/25] debugfs: Disable open() when kernel is locked down

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Date:   Tue, 26 Mar 2019 22:29:41 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Greg KH <gregkh@...uxfoundation.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Matthew Garrett <matthewgarrett@...gle.com>,
        James Morris <jmorris@...ei.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        David Howells <dhowells@...hat.com>,
        Linux API <linux-api@...r.kernel.org>,
        Matthew Garrett <mjg59@...gle.com>
Subject: Re: [PATCH V31 25/25] debugfs: Disable open() when kernel is locked down



> On Mar 26, 2019, at 10:06 PM, Greg KH <gregkh@...uxfoundation.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@...uxfoundation.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@...gle.com> wrote:
>>>>> 
>>>>> From: Matthew Garrett <mjg59@...gle.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?