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From:   Andy Lutomirski <luto@kernel.org>
Date:   Wed, 11 Apr 2018 19:54:12 -0700
Message-ID: <CALCETrWynogzZcaKMLWKce1rMvAC_sViReBDyiEWL-rm21HvAg@mail.gmail.com>
Subject: Re: [PATCH 24/24] debugfs: Restrict debugfs when the kernel is locked down
To:     Greg KH <greg@kroah.com>
Cc:     David Howells <dhowells@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-man <linux-man@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        James Morris <jmorris@namei.org>,
        LKML <linux-kernel@vger.kernel.org>,
        LSM List <linux-security-module@vger.kernel.org>
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On Wed, Apr 11, 2018 at 1:33 PM, Greg KH <greg@kroah.com> wrote:
> On Wed, Apr 11, 2018 at 09:09:16PM +0100, David Howells wrote:
>> Greg KH <greg@kroah.com> wrote:
>>
>> > Why not just disable debugfs entirely?  This half-hearted way to sorta
>> > lock it down is odd, it is meant to not be there at all, nothing in your
>> > normal system should ever depend on it.
>> >
>> > So again just don't allow it to be mounted at all, much simpler and more
>> > obvious as to what is going on.
>>
>> Yeah, I agree - and then I got complaints because it seems that it's been
>> abused to allow drivers and userspace components to communicate.
>
> With in-kernel code?  Please let me know and I'll go fix it up to not
> allow that, as that is not ok.
>
> I do know of some bad examples of out-of-tree code abusing debugfs to do
> crazy things (battery level monitoring?), but that's their own fault...
>
> debugfs is for DEBUGGING!  For anything you all feel should be "secure",
> then just disable it entirely.
>

Debugfs is very, very useful for, ahem, debugging.  I really think
this is an example of why we should split lockdown into the read and
write varieties and allow mounting and reading debugfs when only write
is locked down.