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Subject: Re: [PATCH V31 25/25] debugfs: Disable open() when kernel is locked down
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20190327050615.GA548@kroah.com>
Date:   Tue, 26 Mar 2019 22:29:41 -0700
Cc:     Andy Lutomirski <luto@kernel.org>,
        Matthew Garrett <matthewgarrett@google.com>,
        James Morris <jmorris@namei.org>,
        LSM List <linux-security-module@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        David Howells <dhowells@redhat.com>,
        Linux API <linux-api@vger.kernel.org>,
        Matthew Garrett <mjg59@google.com>
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Message-Id: <16124107-70D3-4CA0-9766-36FC6DC10128@amacapital.net>
References: <20190326182742.16950-1-matthewgarrett@google.com> <20190326182742.16950-26-matthewgarrett@google.com> <CALCETrXDF2c2-gnOyNTnEn-uPAZp0AaSB_PB8e2y8jf7zo=Cyg@mail.gmail.com> <20190327003057.GA27311@kroah.com> <CALCETrVDDf4dj3Md1Nxc5P_NCToXGnAbEO3bGfwgKSfeVmU4Kw@mail.gmail.com> <20190327050615.GA548@kroah.com>
To:     Greg KH <gregkh@linuxfoundation.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On Mar 26, 2019, at 10:06 PM, Greg KH <gregkh@linuxfoundation.org> wrote:
>=20
>> 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> wro=
te:
>>>=20
>>>> 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:
>>>>>=20
>>>>> From: Matthew Garrett <mjg59@google.com>
>>>>>=20
>>>>> 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.
>>>>=20
>>>> Ugh.  Some of those files are very useful.  Could this perhaps still
>>>> allow O_RDONLY if we're in INTEGRITY mode?
>>>=20
>>> 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.
>>>=20
>>=20
>> I semi-regularly read files in debugfs to diagnose things, and I think
>> it would be good for this to work on distro kernels.
>=20
> Doing that for debugging is wonderful.  People who want this type of
> "lock down" are trading potential security for diagnositic ability.
>=20

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