Re: [PATCH 24/24] debugfs: Restrict debugfs when the kernel is locked down

From: Andy Lutomirski <luto@kernel.org>
To: Greg KH <greg@kroah.com>
Cc: Andy Lutomirski <luto@kernel.org>,
	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>
Subject: Re: [PATCH 24/24] debugfs: Restrict debugfs when the kernel is locked down
Date: Thu, 12 Apr 2018 07:19:35 -0700	[thread overview]
Message-ID: <CALCETrVm8c0dN=kymg=5-TQEWbL_zk9yCr4Bv4PKJXNC=6j=Fw@mail.gmail.com> (raw)
In-Reply-To: <20180412082313.GA6054@kroah.com>

On Thu, Apr 12, 2018 at 1:23 AM, Greg KH <greg@kroah.com> wrote:
> On Wed, Apr 11, 2018 at 07:54:12PM -0700, Andy Lutomirski wrote:
>> 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.
>
> Ok, but be sure that there are no "secrets" in those debugging files if
> you really buy into the whole "lock down" mess...
>
> Really, it's easier to just disable the whole thing.
>

I mostly agree with your sentiment.  I'm saying that, for most uses, I
*don't* buy into the idea that a normal secure-boot-supporting distro
should block debugfs.  I sometimes like to ask people who report
problems to send me the contents of such-and-such file in debugfs, and
I think it should keep working.  Blocking write access to debugfs is
mostly sensible for a lockdown system, but blocking read only makes
sense if you're worried about straight-up bugs or if you think that
debugfs contains protection-worthy secrets.

What I want to see is:

lockdown=protect_integrity: debugfs is read-only, bpf and perf are
unrestricted, iopl and ioperm are disabled, etc.

lockdown=protect_integrity_and_secrecy: debugfs is gone, bpf and perf
are restricted, plus all the restrictions from
lockdown=protect_integrity

Distros should strongly prefer lockdown=protect_integrity (or
lockdown=off) by default.  lockdown=protect_integrity_and_secrecy is
for custom setups, embedded applications, etc.

--Andy