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Date:   Tue, 26 Mar 2019 09:31:37 +0900
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Matthew Garrett <matthewgarrett@...gle.com>
Cc:     jmorris@...ei.org, linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org, dhowells@...hat.com,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        acpi4asus-user@...ts.sourceforge.net,
        platform-driver-x86@...r.kernel.org,
        Matthew Garrett <mjg59@...f.ucam.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 25/27] debugfs: Restrict debugfs when the kernel is
 locked down

On Mon, Mar 25, 2019 at 03:09:52PM -0700, Matthew Garrett wrote:
> From: David Howells <dhowells@...hat.com>
> 
> Disallow opening of debugfs files that might be used to muck around when
> the kernel is locked down as various drivers give raw access to hardware
> through debugfs.  Given the effort of auditing all 2000 or so files and
> manually fixing each one as necessary, I've chosen to apply a heuristic
> instead.  The following changes are made:
> 
>  (1) chmod and chown are disallowed on debugfs objects (though the root dir
>      can be modified by mount and remount, but I'm not worried about that).
> 
>  (2) When the kernel is locked down, only files with the following criteria
>      are permitted to be opened:
> 
> 	- The file must have mode 00444
> 	- The file must not have ioctl methods
> 	- The file must not have mmap
> 
>  (3) When the kernel is locked down, files may only be opened for reading.
> 
> Normal device interaction should be done through configfs, sysfs or a
> miscdev, not debugfs.

Then why not just not allow debugfs at all if it is such a "big
problem"?

Actually, it is a big problem, and I am pushing for systems to not mount
debugfs at all these days, but that's a different story...

Just "disabling" it entirely for this crazy dream of a "locked down"
kernel is fine for me.

thanks,

greg k-h

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