linux-kernel - Re: [PATCH 2/2] debugfs: Add access restriction option

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Message-ID: <45741a2f-3763-a41d-62ad-29b5fbb16101@sony.com>
Date:   Wed, 15 Jul 2020 10:59:12 +0000
From:   "Enderborg, Peter" <Peter.Enderborg@...y.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 2/2] debugfs: Add access restriction option

On 7/15/20 12:35 PM, Greg Kroah-Hartman wrote:
> On Wed, Jul 15, 2020 at 10:03:19AM +0000, Enderborg, Peter wrote:
>> On 7/15/20 11:39 AM, Greg Kroah-Hartman wrote:
>>> On Wed, Jul 15, 2020 at 10:42:07AM +0200, Peter Enderborg wrote:
>>>> Since debugfs include sensitive information it need to be treated
>>>> carefully. But it also has many very useful debug functions for userspace.
>>>> With this option we can have same configuration for system with
>>>> need of debugfs and a way to turn it off. This gives a extra protection
>>>> for exposure on systems where user-space services with system
>>>> access are attacked.
>>>>
>>>> It is controlled by a configurable default value that can be override
>>>> with a kernel command line parameter. (debugfs=)
>>>>
>>>> It can be on or off, but also internally on but not seen from user-space.
>>>> This no-fs mode do not register a debugfs as filesystem, but client can
>>>> register their parts in the internal structures. This data can be readed
>>>> with a debugger or saved with a crashkernel. When it is off clients
>>>> get EPERM error when accessing the functions for registering their
>>>> components.
>>>>
>>>> Signed-off-by: Peter Enderborg <peter.enderborg@...y.com>
>>>> ---
>>>>  .../admin-guide/kernel-parameters.txt         | 14 +++++++
>>>>  fs/debugfs/inode.c                            | 37 +++++++++++++++++++
>>>>  fs/debugfs/internal.h                         | 14 +++++++
>>>>  lib/Kconfig.debug                             | 32 ++++++++++++++++
>>>>  4 files changed, 97 insertions(+)
>>>>
>>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>>>> index fb95fad81c79..805aa2e58491 100644
>>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>>> @@ -827,6 +827,20 @@
>>>>  			useful to also enable the page_owner functionality.
>>>>  			on: enable the feature
>>>>  
>>>> +	debugfs=    	[KNL] This parameter enables what is exposed to userspace
>>>> +			and debugfs internal clients.
>>>> +			Format: { on, no-fs, off }
>>>> +			on: 	All functions are enabled.
>>>> +			no-fs: 	Filesystem is not registered but kernel clients can
>>>> +			        access APIs and a crashkernel can be used to read
>>>> +				its content. There is nothing to mount.
>>>> +			off: 	Filesystem is not registered and clients
>>>> +			        get a -EPERM as result when trying to register files
>>>> +				or directories within debugfs.
>>>> +				This is equilivant of the runtime functionality if
>>>> +				debugfs was not enabled in the kernel at all.
>>>> +			Default value is set in build-time with a kernel configuration.
>>> Naming is hard.  In looking at this, should "no-fs" be called
>>> "no-mount"?  That's a better description of what it does, right?
>> I think no-fs cover it better since it does not register a filesystem
>> but provides the interfaces. Mounting is then indirectly stopped.
> But "mounting" is the common term we all know.  "no-fs" doesn't really
> describe what is happening here, right?  Everything works internally
> just fine, but we just are forbidding the filesystem to be mounted.
>
I have no objection but now you know the background. So no-mount then.

I will do a new patch-set.

>> The idea start with a check for mounting but it is much more
>> definitely stopped by prevention of register of the filesystem.
>> I can imagine to have a forth "mode" where it register a fs but
>> not allowing mounting. Such mode maybe useful for some runtime
>> configuration. But this patch is about boot time configuration.
> Preventing the registering of the filesystem does cut out the ability to
> mount the thing quite well :)
>
> We could change it to just prevent the superblock from mounting if you
> want, as maybe we do need the fs to remain in the list of filesystems in
> the kernel at that point in time?  Otherwise we are lying to userspace.

It is all about hide it away for userspace.


> thanks,
>
> greg k-h