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

Enderborg, Peter
2020-07-15T11:47:24Z

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@sony.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