linux-kernel - Re: [PATCH] debugfs: Add mount restriction option

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Date:   Thu, 28 May 2020 12:04:32 +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>
Subject: Re: [PATCH] debugfs: Add mount restriction option

On 5/28/20 11:27 AM, Greg Kroah-Hartman wrote:
> On Thu, May 28, 2020 at 08:39:02AM +0000, Enderborg, Peter wrote:
>> On 5/28/20 10:27 AM, Greg Kroah-Hartman wrote:
>>> On Thu, May 28, 2020 at 10:00:31AM +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. It is needed new
>>>> kernel command line parameter to be activated.
>>> By "configuration" do you mean "kernel configuration"?  What is wrong
>>> with relying on the build option like we do today?
>>>
>>> You might want to reword all of this to make more sense about the
>>> "problem" you are trying to solve here, as I don't really understand it,
>>> sorry.
>>>
>>>
>>>> Signed-off-by: Peter Enderborg <peter.enderborg@...y.com>
>>>> ---
>>>>  fs/debugfs/inode.c | 17 ++++++++++++++++-
>>>>  lib/Kconfig.debug  | 10 ++++++++++
>>>>  2 files changed, 26 insertions(+), 1 deletion(-)
>>> No documentation update?  That's not good :(
>>>
>>>
>>>> diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
>>>> index b7f2e971ecbc..bde37dab77e0 100644
>>>> --- a/fs/debugfs/inode.c
>>>> +++ b/fs/debugfs/inode.c
>>>> @@ -786,10 +786,25 @@ bool debugfs_initialized(void)
>>>>  }
>>>>  EXPORT_SYMBOL_GPL(debugfs_initialized);
>>>>  
>>>> +static int allow_debugfs;
>>>> +
>>>> +static int __init debugfs_kernel(char *str)
>>>> +{
>>>> +	if (str && !strcmp(str, "true"))
>>>> +		allow_debugfs = true;
>>>> +
>>>> +	return 0;
>>>> +
>>>> +}
>>>> +early_param("debugfs", debugfs_kernel);
>>>> +
>>>>  static int __init debugfs_init(void)
>>>>  {
>>>>  	int retval;
>>>> -
>>>> +#ifdef CONFIG_DEBUG_FS_MOUNT_RESTRICTED
>>>> +	if (!allow_debugfs)
>>>> +		return -EPERM;
>>>> +#endif
>>> But you are not restricting the ability to mount it here, you are
>>> removing the ability for it to even start up at all.  What does this
>>> break for code that thinks the filesystem is registered (i.e. the call
>>> to simple_pin_fs() in start_creating() in fs/debugfs/inode.c?
>>>
>> If it does, the lines below is also cause the same problem.
> In a working system, errors in the lines below will never happen :)

I think it is not unreasonable to expect that debug functionality are robust enough to handle
system where not everything is working smoothly.


> thanks,
>
> greg k-h