Re: [PATCH] debugfs: Add mount restriction option

[Greg Kroah-Hartman]

On Thu, May 28, 2020 at 12:04:32PM +0000, Enderborg, Peter wrote:
> 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@xxxxxxxx>
> >>>> ---
> >>>>  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.

I do not disagree, but have you tested this to ensure that your kernel
still works when you disable debugfs in this manner?

If so, great!  Just trying to point out a potential problem...

thanks,

greg k-h