Re: [PATCH] debugfs: Add mount restriction option

["Enderborg, Peter" <Peter.Enderborg@sony.com>]

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@sony.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.


>
>>  	retval = sysfs_create_mount_point(kernel_kobj, "debug");
>>  	if (retval)
>>  		return retval;
>> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
>> index 21d9c5f6e7ec..d3a3338740d2 100644
>> --- a/lib/Kconfig.debug
>> +++ b/lib/Kconfig.debug
>> @@ -443,6 +443,16 @@ config DEBUG_FS
>>  
>>  	  If unsure, say N.
>>  
>> +config DEBUG_FS_MOUNT_RESTRICTED
>> +	bool "Debug Filesystem mount restricted"
>> +	depends on DEBUG_FS
>> +	help
>> +	  This is an additional restriction for mounting debugfs. It allows
>> +	  the kernel to have debugfs compiled, but requires that kernel command
>> +	  line has a debugfs parameter to register as a filesystem.
>> +
>> +	  If unsure, say N.
> No hint as to what the command line option is?  And again, it's not a
> restriction for mounting it, you are preventing it from initializing
> entirely.
>
> Which, as I have found out over time, can had side affects for codepaths
> that try to check the return value of debugfs calls, and if they fail to
> succeed, abort the whole driver/subsystem/whatever initializaion path,
> which is not what I think you want to do here at all.
>
> thanks,
>
> greg k-h