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

Enderborg, Peter
2020-07-21T06:49:00Z

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?
One name that had in mind was Stealth. It is more of what it is intended to do than how.
> Then we can rename the bits to match the documentation so we aren't
> constantly thinking of different things and trying to match them up:
>
>
>> --- a/fs/debugfs/internal.h
>> +++ b/fs/debugfs/internal.h
>> @@ -29,4 +29,18 @@ struct debugfs_fsdata {
>> */
>> #define DEBUGFS_FSDATA_IS_REAL_FOPS_BIT BIT(0)
>>
>> +/* Access BITS */
>> +#define DEBUGFS_ALLOW_API BIT(0)
>> +#define DEBUGFS_ALLOW_FS BIT(1)
> #define DEBUGFS_ALLOW_API BIT(0)
> #define DEBUGFS_ALLOW_MOUNT BIT(1)
>
>> +#ifdef CONFIG_DEBUG_FS_ACCESS_ALL
>> +#define DEFAULT_DEBUGFS_ACCESS_BITS (DEBUGFS_ALLOW_FS | DEBUGFS_ALLOW_API)
>> +#endif
>> +#ifdef CONFIG_DEBUG_FS_ACCESS_NO_FS
>> +#define DEFAULT_DEBUGFS_ACCESS_BITS (DEBUGFS_ALLOW_API)
>> +#endif
>> +#ifdef CONFIG_DEBUG_FS_ACCESS_NONE
>> +#define DEFAULT_DEBUGFS_ACCESS_BITS (0)
>> +#endif
>> +
>> #endif /* _DEBUGFS_INTERNAL_H_ */
>> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
>> index 9ad9210d70a1..b609ad7c1343 100644
>> --- a/lib/Kconfig.debug
>> +++ b/lib/Kconfig.debug
>> @@ -476,6 +476,38 @@ config DEBUG_FS
>>
>> If unsure, say N.
>>
>> +choice
>> + prompt "Debugfs default access"
>> + depends on DEBUG_FS
>> + default DEBUG_FS_ACCESS_ALL
> default DEBUGFS_FS_ALLOW_ALL
>
>> + help
>> + This select the default access restricions for debugfs.
>> + It can be overridden with kernel command line option
>> + debugfs=[on,no-fs,off] The restrictions apply for API access
>> + and filesystem registration. .
>> +
>> +config DEBUG_FS_ACCESS_ALL
> config DEBUG_FS_ALLOW_ALL
>
>> + bool "Access normal"
>> + help
>> + No restrictions applies. Both API and filesystem registration
>> + is on. This is the normal default operation.
>> +
>> +config DEBUG_FS_ACCESS_NO_FS
> config DEBUG_FS_DISALLOW_MOUNT
>
>> + bool "Do not register debugfs as filesystem"
>> + help
>> + The API is open but filesystem not loaded. Client can still do
>> + their work and readed with debug tools that does not need
>> + debugfs filesystem.
>> +
>> +config DEBUG_FS_ACCESS_NONE
> config DEBUG_FS_ALLOW_NONE
>
> Does that sound better?
>
> I'm not trying to be a pain, just trying to name this all correctly as
> it's messy with different config options and bits and mapping that to
> checks in the code without everything called the same.
>
> thanks,
>
> greg k-h