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

Randy Dunlap
2020-07-16T05:12:53Z

Hi Peter,

Here are a few more comments/corrections.

On 7/15/20 9:54 PM, 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-mount 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 | 15 ++++++++
> fs/debugfs/inode.c | 37 +++++++++++++++++++
> fs/debugfs/internal.h | 14 +++++++
> lib/Kconfig.debug | 32 ++++++++++++++++
> 4 files changed, 98 insertions(+)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index fb95fad81c79..779d6cdc9627 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -827,6 +827,21 @@
> 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-mount, off }
> + on: All functions are enabled.
> + no-mount:
> + 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

equivalent

> + debugfs was not enabled in the kernel at all.
> + Default value is set in build-time with a kernel configuration.
> +
> debugpat [X86] Enable PAT debugging
>
> decnet.addr= [HW,NET]
> diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
> index b7f2e971ecbc..02d08b17d0e6 100644
> --- a/fs/debugfs/inode.c
> +++ b/fs/debugfs/inode.c

> @@ -786,10 +808,25 @@ bool debugfs_initialized(void)
> }
> EXPORT_SYMBOL_GPL(debugfs_initialized);
>

I would add some "else"s here:

> +static int __init debugfs_kernel(char *str)
> +{
> + if (str && !strcmp(str, "on"))
> + debugfs_allow = DEBUGFS_ALLOW_API | DEBUGFS_ALLOW_MOUNT;
else if ...

> + if (str && !strcmp(str, "no-mount"))
> + debugfs_allow = DEBUGFS_ALLOW_API;
else if ...

> + if (str && !strcmp(str, "off"))
> + debugfs_allow = 0;
> +
> + return 0;
> +}
> +early_param("debugfs", debugfs_kernel);

> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 9ad9210d70a1..ebe670fdf1bd 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_ALLOW_ALL
> + help
> + This selects the default access restrictions for debugfs.
> + It can be overridden with kernel command line option
> + debugfs=[on,no-mount,off]. The restrictions apply for API access
> + and filesystem registration. .

stray '.' there.

> +
> +config DEBUG_FS_ALLOW_ALL
> + bool "Access normal"
> + help
> + No restrictions apply. Both API and filesystem registration
> + is on. This is the normal default operation.
> +
> +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

but filesystem is not loaded. Clients can still do

> + their work and read with debug tools that do not need
> + debugfs filesystem.
> +
> +config DEBUG_FS_ALLOW_NONE
> + bool "No access"
> + help
> + Access is off. Clients get -PERM when trying to create nodes in
> + debugfs tree and debugfs is not registered as a filesystem.
> + Client can then back-off or continue without debugfs access.
> +
> +endchoice
> +
> source "lib/Kconfig.kgdb"
>
> source "lib/Kconfig.ubsan"
>

thanks.
--
~Randy