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

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Date:   Wed, 15 Jul 2020 22:12:42 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Peter Enderborg <peter.enderborg@...y.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 2/2] debugfs: Add access restriction option

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@...y.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