Re: [PATCH v3] debugfs: Add access restriction option

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

On 6/17/20 4:15 PM, Greg Kroah-Hartman wrote:
> On Wed, Jun 17, 2020 at 03:37:38PM +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.
>>
>> When enabled it is needed a kernel command line parameter to be activated.
>>
>> 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>
>> ---
>> v2. Removed MOUNT as part of restrictions. Added API's restrictions as
>>     separate restriction.
>> v3  Updated Documentation after Randy Dunlap reviews and suggestions.
>>
>>  .../admin-guide/kernel-parameters.txt         | 11 +++++
>>  fs/debugfs/inode.c                            | 47 +++++++++++++++++++
>>  lib/Kconfig.debug                             | 10 ++++
>>  3 files changed, 68 insertions(+)
>>
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index fb95fad81c79..249c86e53bb7 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -827,6 +827,17 @@
>>  			useful to also enable the page_owner functionality.
>>  			on: enable the feature
>>  
>> +	debugfs=    	[KNL] When CONFIG_DEBUG_FS_RESTRICTED is set, this parameter
>> +			enables what is exposed to userspace.
>> +			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
>> +				it's content. There its nothing to mount.
>> +			off: 	(default) Filesystem is not registered and clients
>> +			        get a -EPERM as result when trying to register files
>> +				or directories within debugfs.
>> +
>>  	debugpat	[X86] Enable PAT debugging
>>  
>>  	decnet.addr=	[HW,NET]
>> diff --git a/fs/debugfs/inode.c b/fs/debugfs/inode.c
>> index b7f2e971ecbc..2bd80a932ae1 100644
>> --- a/fs/debugfs/inode.c
>> +++ b/fs/debugfs/inode.c
>> @@ -31,10 +31,17 @@
>>  #include "internal.h"
>>  
>>  #define DEBUGFS_DEFAULT_MODE	0700
>> +#ifdef CONFIG_DEBUG_FS_RESTRICTED
>> +#define DEBUGFS_ALLOW_API 0x2
>> +#define DEBUGFS_ALLOW_FS 0x1
> BIT()?
>
> And a tab?
>
> And why a #ifdef?
To get it as least intrusive as possible. A solid Opt-In.
>
>> +#endif
>>  
>>  static struct vfsmount *debugfs_mount;
>>  static int debugfs_mount_count;
>>  static bool debugfs_registered;
>> +#ifdef CONFIG_DEBUG_FS_RESTRICTED
>> +static unsigned int debugfs_allow;
>> +#endif
> Why #ifdef?
>
>>  
>>  /*
>>   * Don't allow access attributes to be changed whilst the kernel is locked down
>> @@ -266,6 +273,10 @@ static struct dentry *debug_mount(struct file_system_type *fs_type,
>>  			int flags, const char *dev_name,
>>  			void *data)
>>  {
>> +#ifdef CONFIG_DEBUG_FS_RESTRICTED
>> +	if (!(debugfs_allow & DEBUGFS_ALLOW_API))
>> +		return ERR_PTR(-EPERM);
>> +#endif
> Ick, all of this #ifdef is a mess, and can be totally avoided if you do
> the logic right here.  Please make it so that the functions and almost
> all of the .c code does not have #ifdef CONFIG_DEBUG_FS_RESTRICTED at
> all.
>
Is it ok to remove the #ifdefs and let code always be there and let the config set the default value?


>>  	return mount_single(fs_type, flags, data, debug_fill_super);
>>  }
>>  
>> @@ -385,6 +396,12 @@ static struct dentry *__debugfs_create_file(const char *name, umode_t mode,
>>  	if (IS_ERR(dentry))
>>  		return dentry;
>>  
>> +#ifdef CONFIG_DEBUG_FS_RESTRICTED
>> +	if (!(debugfs_allow & DEBUGFS_ALLOW_API)) {
>> +		failed_creating(dentry);
>> +		return ERR_PTR(-EPERM);
>> +	}
>> +#endif
>>  	inode = debugfs_get_inode(dentry->d_sb);
>>  	if (unlikely(!inode)) {
>>  		pr_err("out of free dentries, can not create file '%s'\n",
>> @@ -541,6 +558,12 @@ struct dentry *debugfs_create_dir(const char *name, struct dentry *parent)
>>  	if (IS_ERR(dentry))
>>  		return dentry;
>>  
>> +#ifdef CONFIG_DEBUG_FS_RESTRICTED
>> +	if (!(debugfs_allow & DEBUGFS_ALLOW_API)) {
>> +		failed_creating(dentry);
>> +		return ERR_PTR(-EPERM);
>> +	}
>> +#endif
>>  	inode = debugfs_get_inode(dentry->d_sb);
>>  	if (unlikely(!inode)) {
>>  		pr_err("out of free dentries, can not create directory '%s'\n",
>> @@ -583,6 +606,12 @@ struct dentry *debugfs_create_automount(const char *name,
>>  	if (IS_ERR(dentry))
>>  		return dentry;
>>  
>> +#ifdef CONFIG_DEBUG_FS_RESTRICTED
>> +	if (!(debugfs_allow & DEBUGFS_ALLOW_API)) {
>> +		failed_creating(dentry);
>> +		return ERR_PTR(-EPERM);
>> +	}
>> +#endif
>>  	inode = debugfs_get_inode(dentry->d_sb);
>>  	if (unlikely(!inode)) {
>>  		pr_err("out of free dentries, can not create automount '%s'\n",
>> @@ -786,10 +815,28 @@ bool debugfs_initialized(void)
>>  }
>>  EXPORT_SYMBOL_GPL(debugfs_initialized);
>>  
>> +static int __init debugfs_kernel(char *str)
>> +{
>> +#ifdef CONFIG_DEBUG_FS_RESTRICTED
>> +	if (str && !strcmp(str, "on"))
>> +		debugfs_allow = DEBUGFS_ALLOW_API | DEBUGFS_ALLOW_FS;
>> +	if (str && !strcmp(str, "no-fs"))
>> +		debugfs_allow |= DEBUGFS_ALLOW_API;
>> +	if (str && !strcmp(str, "off"))
>> +		debugfs_allow = 0;
> It's set to 0 by default, no need to set it again, right?
I think there have been some issues with the same parameter more than once.
>
>> +#endif
>> +	return 0;
>> +
>> +}
>> +early_param("debugfs", debugfs_kernel);
> Why is this a valid parm even if the option is not enabled?  Do you mean
> to do that?  Why?

I did not find any good usage where it was config dependent, when it is there; it is  "reserve" the name.

It will always be there if we remove the #ifdefs.

> thanks,
>
> greg k-h