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

Greg Kroah-Hartman
2020-06-17T14:15:46Z

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?

> +#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.

> 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?

> +#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?

thanks,

greg k-h