Re: [PATCH v1 01/11] landlock: Define access_mask_t to enforce a consistent access mask size

[Paul Moore <paul@paul-moore.com>]

On Mon, Feb 21, 2022 at 4:15 PM Mickaël Salaün <mic@digikod.net> wrote:
>
> From: Mickaël Salaün <mic@linux.microsoft.com>
>
> Create and use the access_mask_t typedef to enforce a consistent access
> mask size and uniformly use a 16-bits type.  This will helps transition
> to a 32-bits value one day.
>
> Add a build check to make sure all (filesystem) access rights fit in.
> This will be extended with a following commit.
>
> Signed-off-by: Mickaël Salaün <mic@linux.microsoft.com>
> Link: https://lore.kernel.org/r/20220221212522.320243-2-mic@digikod.net
> ---
>  security/landlock/fs.c      | 19 ++++++++++---------
>  security/landlock/fs.h      |  2 +-
>  security/landlock/limits.h  |  2 ++
>  security/landlock/ruleset.c |  6 ++++--
>  security/landlock/ruleset.h | 17 +++++++++++++----
>  5 files changed, 30 insertions(+), 16 deletions(-)
>
> diff --git a/security/landlock/fs.c b/security/landlock/fs.c
> index 97b8e421f617..9de2a460a762 100644
> --- a/security/landlock/fs.c
> +++ b/security/landlock/fs.c
> @@ -150,7 +150,7 @@ static struct landlock_object *get_inode_object(struct inode *const inode)
>   * @path: Should have been checked by get_path_from_fd().
>   */
>  int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
> -               const struct path *const path, u32 access_rights)
> +               const struct path *const path, access_mask_t access_rights)
>  {
>         int err;
>         struct landlock_object *object;
> @@ -182,8 +182,8 @@ int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
>
>  static inline u64 unmask_layers(
>                 const struct landlock_ruleset *const domain,
> -               const struct path *const path, const u32 access_request,
> -               u64 layer_mask)
> +               const struct path *const path,
> +               const access_mask_t access_request, u64 layer_mask)
>  {
>         const struct landlock_rule *rule;
>         const struct inode *inode;
> @@ -223,7 +223,8 @@ static inline u64 unmask_layers(
>  }
>
>  static int check_access_path(const struct landlock_ruleset *const domain,
> -               const struct path *const path, u32 access_request)
> +               const struct path *const path,
> +               const access_mask_t access_request)
>  {
>         bool allowed = false;
>         struct path walker_path;
> @@ -308,7 +309,7 @@ static int check_access_path(const struct landlock_ruleset *const domain,
>  }
>
>  static inline int current_check_access_path(const struct path *const path,
> -               const u32 access_request)
> +               const access_mask_t access_request)
>  {
>         const struct landlock_ruleset *const dom =
>                 landlock_get_current_domain();
> @@ -511,7 +512,7 @@ static int hook_sb_pivotroot(const struct path *const old_path,
>
>  /* Path hooks */
>
> -static inline u32 get_mode_access(const umode_t mode)
> +static inline access_mask_t get_mode_access(const umode_t mode)
>  {
>         switch (mode & S_IFMT) {
>         case S_IFLNK:
> @@ -563,7 +564,7 @@ static int hook_path_link(struct dentry *const old_dentry,
>                         get_mode_access(d_backing_inode(old_dentry)->i_mode));
>  }
>
> -static inline u32 maybe_remove(const struct dentry *const dentry)
> +static inline access_mask_t maybe_remove(const struct dentry *const dentry)
>  {
>         if (d_is_negative(dentry))
>                 return 0;
> @@ -631,9 +632,9 @@ static int hook_path_rmdir(const struct path *const dir,
>
>  /* File hooks */
>
> -static inline u32 get_file_access(const struct file *const file)
> +static inline access_mask_t get_file_access(const struct file *const file)
>  {
> -       u32 access = 0;
> +       access_mask_t access = 0;
>
>         if (file->f_mode & FMODE_READ) {
>                 /* A directory can only be opened in read mode. */
> diff --git a/security/landlock/fs.h b/security/landlock/fs.h
> index 187284b421c9..74be312aad96 100644
> --- a/security/landlock/fs.h
> +++ b/security/landlock/fs.h
> @@ -65,6 +65,6 @@ static inline struct landlock_superblock_security *landlock_superblock(
>  __init void landlock_add_fs_hooks(void);
>
>  int landlock_append_fs_rule(struct landlock_ruleset *const ruleset,
> -               const struct path *const path, u32 access_hierarchy);
> +               const struct path *const path, access_mask_t access_hierarchy);
>
>  #endif /* _SECURITY_LANDLOCK_FS_H */
> diff --git a/security/landlock/limits.h b/security/landlock/limits.h
> index 2a0a1095ee27..458d1de32ed5 100644
> --- a/security/landlock/limits.h
> +++ b/security/landlock/limits.h
> @@ -9,6 +9,7 @@
>  #ifndef _SECURITY_LANDLOCK_LIMITS_H
>  #define _SECURITY_LANDLOCK_LIMITS_H
>
> +#include <linux/bitops.h>
>  #include <linux/limits.h>
>  #include <uapi/linux/landlock.h>
>
> @@ -17,5 +18,6 @@
>
>  #define LANDLOCK_LAST_ACCESS_FS                LANDLOCK_ACCESS_FS_MAKE_SYM
>  #define LANDLOCK_MASK_ACCESS_FS                ((LANDLOCK_LAST_ACCESS_FS << 1) - 1)
> +#define LANDLOCK_NUM_ACCESS_FS         __const_hweight64(LANDLOCK_MASK_ACCESS_FS)

The line above, and the static_assert() in ruleset.h are clever.  I'll
admit I didn't even know the hweightX() macros existed until looking
at this code :)

However, the LANDLOCK_NUM_ACCESS_FS is never really going to be used
outside the static_assert() in ruleset.h is it?  I wonder if it would
be better to skip the extra macro and rewrite the static_assert like
this:

static_assert(BITS_PER_TYPE(access_mask_t) >=
__const_hweight64(LANDLOCK_MASK_ACCESS_FS));

If not, I might suggest changing LANDLOCK_NUM_ACCESS_FS to
LANDLOCK_BITS_ACCESS_FS or something similar.


> diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h
> index 2d3ed7ec5a0a..7e7cac68e443 100644
> --- a/security/landlock/ruleset.h
> +++ b/security/landlock/ruleset.h
> @@ -9,13 +9,20 @@
>  #ifndef _SECURITY_LANDLOCK_RULESET_H
>  #define _SECURITY_LANDLOCK_RULESET_H
>
> +#include <linux/bitops.h>
> +#include <linux/build_bug.h>
>  #include <linux/mutex.h>
>  #include <linux/rbtree.h>
>  #include <linux/refcount.h>
>  #include <linux/workqueue.h>
>
> +#include "limits.h"
>  #include "object.h"
>
> +typedef u16 access_mask_t;
> +/* Makes sure all filesystem access rights can be stored. */
> +static_assert(BITS_PER_TYPE(access_mask_t) >= LANDLOCK_NUM_ACCESS_FS);

--
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