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Message-ID: <Zy-n_GtpJPV-5LrB@google.com>
Date: Sat, 9 Nov 2024 19:21:04 +0100
From: "Günther Noack" <gnoack@...gle.com>
To: "Mickaël Salaün" <mic@...ikod.net>
Cc: Mikhail Ivanov <ivanov.mikhail1@...wei-partners.com>,
Konstantin Meskhidze <konstantin.meskhidze@...wei.com>, Paul Moore <paul@...l-moore.com>,
Tahera Fahimi <fahimitahera@...il.com>, linux-kernel@...r.kernel.org,
linux-security-module@...r.kernel.org
Subject: Re: [PATCH v4 1/3] landlock: Refactor filesystem access mask management
On Sat, Nov 09, 2024 at 12:08:54PM +0100, Mickaël Salaün wrote:
> Replace get_raw_handled_fs_accesses() with a generic
> landlock_merge_access_masks(), and replace get_fs_domain() with a
> generic landlock_match_ruleset(). These helpers will also be useful for
> other types of access.
>
> Cc: Günther Noack <gnoack@...gle.com>
> Cc: Mikhail Ivanov <ivanov.mikhail1@...wei-partners.com>
> Signed-off-by: Mickaël Salaün <mic@...ikod.net>
> Link: https://lore.kernel.org/r/20241109110856.222842-2-mic@digikod.net
> ---
>
> Changes since v3:
> * Rename landlock_merge_access_masks() to landlock_union_access_masks(),
> suggested by Günther.
> * Rename landlock_match_ruleset() to landlock_get_applicable_domain(),
> suggested by Günther.
> * Rename the "ruleset" arguments to "domain" for consistency with these
> new helpers.
> * Use typeof_member(), suggested by Günther. Also include kernel.h
> (instead of the new container_of.h to ease backports).
>
> Changes since v2:
> * Create a new type union access_masks_all instead of changing struct
> acces_masks.
> * Replace get_fs_domain() with an explicit call to
> landlock_match_ruleset().
>
> Changes since v1:
> * Rename landlock_filter_access_masks() to landlock_match_ruleset().
> * Rename local variables from domain to ruleset to mach helpers'
> semantic. We'll rename and move these helpers when we'll have a
> dedicated domain struct type.
> * Rename the all_fs mask to any_fs.
> * Add documentation, as suggested by Günther.
> ---
> security/landlock/fs.c | 31 ++++-----------
> security/landlock/ruleset.h | 73 ++++++++++++++++++++++++++++++++----
> security/landlock/syscalls.c | 2 +-
> 3 files changed, 74 insertions(+), 32 deletions(-)
>
> diff --git a/security/landlock/fs.c b/security/landlock/fs.c
> index 7d79fc8abe21..e31b97a9f175 100644
> --- a/security/landlock/fs.c
> +++ b/security/landlock/fs.c
> @@ -388,38 +388,22 @@ static bool is_nouser_or_private(const struct dentry *dentry)
> unlikely(IS_PRIVATE(d_backing_inode(dentry))));
> }
>
> -static access_mask_t
> -get_raw_handled_fs_accesses(const struct landlock_ruleset *const domain)
> -{
> - access_mask_t access_dom = 0;
> - size_t layer_level;
> -
> - for (layer_level = 0; layer_level < domain->num_layers; layer_level++)
> - access_dom |=
> - landlock_get_raw_fs_access_mask(domain, layer_level);
> - return access_dom;
> -}
> -
> static access_mask_t
> get_handled_fs_accesses(const struct landlock_ruleset *const domain)
> {
> /* Handles all initially denied by default access rights. */
> - return get_raw_handled_fs_accesses(domain) |
> + return landlock_union_access_masks(domain).fs |
> LANDLOCK_ACCESS_FS_INITIALLY_DENIED;
> }
>
> -static const struct landlock_ruleset *
> -get_fs_domain(const struct landlock_ruleset *const domain)
> -{
> - if (!domain || !get_raw_handled_fs_accesses(domain))
> - return NULL;
> -
> - return domain;
> -}
> +static const struct access_masks any_fs = {
> + .fs = ~0,
> +};
>
> static const struct landlock_ruleset *get_current_fs_domain(void)
> {
> - return get_fs_domain(landlock_get_current_domain());
> + return landlock_get_applicable_domain(landlock_get_current_domain(),
> + any_fs);
> }
>
> /*
> @@ -1517,7 +1501,8 @@ static int hook_file_open(struct file *const file)
> access_mask_t open_access_request, full_access_request, allowed_access,
> optional_access;
> const struct landlock_ruleset *const dom =
> - get_fs_domain(landlock_cred(file->f_cred)->domain);
> + landlock_get_applicable_domain(
> + landlock_cred(file->f_cred)->domain, any_fs);
>
> if (!dom)
> return 0;
> diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h
> index 61bdbc550172..698fa3114cf4 100644
> --- a/security/landlock/ruleset.h
> +++ b/security/landlock/ruleset.h
> @@ -11,6 +11,7 @@
>
> #include <linux/bitops.h>
> #include <linux/build_bug.h>
> +#include <linux/kernel.h>
> #include <linux/mutex.h>
> #include <linux/rbtree.h>
> #include <linux/refcount.h>
> @@ -47,6 +48,15 @@ struct access_masks {
> access_mask_t scope : LANDLOCK_NUM_SCOPE;
> };
>
> +union access_masks_all {
> + struct access_masks masks;
> + u32 all;
> +};
> +
> +/* Makes sure all fields are covered. */
> +static_assert(sizeof(typeof_member(union access_masks_all, masks)) ==
> + sizeof(typeof_member(union access_masks_all, all)));
> +
> typedef u16 layer_mask_t;
> /* Makes sure all layers can be checked. */
> static_assert(BITS_PER_TYPE(layer_mask_t) >= LANDLOCK_MAX_NUM_LAYERS);
> @@ -260,6 +270,60 @@ static inline void landlock_get_ruleset(struct landlock_ruleset *const ruleset)
> refcount_inc(&ruleset->usage);
> }
>
> +/**
> + * landlock_union_access_masks - Return all access rights handled in the
> + * domain
> + *
> + * @domain: Landlock ruleset (used as a domain)
> + *
> + * Returns: an access_masks result of the OR of all the domain's access masks.
> + */
> +static inline struct access_masks
> +landlock_union_access_masks(const struct landlock_ruleset *const domain)
> +{
> + union access_masks_all matches = {};
> + size_t layer_level;
> +
> + for (layer_level = 0; layer_level < domain->num_layers; layer_level++) {
> + union access_masks_all layer = {
> + .masks = domain->access_masks[layer_level],
> + };
> +
> + matches.all |= layer.all;
> + }
> +
> + return matches.masks;
> +}
> +
> +/**
> + * landlock_get_applicable_domain - Return @domain if it applies to (handles)
> + * the access rights specified in @masks
Very minor wording suggestion: For clarity, I would suggest "at least one of":
Return @domain if it applies to (handles)
at least one of the access rights specified in @masks
Otherwise this can be interpreted as "...all of the access rights specified in
@masks".
> + *
> + * @domain: Landlock ruleset (used as a domain)
> + * @masks: access masks
> + *
> + * Returns: @domain if any access rights specified in @masks is handled, or
> + * NULL otherwise.
> + */
> +static inline const struct landlock_ruleset *
> +landlock_get_applicable_domain(const struct landlock_ruleset *const domain,
> + const struct access_masks masks)
> +{
> + const union access_masks_all masks_all = {
> + .masks = masks,
> + };
> + union access_masks_all merge = {};
> +
> + if (!domain)
> + return NULL;
> +
> + merge.masks = landlock_union_access_masks(domain);
> + if (merge.all & masks_all.all)
> + return domain;
> +
> + return NULL;
> +}
> +
> static inline void
> landlock_add_fs_access_mask(struct landlock_ruleset *const ruleset,
> const access_mask_t fs_access_mask,
> @@ -295,19 +359,12 @@ landlock_add_scope_mask(struct landlock_ruleset *const ruleset,
> ruleset->access_masks[layer_level].scope |= mask;
> }
>
> -static inline access_mask_t
> -landlock_get_raw_fs_access_mask(const struct landlock_ruleset *const ruleset,
> - const u16 layer_level)
> -{
> - return ruleset->access_masks[layer_level].fs;
> -}
> -
> static inline access_mask_t
> landlock_get_fs_access_mask(const struct landlock_ruleset *const ruleset,
> const u16 layer_level)
> {
> /* Handles all initially denied by default access rights. */
> - return landlock_get_raw_fs_access_mask(ruleset, layer_level) |
> + return ruleset->access_masks[layer_level].fs |
> LANDLOCK_ACCESS_FS_INITIALLY_DENIED;
> }
>
> diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
> index f5a0e7182ec0..c097d356fa45 100644
> --- a/security/landlock/syscalls.c
> +++ b/security/landlock/syscalls.c
> @@ -329,7 +329,7 @@ static int add_rule_path_beneath(struct landlock_ruleset *const ruleset,
> return -ENOMSG;
>
> /* Checks that allowed_access matches the @ruleset constraints. */
> - mask = landlock_get_raw_fs_access_mask(ruleset, 0);
> + mask = ruleset->access_masks[0].fs;
> if ((path_beneath_attr.allowed_access | mask) != mask)
> return -EINVAL;
>
> --
> 2.47.0
>
Reviewed-by: Günther Noack <gnoack@...gle.com>
Looks good, thanks! Names are much clearer now, IMHO. 👍
—Günther
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