[PATCH v5 11/24] landlock: Log mount-related denials

Mickaël Salaün
2025-01-31T16:31:35Z

Add audit support for sb_mount, move_mount, sb_umount, sb_remount, and
sb_pivot_root hooks.

The new related blocker is "fs.change_layout".

Audit event sample:

type=LANDLOCK_DENY msg=audit(1729738800.349:44): domain=195ba459b blockers=fs.change_layout name="/" dev="tmpfs" ino=1

Remove landlock_get_applicable_domain() and get_current_fs_domain()
which are now fully replaced with landlock_get_applicable_subject().

Cc: Günther Noack <gnoack@google.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20250131163059.1139617-12-mic@digikod.net
---

Changes since v4:
- Rebase on top of the landlock_log_denial() and subject type changes.
- Fix off-by-one error in landlock_match_layer_level(), now merged into
landlock_get_applicable_domain().

Changes since v3:
- Cosmetic change to the "fs.change_layout" name.

Changes since v2:
- Log the domain that denied the action because not all layers block FS
layout changes.
- Fix landlock_match_layer_level().

Changes since v1:
- Rebased on the TCP patch series.
- Don't log missing permissions, only domain layer, and then remove the
permission word (suggested by Günther)
---
security/landlock/audit.c | 3 ++
security/landlock/audit.h | 1 +
security/landlock/fs.c | 85 ++++++++++++++++++++++++++++++++-----
security/landlock/ruleset.h | 30 -------------
4 files changed, 78 insertions(+), 41 deletions(-)

diff --git a/security/landlock/audit.c b/security/landlock/audit.c
index a5b055306757..091b7cba7e9f 100644
--- a/security/landlock/audit.c
+++ b/security/landlock/audit.c
@@ -21,6 +21,9 @@ static const char *get_blocker(const enum landlock_request_type type)
switch (type) {
case LANDLOCK_REQUEST_PTRACE:
return "ptrace";
+
+ case LANDLOCK_REQUEST_FS_CHANGE_LAYOUT:
+ return "fs.change_layout";
}

WARN_ON_ONCE(1);
diff --git a/security/landlock/audit.h b/security/landlock/audit.h
index 0608241eb7e1..258b7e3cd9a5 100644
--- a/security/landlock/audit.h
+++ b/security/landlock/audit.h
@@ -16,6 +16,7 @@

enum landlock_request_type {
LANDLOCK_REQUEST_PTRACE = 1,
+ LANDLOCK_REQUEST_FS_CHANGE_LAYOUT,
};

/*
diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index 276cbcffe6f5..1370641eb0a4 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -23,6 +23,7 @@
#include <linux/kernel.h>
#include <linux/limits.h>
#include <linux/list.h>
+#include <linux/lsm_audit.h>
#include <linux/lsm_hooks.h>
#include <linux/mount.h>
#include <linux/namei.h>
@@ -37,6 +38,7 @@
#include <uapi/linux/landlock.h>

#include "access.h"
+#include "audit.h"
#include "common.h"
#include "cred.h"
#include "fs.h"
@@ -393,12 +395,6 @@ static const struct access_masks any_fs = {
.fs = ~0,
};

-static const struct landlock_ruleset *get_current_fs_domain(void)
-{
- return landlock_get_applicable_domain(landlock_get_current_domain(),
- any_fs);
-}
-
/*
* Check that a destination file hierarchy has more restrictions than a source
* file hierarchy. This is only used for link and rename actions.
@@ -1333,6 +1329,38 @@ static void hook_sb_delete(struct super_block *const sb)
!atomic_long_read(&landlock_superblock(sb)->inode_refs));
}

+static void
+log_fs_change_layout_path(const struct landlock_cred_security *const subject,
+ size_t handle_layer, const struct path *const path)
+{
+ const struct landlock_request request = {
+ .type = LANDLOCK_REQUEST_FS_CHANGE_LAYOUT,
+ .audit = {
+ .type = LSM_AUDIT_DATA_PATH,
+ .u.path = *path,
+ },
+ .layer_plus_one = handle_layer + 1,
+ };
+
+ landlock_log_denial(subject, &request);
+}
+
+static void
+log_fs_change_layout_dentry(const struct landlock_cred_security *const subject,
+ size_t handle_layer, struct dentry *const dentry)
+{
+ const struct landlock_request request = {
+ .type = LANDLOCK_REQUEST_FS_CHANGE_LAYOUT,
+ .audit = {
+ .type = LSM_AUDIT_DATA_DENTRY,
+ .u.dentry = dentry,
+ },
+ .layer_plus_one = handle_layer + 1,
+ };
+
+ landlock_log_denial(subject, &request);
+}
+
/*
* Because a Landlock security policy is defined according to the filesystem
* topology (i.e. the mount namespace), changing it may grant access to files
@@ -1355,16 +1383,30 @@ static int hook_sb_mount(const char *const dev_name,
const struct path *const path, const char *const type,
const unsigned long flags, void *const data)
{
- if (!get_current_fs_domain())
+ size_t handle_layer;
+ const struct landlock_cred_security *const subject =
+ landlock_get_applicable_subject(current_cred(), any_fs,
+ &handle_layer);
+
+ if (!subject)
return 0;
+
+ log_fs_change_layout_path(subject, handle_layer, path);
return -EPERM;
}

static int hook_move_mount(const struct path *const from_path,
const struct path *const to_path)
{
- if (!get_current_fs_domain())
+ size_t handle_layer;
+ const struct landlock_cred_security *const subject =
+ landlock_get_applicable_subject(current_cred(), any_fs,
+ &handle_layer);
+
+ if (!subject)
return 0;
+
+ log_fs_change_layout_path(subject, handle_layer, to_path);
return -EPERM;
}

@@ -1374,15 +1416,29 @@ static int hook_move_mount(const struct path *const from_path,
*/
static int hook_sb_umount(struct vfsmount *const mnt, const int flags)
{
- if (!get_current_fs_domain())
+ size_t handle_layer;
+ const struct landlock_cred_security *const subject =
+ landlock_get_applicable_subject(current_cred(), any_fs,
+ &handle_layer);
+
+ if (!subject)
return 0;
+
+ log_fs_change_layout_dentry(subject, handle_layer, mnt->mnt_root);
return -EPERM;
}

static int hook_sb_remount(struct super_block *const sb, void *const mnt_opts)
{
- if (!get_current_fs_domain())
+ size_t handle_layer;
+ const struct landlock_cred_security *const subject =
+ landlock_get_applicable_subject(current_cred(), any_fs,
+ &handle_layer);
+
+ if (!subject)
return 0;
+
+ log_fs_change_layout_dentry(subject, handle_layer, sb->s_root);
return -EPERM;
}

@@ -1397,8 +1453,15 @@ static int hook_sb_remount(struct super_block *const sb, void *const mnt_opts)
static int hook_sb_pivotroot(const struct path *const old_path,
const struct path *const new_path)
{
- if (!get_current_fs_domain())
+ size_t handle_layer;
+ const struct landlock_cred_security *const subject =
+ landlock_get_applicable_subject(current_cred(), any_fs,
+ &handle_layer);
+
+ if (!subject)
return 0;
+
+ log_fs_change_layout_path(subject, handle_layer, new_path);
return -EPERM;
}

diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h
index bbb5996545d2..27a4f92ae82c 100644
--- a/security/landlock/ruleset.h
+++ b/security/landlock/ruleset.h
@@ -242,36 +242,6 @@ landlock_union_access_masks(const struct landlock_ruleset *const domain)
return matches.masks;
}

-/**
- * landlock_get_applicable_domain - Return @domain if it applies to (handles)
- * at least one 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,
--
2.48.1