[PATCH v4 10/30] landlock: Log mount-related denials

["Mickaël Salaün" <mic@digikod.net>]

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".

Add and use a new landlock_match_layer_level() helper.

Audit event sample:

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

Cc: Günther Noack <gnoack@google.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20250108154338.1129069-11-mic@digikod.net
---

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      | 64 ++++++++++++++++++++++++++++++++++---
 security/landlock/ruleset.h | 31 ++++++++++++++++++
 4 files changed, 94 insertions(+), 5 deletions(-)

diff --git a/security/landlock/audit.c b/security/landlock/audit.c
index ccc591146f8a..aae93b2c994b 100644
--- a/security/landlock/audit.c
+++ b/security/landlock/audit.c
@@ -20,6 +20,9 @@ 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 7bc67ca2e470..745dcf7b1b6d 100644
--- a/security/landlock/audit.h
+++ b/security/landlock/audit.h
@@ -15,6 +15,7 @@
 
 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 9779170d9199..171012efb559 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"
@@ -1301,6 +1303,38 @@ 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_ruleset *const domain,
+			  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 = landlock_match_layer_level(domain, any_fs) + 1,
+	};
+
+	landlock_log_denial(domain, &request);
+}
+
+static void
+log_fs_change_layout_dentry(const struct landlock_ruleset *const domain,
+			    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 = landlock_match_layer_level(domain, any_fs) + 1,
+	};
+
+	landlock_log_denial(domain, &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
@@ -1323,16 +1357,24 @@ 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())
+	const struct landlock_ruleset *const dom = get_current_fs_domain();
+
+	if (!dom)
 		return 0;
+
+	log_fs_change_layout_path(dom, 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())
+	const struct landlock_ruleset *const dom = get_current_fs_domain();
+
+	if (!dom)
 		return 0;
+
+	log_fs_change_layout_path(dom, to_path);
 	return -EPERM;
 }
 
@@ -1342,15 +1384,23 @@ 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())
+	const struct landlock_ruleset *const dom = get_current_fs_domain();
+
+	if (!dom)
 		return 0;
+
+	log_fs_change_layout_dentry(dom, mnt->mnt_root);
 	return -EPERM;
 }
 
 static int hook_sb_remount(struct super_block *const sb, void *const mnt_opts)
 {
-	if (!get_current_fs_domain())
+	const struct landlock_ruleset *const dom = get_current_fs_domain();
+
+	if (!dom)
 		return 0;
+
+	log_fs_change_layout_dentry(dom, sb->s_root);
 	return -EPERM;
 }
 
@@ -1365,8 +1415,12 @@ 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())
+	const struct landlock_ruleset *const dom = get_current_fs_domain();
+
+	if (!dom)
 		return 0;
+
+	log_fs_change_layout_path(dom, new_path);
 	return -EPERM;
 }
 
diff --git a/security/landlock/ruleset.h b/security/landlock/ruleset.h
index 39169b6860e3..7dba8bf960f6 100644
--- a/security/landlock/ruleset.h
+++ b/security/landlock/ruleset.h
@@ -267,6 +267,37 @@ landlock_get_applicable_domain(const struct landlock_ruleset *const domain,
 	return NULL;
 }
 
+/**
+ * landlock_match_layer_level - Return the layer level restricting @masks
+ *
+ * @domain: Landlock ruleset (used as a domain)
+ * @masks: access masks
+ *
+ * Returns: the number of the layer restricting/handling any right of @access,
+ * or return 0 (i.e. first layer) otherwise.
+ */
+static inline size_t
+landlock_match_layer_level(const struct landlock_ruleset *const domain,
+			   const struct access_masks masks)
+{
+	const union access_masks_all masks_all = {
+		.masks = masks,
+	};
+	ssize_t layer_level;
+
+	for (layer_level = domain->num_layers; layer_level >= 0;
+	     layer_level--) {
+		union access_masks_all layer = {
+			.masks = domain->access_masks[layer_level],
+		};
+
+		if (masks_all.all & layer.all)
+			return layer_level;
+	}
+
+	return 0;
+}
+
 static inline void
 landlock_add_fs_access_mask(struct landlock_ruleset *const ruleset,
 			    const access_mask_t fs_access_mask,
-- 
2.47.1