[PATCH v6 15/26] landlock: Log scoped denials

["Mickaël Salaün" <mic@digikod.net>]

Add audit support for unix_stream_connect, unix_may_send, task_kill, and
file_send_sigiotask hooks.

The related blockers are:
- scope.abstract_unix_socket
- scope.signal

Audit event sample for abstract unix socket:

  type=LANDLOCK_DENY msg=audit(1729738800.268:30): domain=195ba459b blockers=scope.abstract_unix_socket path=00666F6F

Audit event sample for signal:

  type=LANDLOCK_DENY msg=audit(1729738800.291:31): domain=195ba459b blockers=scope.signal opid=1 ocomm="systemd"

Refactor and simplify error handling in LSM hooks.

Extend struct landlock_file_security with fown_layer and use it to log
the blocking domain.  The struct aligned size is still 16 bytes.

Cc: Günther Noack <gnoack@google.com>
Cc: Tahera Fahimi <fahimitahera@gmail.com>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20250308184422.2159360-16-mic@digikod.net
---

Changes since v5:
- Move request declarations in the landlock_log_denial() calls to
  not impact allowed requests with audit, and return as soon as
  possible when access is allowed.
- Store a fown_layer per file and use it to log the blocking domain.
- Refactor and simplify error handling in LSM hooks.

Changes since v4:
- Rebase on top of the landlock_log_denial() and subject type changes.

Changes since v3:
- Cosmetic change to the "scope.*" blocker names.
- Extend commit message.

Changes since v1:
- New patch.
---
 security/landlock/audit.c |  8 ++++
 security/landlock/audit.h |  2 +
 security/landlock/fs.c    |  7 +++-
 security/landlock/fs.h    | 14 +++++++
 security/landlock/task.c  | 79 +++++++++++++++++++++++++++++++--------
 5 files changed, 92 insertions(+), 18 deletions(-)

diff --git a/security/landlock/audit.c b/security/landlock/audit.c
index 6c34758b9ff2..d0eafb946f31 100644
--- a/security/landlock/audit.c
+++ b/security/landlock/audit.c
@@ -71,6 +71,14 @@ get_blocker(const enum landlock_request_type type,
 		if (WARN_ON_ONCE(access_bit >= ARRAY_SIZE(net_access_strings)))
 			return "unknown";
 		return net_access_strings[access_bit];
+
+	case LANDLOCK_REQUEST_SCOPE_ABSTRACT_UNIX_SOCKET:
+		WARN_ON_ONCE(access_bit != -1);
+		return "scope.abstract_unix_socket";
+
+	case LANDLOCK_REQUEST_SCOPE_SIGNAL:
+		WARN_ON_ONCE(access_bit != -1);
+		return "scope.signal";
 	}
 
 	WARN_ON_ONCE(1);
diff --git a/security/landlock/audit.h b/security/landlock/audit.h
index eeff2c5bfa4f..aaf21b31baa8 100644
--- a/security/landlock/audit.h
+++ b/security/landlock/audit.h
@@ -20,6 +20,8 @@ enum landlock_request_type {
 	LANDLOCK_REQUEST_FS_CHANGE_LAYOUT,
 	LANDLOCK_REQUEST_FS_ACCESS,
 	LANDLOCK_REQUEST_NET_ACCESS,
+	LANDLOCK_REQUEST_SCOPE_ABSTRACT_UNIX_SOCKET,
+	LANDLOCK_REQUEST_SCOPE_SIGNAL,
 };
 
 /*
diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index 14f9c6d9903f..34f316031ecf 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -1798,6 +1798,7 @@ static void hook_file_set_fowner(struct file *file)
 	const struct landlock_cred_security *new_subject;
 	struct landlock_cred_security *fown_subject;
 	struct landlock_ruleset *prev_dom;
+	size_t handle_layer;
 
 	/*
 	 * Lock already held by __f_setown(), see commit 26f204380a3c ("fs: Fix
@@ -1807,15 +1808,17 @@ static void hook_file_set_fowner(struct file *file)
 
 	fown_subject = &landlock_file(file)->fown_subject;
 	prev_dom = fown_subject->domain;
-	new_subject = landlock_get_applicable_subject(current_cred(),
-						      signal_scope, NULL);
+	new_subject = landlock_get_applicable_subject(
+		current_cred(), signal_scope, &handle_layer);
 	if (new_subject) {
 		landlock_get_ruleset(new_subject->domain);
 		*fown_subject = *new_subject;
+		landlock_file(file)->fown_layer = handle_layer;
 	} else {
 		static const struct landlock_cred_security empty = {};
 
 		*fown_subject = empty;
+		landlock_file(file)->fown_layer = 0;
 	}
 
 	/* Called in an RCU read-side critical section. */
diff --git a/security/landlock/fs.h b/security/landlock/fs.h
index 3a09ba985b74..a2a2014d68ae 100644
--- a/security/landlock/fs.h
+++ b/security/landlock/fs.h
@@ -9,6 +9,7 @@
 #ifndef _SECURITY_LANDLOCK_FS_H
 #define _SECURITY_LANDLOCK_FS_H
 
+#include <linux/build_bug.h>
 #include <linux/fs.h>
 #include <linux/init.h>
 #include <linux/rcupdate.h>
@@ -61,6 +62,11 @@ struct landlock_file_security {
 	 * _LANDLOCK_ACCESS_FS_OPTIONAL).
 	 */
 	deny_masks_t deny_masks;
+	/**
+	 * @fown_layer: Layer level of @fown_subject->domain with
+	 * LANDLOCK_SCOPE_SIGNAL.
+	 */
+	u8 fown_layer;
 #endif /* CONFIG_AUDIT */
 
 	/**
@@ -73,6 +79,14 @@ struct landlock_file_security {
 	struct landlock_cred_security fown_subject;
 };
 
+#ifdef CONFIG_AUDIT
+
+/* Makes sure all layers can be identified. */
+static_assert((typeof_member(struct landlock_file_security, fown_layer))~0 >=
+	      LANDLOCK_MAX_NUM_LAYERS);
+
+#endif /* CONFIG_AUDIT */
+
 /**
  * struct landlock_superblock_security - Superblock security blob
  *
diff --git a/security/landlock/task.c b/security/landlock/task.c
index e8a0b4ead381..f6359cf4c11b 100644
--- a/security/landlock/task.c
+++ b/security/landlock/task.c
@@ -264,26 +264,41 @@ static int hook_unix_stream_connect(struct sock *const sock,
 				    struct sock *const other,
 				    struct sock *const newsk)
 {
+	size_t handle_layer;
 	const struct landlock_cred_security *const subject =
 		landlock_get_applicable_subject(current_cred(), unix_scope,
-						NULL);
+						&handle_layer);
 
 	/* Quick return for non-landlocked tasks. */
 	if (!subject)
 		return 0;
 
-	if (is_abstract_socket(other) && sock_is_scoped(other, subject->domain))
-		return -EPERM;
+	if (!is_abstract_socket(other))
+		return 0;
+
+	if (!sock_is_scoped(other, subject->domain))
+		return 0;
 
-	return 0;
+	landlock_log_denial(subject, &(struct landlock_request) {
+		.type = LANDLOCK_REQUEST_SCOPE_ABSTRACT_UNIX_SOCKET,
+		.audit = {
+			.type = LSM_AUDIT_DATA_NET,
+			.u.net = &(struct lsm_network_audit) {
+				.sk = other,
+			},
+		},
+		.layer_plus_one = handle_layer + 1,
+	});
+	return -EPERM;
 }
 
 static int hook_unix_may_send(struct socket *const sock,
 			      struct socket *const other)
 {
+	size_t handle_layer;
 	const struct landlock_cred_security *const subject =
 		landlock_get_applicable_subject(current_cred(), unix_scope,
-						NULL);
+						&handle_layer);
 
 	if (!subject)
 		return 0;
@@ -295,11 +310,23 @@ static int hook_unix_may_send(struct socket *const sock,
 	if (unix_peer(sock->sk) == other->sk)
 		return 0;
 
-	if (is_abstract_socket(other->sk) &&
-	    sock_is_scoped(other->sk, subject->domain))
-		return -EPERM;
+	if (!is_abstract_socket(other->sk))
+		return 0;
+
+	if (!sock_is_scoped(other->sk, subject->domain))
+		return 0;
 
-	return 0;
+	landlock_log_denial(subject, &(struct landlock_request) {
+		.type = LANDLOCK_REQUEST_SCOPE_ABSTRACT_UNIX_SOCKET,
+		.audit = {
+			.type = LSM_AUDIT_DATA_NET,
+			.u.net = &(struct lsm_network_audit) {
+				.sk = other->sk,
+			},
+		},
+		.layer_plus_one = handle_layer + 1,
+	});
+	return -EPERM;
 }
 
 static const struct access_masks signal_scope = {
@@ -311,13 +338,15 @@ static int hook_task_kill(struct task_struct *const p,
 			  const struct cred *cred)
 {
 	bool is_scoped;
+	size_t handle_layer;
 	const struct landlock_cred_security *subject;
 
 	if (!cred)
 		/* Not dealing with USB IO. */
 		cred = current_cred();
 
-	subject = landlock_get_applicable_subject(cred, signal_scope, NULL);
+	subject = landlock_get_applicable_subject(cred, signal_scope,
+						  &handle_layer);
 
 	/* Quick return for non-landlocked tasks. */
 	if (!subject)
@@ -329,10 +358,19 @@ static int hook_task_kill(struct task_struct *const p,
 					     landlock_get_task_domain(p),
 					     signal_scope.scope);
 	}
-	if (is_scoped)
-		return -EPERM;
 
-	return 0;
+	if (!is_scoped)
+		return 0;
+
+	landlock_log_denial(subject, &(struct landlock_request) {
+		.type = LANDLOCK_REQUEST_SCOPE_SIGNAL,
+		.audit = {
+			.type = LSM_AUDIT_DATA_TASK,
+			.u.tsk = p,
+		},
+		.layer_plus_one = handle_layer + 1,
+	});
+	return -EPERM;
 }
 
 static int hook_file_send_sigiotask(struct task_struct *tsk,
@@ -361,10 +399,19 @@ static int hook_file_send_sigiotask(struct task_struct *tsk,
 					     landlock_get_task_domain(tsk),
 					     signal_scope.scope);
 	}
-	if (is_scoped)
-		return -EPERM;
 
-	return 0;
+	if (!is_scoped)
+		return 0;
+
+	landlock_log_denial(subject, &(struct landlock_request) {
+		.type = LANDLOCK_REQUEST_SCOPE_SIGNAL,
+		.audit = {
+			.type = LSM_AUDIT_DATA_TASK,
+			.u.tsk = tsk,
+		},
+		.layer_plus_one = landlock_file(fown->file)->fown_layer + 1,
+	});
+	return -EPERM;
 }
 
 static struct security_hook_list landlock_hooks[] __ro_after_init = {
-- 
2.48.1