landlock: Add signal scoping

Currently, a sandbox process is not restricted to sending a signal (e.g.
SIGKILL) to a process outside the sandbox environment. The ability to
send a signal for a sandboxed process should be scoped the same way
abstract UNIX sockets are scoped. Therefore, we extend the "scoped"
field in a ruleset with LANDLOCK_SCOPE_SIGNAL to specify that a ruleset
will deny sending any signal from within a sandbox process to its parent
(i.e. any parent sandbox or non-sandboxed processes).

This patch adds file_set_fowner and file_free_security hooks to set and
release a pointer to the file owner's domain. This pointer, fown_domain
in landlock_file_security will be used in file_send_sigiotask to check
if the process can send a signal.

The ruleset_with_unknown_scope test is updated to support
LANDLOCK_SCOPE_SIGNAL.

This depends on two new changes:
- commit 1934b212615d ("file: reclaim 24 bytes from f_owner"): replace
container_of(fown, struct file, f_owner) with fown->file .
- commit 26f204380a3c ("fs: Fix file_set_fowner LSM hook
inconsistencies"): lock before calling the hook.

Signed-off-by: Tahera Fahimi <[e-mail]>
Closes: https://github.com/landlock-lsm/linux/issues/8
Link: https://lore.kernel.org/r/df2b[e-mail]
[mic: Update landlock_get_current_domain()'s return type, improve and
fix locking in hook_file_set_fowner(), simplify and fix sleepable call
and locking issue in hook_file_send_sigiotask() and rebase on the latest
VFS tree, simplify hook_task_kill() and quickly return when not
sandboxed, improve comments, rename LANDLOCK_SCOPED_SIGNAL]
Co-developed-by: Mickaël Salaün <[e-mail]>
Signed-off-by: Mickaël Salaün <[e-mail]>

include/uapi/linux/landlock.h

···
  * - %LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET: Restrict a sandboxed process from
  *   connecting to an abstract UNIX socket created by a process outside the
  *   related Landlock domain (e.g. a parent domain or a non-sandboxed process).
+ * - %LANDLOCK_SCOPE_SIGNAL: Restrict a sandboxed process from sending a signal
+ *   to another process outside the domain.
  */
 /* clang-format off */
 #define LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET		(1ULL << 0)
+#define LANDLOCK_SCOPE_SIGNAL		                (1ULL << 1)
 /* clang-format on*/
 
 #endif /* _UAPI_LINUX_LANDLOCK_H */

security/landlock/cred.h

···
 	return cred->security + landlock_blob_sizes.lbs_cred;
 }
 
-static inline const struct landlock_ruleset *landlock_get_current_domain(void)
+static inline struct landlock_ruleset *landlock_get_current_domain(void)
 {
 	return landlock_cred(current_cred())->domain;
 }

security/landlock/fs.c

···
 	return -EACCES;
 }
 
+static void hook_file_set_fowner(struct file *file)
+{
+	struct landlock_ruleset *new_dom, *prev_dom;
+
+	/*
+	 * Lock already held by __f_setown(), see commit 26f204380a3c ("fs: Fix
+	 * file_set_fowner LSM hook inconsistencies").
+	 */
+	lockdep_assert_held(&file_f_owner(file)->lock);
+	new_dom = landlock_get_current_domain();
+	landlock_get_ruleset(new_dom);
+	prev_dom = landlock_file(file)->fown_domain;
+	landlock_file(file)->fown_domain = new_dom;
+
+	/* Called in an RCU read-side critical section. */
+	landlock_put_ruleset_deferred(prev_dom);
+}
+
+static void hook_file_free_security(struct file *file)
+{
+	landlock_put_ruleset_deferred(landlock_file(file)->fown_domain);
+}
+
 static struct security_hook_list landlock_hooks[] __ro_after_init = {
 	LSM_HOOK_INIT(inode_free_security_rcu, hook_inode_free_security_rcu),
 
···
 	LSM_HOOK_INIT(file_truncate, hook_file_truncate),
 	LSM_HOOK_INIT(file_ioctl, hook_file_ioctl),
 	LSM_HOOK_INIT(file_ioctl_compat, hook_file_ioctl_compat),
+	LSM_HOOK_INIT(file_set_fowner, hook_file_set_fowner),
+	LSM_HOOK_INIT(file_free_security, hook_file_free_security),
 };
 
 __init void landlock_add_fs_hooks(void)

security/landlock/fs.h

···
 	 * needed to authorize later operations on the open file.
 	 */
 	access_mask_t allowed_access;
+	/**
+	 * @fown_domain: Domain of the task that set the PID that may receive a
+	 * signal e.g., SIGURG when writing MSG_OOB to the related socket.
+	 * This pointer is protected by the related file->f_owner->lock, as for
+	 * fown_struct's members: pid, uid, and euid.
+	 */
+	struct landlock_ruleset *fown_domain;
 };
 
 /**

security/landlock/limits.h

···
 #define LANDLOCK_MASK_ACCESS_NET	((LANDLOCK_LAST_ACCESS_NET << 1) - 1)
 #define LANDLOCK_NUM_ACCESS_NET		__const_hweight64(LANDLOCK_MASK_ACCESS_NET)
 
-#define LANDLOCK_LAST_SCOPE		LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET
+#define LANDLOCK_LAST_SCOPE		LANDLOCK_SCOPE_SIGNAL
 #define LANDLOCK_MASK_SCOPE		((LANDLOCK_LAST_SCOPE << 1) - 1)
 #define LANDLOCK_NUM_SCOPE		__const_hweight64(LANDLOCK_MASK_SCOPE)
 /* clang-format on */

security/landlock/task.c

···
 
 #include "common.h"
 #include "cred.h"
+#include "fs.h"
 #include "ruleset.h"
 #include "setup.h"
 #include "task.h"
···
 	return 0;
 }
 
+static int hook_task_kill(struct task_struct *const p,
+			  struct kernel_siginfo *const info, const int sig,
+			  const struct cred *const cred)
+{
+	bool is_scoped;
+	const struct landlock_ruleset *dom;
+
+	if (cred) {
+		/* Dealing with USB IO. */
+		dom = landlock_cred(cred)->domain;
+	} else {
+		dom = landlock_get_current_domain();
+	}
+
+	/* Quick return for non-landlocked tasks. */
+	if (!dom)
+		return 0;
+
+	rcu_read_lock();
+	is_scoped = domain_is_scoped(dom, landlock_get_task_domain(p),
+				     LANDLOCK_SCOPE_SIGNAL);
+	rcu_read_unlock();
+	if (is_scoped)
+		return -EPERM;
+
+	return 0;
+}
+
+static int hook_file_send_sigiotask(struct task_struct *tsk,
+				    struct fown_struct *fown, int signum)
+{
+	const struct landlock_ruleset *dom;
+	bool is_scoped = false;
+
+	/* Lock already held by send_sigio() and send_sigurg(). */
+	lockdep_assert_held(&fown->lock);
+	dom = landlock_file(fown->file)->fown_domain;
+
+	/* Quick return for unowned socket. */
+	if (!dom)
+		return 0;
+
+	rcu_read_lock();
+	is_scoped = domain_is_scoped(dom, landlock_get_task_domain(tsk),
+				     LANDLOCK_SCOPE_SIGNAL);
+	rcu_read_unlock();
+	if (is_scoped)
+		return -EPERM;
+
+	return 0;
+}
+
 static struct security_hook_list landlock_hooks[] __ro_after_init = {
 	LSM_HOOK_INIT(ptrace_access_check, hook_ptrace_access_check),
 	LSM_HOOK_INIT(ptrace_traceme, hook_ptrace_traceme),
 
 	LSM_HOOK_INIT(unix_stream_connect, hook_unix_stream_connect),
 	LSM_HOOK_INIT(unix_may_send, hook_unix_may_send),
+
+	LSM_HOOK_INIT(task_kill, hook_task_kill),
+	LSM_HOOK_INIT(file_send_sigiotask, hook_file_send_sigiotask),
 };
 
 __init void landlock_add_task_hooks(void)

tools/testing/selftests/landlock/scoped_test.c

···
 
 #include "common.h"
 
-#define ACCESS_LAST LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET
+#define ACCESS_LAST LANDLOCK_SCOPE_SIGNAL
 
 TEST(ruleset_with_unknown_scope)
 {