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 <fahimitahera@gmail.com>
Closes: https://github.com/landlock-lsm/linux/issues/8
Link: https://lore.kernel.org/r/df2b4f880a2ed3042992689a793ea0951f6798a5.1725657727.git.fahimitahera@gmail.com
[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 <mic@digikod.net>
Signed-off-by: Mickaël Salaün <mic@digikod.net>

1 files changed, 56 insertions, 0 deletions

diff --git a/security/landlock/task.c b/security/landlock/task.c
index 4f8013ca412e..4acbd7c40eee 100644
--- a/security/landlock/task.c
+++ b/security/landlock/task.c
@@ -18,6 +18,7 @@
 
 #include "common.h"
 #include "cred.h"
+#include "fs.h"
 #include "ruleset.h"
 #include "setup.h"
 #include "task.h"
@@ -242,12 +243,67 @@ static int hook_unix_may_send(struct socket *const sock,
 	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)