[PATCH v4 1/2] landlock: fix LANDLOCK_SCOPE_SIGNAL bypass on the SIGIO path

[Bryam Vargas <hexlabsecurity@proton.me>]

LANDLOCK_SCOPE_SIGNAL must prevent a sandboxed process from signaling
processes outside its Landlock domain.  It can be bypassed through the
asynchronous SIGIO delivery path.

A sandboxed process that owns any file or socket can arm it with
fcntl(F_SETOWN, fd, -pgid), fcntl(F_SETSIG, fd, SIGKILL) and O_ASYNC, so
that an I/O event makes the kernel deliver the chosen signal to the whole
process group.  As the head of its own process group -- the default right
after fork() -- that group also holds the non-sandboxed process that
launched it, e.g. a supervisor or a security monitor.  The sandbox can
thus kill or repeatedly signal exactly the processes SCOPE_SIGNAL is meant
to protect from it.

The scope is enforced in hook_file_send_sigiotask() against the Landlock
domain recorded at F_SETOWN time, not the live domain of the sender.
control_current_fowner() decides whether to record that domain and skips
recording it when the fowner target is in the caller's thread group --
safe only when the target is a single process sharing the caller's
credentials (PIDTYPE_PID, PIDTYPE_TGID).  For a process group
(PIDTYPE_PGID) the target resolves to the caller itself when it is the
group head, recording is skipped, and hook_file_send_sigiotask() then lets
the signal fan out to the whole group unchecked.

Skip the recording only for the single-process target types, so the scope
is enforced against every group member at delivery time.  The direct
kill() path (hook_task_kill) already evaluates the live domain and is
unaffected.

Fixes: 18eb75f3af40 ("landlock: Always allow signals between threads of the same process")
Cc: stable@vger.kernel.org
Tested-by: Justin Suess <utilityemal77@gmail.com>
Signed-off-by: Bryam Vargas <hexlabsecurity@proton.me>
---
 security/landlock/fs.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/security/landlock/fs.c b/security/landlock/fs.c
index c1ecfe239032..2ebad70a956d 100644
--- a/security/landlock/fs.c
+++ b/security/landlock/fs.c
@@ -1909,6 +1909,15 @@ static bool control_current_fowner(struct fown_struct *const fown)
 	if (!p)
 		return true;
 
+	/*
+	 * A process-group fowner fans the signal out to every member at
+	 * delivery time, so record the domain for any non single-process
+	 * target -- even when it resolves to current as the group head --
+	 * and let hook_file_send_sigiotask() check the live scope.
+	 */
+	if (fown->pid_type != PIDTYPE_PID && fown->pid_type != PIDTYPE_TGID)
+		return true;
+
 	return !same_thread_group(p, current);
 }
 
-- 
2.43.0