[PATCH v4 1/2] landlock: Serialize TSYNC thread restriction

[Yihan Ding <dingyihan@uniontech.com>]

syzbot found a deadlock in landlock_restrict_sibling_threads().
When multiple threads concurrently call landlock_restrict_self() with
sibling thread restriction enabled, they can deadlock by mutually
queueing task_works on each other and then blocking in kernel space
(waiting for the other to finish).

Fix this by serializing the TSYNC operations within the same process
using the exec_update_lock. This prevents concurrent invocations
from deadlocking.

We use down_write_trylock() and return -ERESTARTNOINTR if the lock
cannot be acquired immediately. This ensures that if a thread fails
to get the lock, it will return to userspace, allowing it to process
any pending TSYNC task_works from the lock holder, and then
transparently restart the syscall.

Fixes: 42fc7e6543f6 ("landlock: Multithreading support for landlock_restrict_self()")
Reported-by: syzbot+7ea2f5e9dfd468201817@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=7ea2f5e9dfd468201817
Suggested-by: Günther Noack <gnoack3000@gmail.com>
Signed-off-by: Yihan Ding <dingyihan@uniontech.com>
---
Changes in v4:
- Use restart_syscall() instead of returning -ERESTARTNOINTR.
  This ensures the syscall is properly restarted without leaking the
  internal error code to userspace, fixing a test failure in
  tsync_test.competing_enablement. (Caught by Justin Suess, suggested
  by Tingmao Wang).

Changes in v3:
- Replaced down_write_killable() with down_write_trylock() and
  returned -ERESTARTNOINTR to avoid a secondary deadlock caused by
  blocking the execution of task_works. (Caught by Günther Noack).

---
 security/landlock/tsync.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/security/landlock/tsync.c b/security/landlock/tsync.c
index de01aa899751..1f460b9ec833 100644
--- a/security/landlock/tsync.c
+++ b/security/landlock/tsync.c
@@ -446,6 +446,15 @@ int landlock_restrict_sibling_threads(const struct cred *old_cred,
 	shared_ctx.old_cred = old_cred;
 	shared_ctx.new_cred = new_cred;
 	shared_ctx.set_no_new_privs = task_no_new_privs(current);
+		/*
+		 * Serialize concurrent TSYNC operations to prevent deadlocks when
+		 * multiple threads call landlock_restrict_self() simultaneously.
+		 * If the lock is already held, we gracefully yield by restarting the
+		 * syscall. This allows the current thread to process pending
+		 * task_works before retrying.
+		 */
+	if (!down_write_trylock(&current->signal->exec_update_lock))
+		return restart_syscall();
 
 	/*
 	 * We schedule a pseudo-signal task_work for each of the calling task's
@@ -556,6 +565,6 @@ int landlock_restrict_sibling_threads(const struct cred *old_cred,
 		wait_for_completion(&shared_ctx.all_finished);
 
 	tsync_works_release(&works);
-
+	up_write(&current->signal->exec_update_lock);
 	return atomic_read(&shared_ctx.preparation_error);
 }
-- 
2.20.1