landlock: Serialize TSYNC thread restriction

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 restart the syscall 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>
Suggested-by: Tingmao Wang <m@maowtm.org>
Tested-by: Justin Suess <utilityemal77@gmail.com>
Signed-off-by: Yihan Ding <dingyihan@uniontech.com>
Tested-by: Günther Noack <gnoack3000@gmail.com>
Reviewed-by: Günther Noack <gnoack3000@gmail.com>
Link: https://lore.kernel.org/r/20260306021651.744723-2-dingyihan@uniontech.com
Signed-off-by: Mickaël Salaün <mic@digikod.net>

diff --git a/security/landlock/tsync.c b/security/landlock/tsync.c
index 1e738ef2cff51369427939ddd07f9e2e09c5ae74..f0e5f8102001afa44452aba90fbd5ea7d0fe3c28 100644 (file)
--- a/security/landlock/tsync.c
+++ b/security/landlock/tsync.c
@@ -485,6 +485,16 @@ int landlock_restrict_sibling_threads(const struct 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
         * sibling threads.  In the task work, each thread:
@@ -594,6 +604,6 @@ 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);
 }