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Subject: Re: [syzbot] [kernel?] INFO: task hung in restrict_one_thread_callback
Author: dingyihan@uniontech.com

#syz test

diff --git a/security/landlock/tsync.c b/security/landlock/tsync.c
index 0d2b9c646030..4a6b9d99e594 100644
--- a/security/landlock/tsync.c
+++ b/security/landlock/tsync.c
@@ -447,6 +447,19 @@ 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);
 
+       /*
+        * We must serialize concurrent TSYNC requests within the same thread group.
+        * However, we cannot use a blocking mutex_lock() here. If Thread A holds
+        * the lock and queues task_work on Thread B, and Thread B calls TSYNC and
+        * blocks on the lock, Thread B will never return to user-mode to execute
+        * Thread A's task_work, resulting in a deadlock.
+        *
+        * Using mutex_trylock() allows us to fail gracefully, returning
+        * -ERESTARTNOINTR to process pending task_work and retry the syscall.
+        */
+       if (!mutex_trylock(&current->signal->cred_guard_mutex))
+               return -ERESTARTNOINTR;
+
        /*
         * We schedule a pseudo-signal task_work for each of the calling task's
         * sibling threads.  In the task work, each thread:
@@ -557,5 +570,7 @@ int landlock_restrict_sibling_threads(const struct cred *old_cred,
 
        tsync_works_release(&works);
 
+       mutex_unlock(&current->signal->cred_guard_mutex);
+
        return atomic_read(&shared_ctx.preparation_error);
 }