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

[Justin Suess]

On Wed, Mar 04, 2026 at 05:54:17PM +0800, Yihan Ding wrote:
> 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=7ea2f5e9dfd468201817
> Suggested-by: Günther Noack <gnoack3000@xxxxxxxxx>
> Signed-off-by: Yihan Ding <dingyihan@xxxxxxxxxxxxx>
> ---
> 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
>

Tested-by: Justin Suess <utilityemal77@xxxxxxxxx>