[PATCH v3 1/2] landlock: Serialize TSYNC thread restriction

[Günther Noack]

On Thu, Feb 26, 2026 at 09:59:02AM +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 at syzkaller.appspotmail.com
> Closes: https://syzkaller.appspot.com/bug?extid=7ea2f5e9dfd468201817
> Suggested-by: Günther Noack <gnoack3000 at gmail.com>
> Signed-off-by: Yihan Ding <dingyihan at uniontech.com>
> ---
> 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).
> 
> Changes in v2:
> - Use down_write_killable() instead of down_write().
> - Split the interrupt path cleanup into a separate patch.
> ---
>  security/landlock/tsync.c | 8 ++++++++
>  1 file changed, 8 insertions(+)
> 
> diff --git a/security/landlock/tsync.c b/security/landlock/tsync.c
> index de01aa899751..xxxxxxxxxxxx 100644
> --- a/security/landlock/tsync.c
> +++ b/security/landlock/tsync.c
> @@ -447,6 +447,13 @@ 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 (!down_write_trylock(&current->signal->exec_update_lock))
> +		return -ERESTARTNOINTR;
> +
>  	/*
>  	 * We schedule a pseudo-signal task_work for each of the calling task's
>  	 * sibling threads.  In the task work, each thread:
> @@ -556,6 +563,7 @@ 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);
>  }
> -- 
> 2.51.0
> 

Reviewed-by: Günther Noack <gnoack3000 at gmail.com>