Re: [PATCH] fork/pid: Fix use-after-free in __task_pid_nr_ns

[Andrew Morton <akpm@linux-foundation.org>]

On Mon,  5 Jan 2026 12:36:27 +0800 Qing Wang <wangqing7171@gmail.com> wrote:

> Syzbot reported a slab-use-after-free issue in __task_pid_nr_ns:
> 
>     BUG: KASAN: slab-use-after-free in __task_pid_nr_ns+0x1e4/0x490...
>     Read of size 8 at addr ffff88807f8058a8 by task syz.1.574/8108
> 
> The race condition occurs between the failure path of copy_process() and
> getting the PIDTYPE_TGID via __task_pid_nr_ns().
> 
> Bug timeline:
>                                     Task B
>                                     perf_event_open()
> Task A <--------------------------- clone()
> copy_process()
>     perf_event_init_task()
>     ...
>     one copy failed
>     free_signal_struct()            close(event_fd)
>                                         perf_child_detach()
>                                             __task_pid_nr_ns()
>                                                 access child task->signal
> 
> This is fixed by:
> 1. Setting task->signal = NULL in the failure cleanup path of copy_process.
> 2. Adding a null check for task->signal before accessing PIDTYPE_TGID from
> task->signal.
> 
> Note: This bug was reported by syzbot without a reproducer.
> The fix is based on code inspection and race condition analysis.

Thanks.

> 
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -329,9 +329,9 @@ EXPORT_SYMBOL_GPL(find_vpid);
>  
>  static struct pid **task_pid_ptr(struct task_struct *task, enum pid_type type)
>  {
> -	return (type == PIDTYPE_PID) ?
> -		&task->thread_pid :
> -		&task->signal->pids[type];
> +	if (type == PIDTYPE_PID)
> +		return &task->thread_pid;
> +	return task->signal ? &task->signal->pids[type] : NULL;
>  }

It might be helpful to have a comment here telling readers how
task->signal can be zero.

Also, what in here prevents task->signal from being zeroed after we've
tested it and before we dereference it?