Re: [PATCH v6 12/12] kernel/kthread: show a warning if kthread's comm is truncated

[Kees Cook]

On Mon, Oct 25, 2021 at 08:33:15AM +0000, Yafang Shao wrote:
> Show a warning if task comm is truncated. Below is the result
> of my test case:
> 
> truncated kthread comm:I-am-a-kthread-with-lon, pid:14 by 6 characters
> 
> Suggested-by: Petr Mladek <pmladek@xxxxxxxx>
> Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> Cc: Arnaldo Carvalho de Melo <arnaldo.melo@xxxxxxxxx>
> Cc: Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Petr Mladek <pmladek@xxxxxxxx>
> ---
>  kernel/kthread.c | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 5b37a8567168..46b924c92078 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -399,12 +399,17 @@ struct task_struct *__kthread_create_on_node(int (*threadfn)(void *data),
>  	if (!IS_ERR(task)) {
>  		static const struct sched_param param = { .sched_priority = 0 };
>  		char name[TASK_COMM_LEN];
> +		int len;
>  
>  		/*
>  		 * task is already visible to other tasks, so updating
>  		 * COMM must be protected.
>  		 */
> -		vsnprintf(name, sizeof(name), namefmt, args);
> +		len = vsnprintf(name, sizeof(name), namefmt, args);
> +		if (len >= TASK_COMM_LEN) {

And since this failure case is slow-path, we could improve the warning
as other had kind of suggested earlier with something like this instead:

			char *full_comm;

			full_comm = kvasprintf(GFP_KERNEL, namefmt, args);
			pr_warn("truncated kthread comm '%s' to '%s' (pid:%d)\n",
				full_comm, name);

			kfree(full_comm);
		}
>  		set_task_comm(task, name);
>  		/*
>  		 * root may have changed our (kthreadd's) priority or CPU mask.
> -- 
> 2.17.1
> 

-- 
Kees Cook