Re: [PATCH] sched: Fix NULL user_cpus_ptr check in dup_user_cpus_ptr()

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Nov 22, 2022 at 02:06:53PM -0500, Waiman Long wrote:
> In general, a non-null user_cpus_ptr will remain set until the task dies.
> A possible exception to this is the fact that do_set_cpus_allowed()
> will clear a non-null user_cpus_ptr. To allow this possible racing
> condition, we need to check for NULL user_cpus_ptr under the pi_lock
> before duping the user mask.
> 
> Fixes: 851a723e45d1 ("sched: Always clear user_cpus_ptr in do_set_cpus_allowed()")
> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/sched/core.c | 23 +++++++++++++++++++++--
>  1 file changed, 21 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8df51b08bb38..f447a6285ea2 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2625,7 +2625,14 @@ int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
>  		      int node)
>  {
>  	unsigned long flags;
> +	cpumask_t *user_mask = NULL;

The inverse xmas tree is sad :-(

>  
> +	/*
> +	 * If there is a concurrent sched_setaffinity(), we may miss the
> +	 * newly updated user_cpus_ptr. However, a non-NULL user_cpus_ptr
> +	 * is relatively unlikely and it is not worth the extra overhead
> +	 * of taking the pi_lock on every fork/clone.
> +	 */

I think the correct argument is saying the thing is racy and loosing the
race is a valid situation. IOW, this is the same as if the concurrent
sched_setaffinity() happens after fork().

>  	if (!src->user_cpus_ptr)
>  		return 0;
>  
> @@ -2633,10 +2640,22 @@ int dup_user_cpus_ptr(struct task_struct *dst, struct task_struct *src,
>  	if (!dst->user_cpus_ptr)
>  		return -ENOMEM;
>  
> -	/* Use pi_lock to protect content of user_cpus_ptr */
> +	/*
> +	 * Use pi_lock to protect content of user_cpus_ptr
> +	 *
> +	 * Though unlikely, user_cpus_ptr can be reset to NULL by a concurrent
> +	 * do_set_cpus_allowed().
> +	 */
>  	raw_spin_lock_irqsave(&src->pi_lock, flags);
> +	if (src->user_cpus_ptr)
> +		cpumask_copy(dst->user_cpus_ptr, src->user_cpus_ptr);
> +	else
> +		swap(dst->user_cpus_ptr, user_mask);

Uhhhh, did you mean to write:

	if (src->user_cpus_ptr) {
		cpumask_copy(user_mask, src->user_cpus_ptr);
		swap(dst->user_cpus_ptr, user_mask);
	}

?

>  	raw_spin_unlock_irqrestore(&src->pi_lock, flags);
> +
> +	if (unlikely(user_mask))
> +		kfree(user_mask);
> +
>  	return 0;
>  }
>  
> -- 
> 2.31.1
>