Re: [PATCH v2] taskstats: don't allow duplicate entries in listener mode

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

On Thu, 16 Jun 2011 19:55:59 +0400
Vasiliy Kulikov <segoon@openwall.com> wrote:

> Currently a single process may register exit handlers unlimited times.
> It may lead to a bloated listeners chain and very slow process terminations.
> E.g. after 10KK sent TASKSTATS_CMD_ATTR_REGISTER_CPUMASKs ~300 Mb of
> kernel memory is stolen for the handlers chain and "time id" shows 2-7
> seconds instead of normal 0.003.  It makes it possible to exhaust all
> kernel memory and to eat much of CPU time by triggerring numerous exits
> on a single CPU.
> 
> The patch limits the number of times a single process may register
> itself on a single CPU to one.
> 
> One little issue is kept unfixed - as taskstats_exit() is called before
> exit_files() in do_exit(), the orphaned listener entry (if it was not
> explicitly deregistered) is kept until the next someone's exit() and
> implicit deregistration in send_cpu_listeners().  So, if a process
> registered itself as a listener exits and the next spawned process gets
> the same pid, it would inherit taskstats attributes.

ow. That's an unprivileged operation, isn't it?

> 
> diff --git a/kernel/taskstats.c b/kernel/taskstats.c
> index 9ffea36..fc0f220 100644
> --- a/kernel/taskstats.c
> +++ b/kernel/taskstats.c
> @@ -285,16 +285,18 @@ ret:
>  static int add_del_listener(pid_t pid, const struct cpumask *mask, int isadd)
>  {
>  	struct listener_list *listeners;
> -	struct listener *s, *tmp;
> +	struct listener *s, *tmp, *s2;
>  	unsigned int cpu;
>  
>  	if (!cpumask_subset(mask, cpu_possible_mask))
>  		return -EINVAL;
>  
> +	s = NULL;
>  	if (isadd == REGISTER) {
>  		for_each_cpu(cpu, mask) {
> -			s = kmalloc_node(sizeof(struct listener), GFP_KERNEL,
> -					 cpu_to_node(cpu));
> +			if (!s)
> +				s = kmalloc_node(sizeof(struct listener),
> +						 GFP_KERNEL, cpu_to_node(cpu));
>  			if (!s)
>  				goto cleanup;
>  			s->pid = pid;
> @@ -303,9 +305,16 @@ static int add_del_listener(pid_t pid, const struct cpumask *mask, int isadd)
>  
>  			listeners = &per_cpu(listener_array, cpu);
>  			down_write(&listeners->sem);
> +			list_for_each_entry_safe(s2, tmp, &listeners->list, list) {
> +				if (s2->pid == pid)
> +					goto next_cpu;
> +			}
>  			list_add(&s->list, &listeners->list);
> +			s = NULL;
> +next_cpu:
>  			up_write(&listeners->sem);
>  		}
> +		kfree(s);
>  		return 0;
>  	}
>