Re: [PATCH 1/2] kernel/fork: handle put_user errors for CLONE_CHILD_SETTID/CLEARTID

[Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>]

On 02/06, Konstantin Khlebnikov wrote:
>
> Whole sequence looks like: task calls fork, glibc calls syscall clone with
> CLONE_CHILD_SETTID and passes pointer to TLS THREAD_SELF->tid as argument.
> Child task gets read-only copy of VM including TLS. Child calls put_user()
> to handle CLONE_CHILD_SETTID from schedule_tail(). put_user() trigger page
> fault and it fails because do_wp_page()  hits memcg limit without invoking
> OOM-killer because this is page-fault from kernel-space.

Because of !FAULT_FLAG_USER?

Perhaps we should fix this? Say mem_cgroup_oom_enable/disable around put_user(),
I dunno.

> Put_user returns
> -EFAULT, which is ignored.  Child returns into user-space and catches here
> assert (THREAD_GETMEM (self, tid) != ppid),

If only I understood why else we need CLONE_CHILD_SETTID ;)

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2312,8 +2312,20 @@ asmlinkage __visible void schedule_tail(struct task_struct *prev)
>  	post_schedule(rq);
>  	preempt_enable();
>  
> -	if (current->set_child_tid)
> -		put_user(task_pid_vnr(current), current->set_child_tid);
> +	if (current->set_child_tid &&
> +	    unlikely(put_user(task_pid_vnr(current), current->set_child_tid))) {
> +		int dummy;
> +
> +		/*
> +		 * If this address is unreadable then userspace has not set
> +		 * proper pointer. Application either doesn't care or will
> +		 * notice this soon. If this address is readable then task
> +		 * will be mislead about its own tid. It's better to die.
> +		 */
> +		if (!get_user(dummy, current->set_child_tid) &&
> +		    !fatal_signal_pending(current))
> +			force_sig(SIGSEGV, current);
> +	}

Well, get_user() can fail the same way? The page we need to cow can be
swapped out.

At first glance, to me this problem should be solved somewhere else...
I'll try to reread this all tomorrow.

Oleg.