Re: [RFC v2 01/26] mm/x86: Introduce kernel address space isolation

[Thomas Gleixner]

On Thu, 11 Jul 2019, Alexandre Chartre wrote:
> +/*
> + * When isolation is active, the address space doesn't necessarily map
> + * the percpu offset value (this_cpu_off) which is used to get pointers
> + * to percpu variables. So functions which can be invoked while isolation
> + * is active shouldn't be getting pointers to percpu variables (i.e. with
> + * get_cpu_var() or this_cpu_ptr()). Instead percpu variable should be
> + * directly read or written to (i.e. with this_cpu_read() or
> + * this_cpu_write()).
> + */
> +
> +int asi_enter(struct asi *asi)
> +{
> +	enum asi_session_state state;
> +	struct asi *current_asi;
> +	struct asi_session *asi_session;
> +
> +	state = this_cpu_read(cpu_asi_session.state);
> +	/*
> +	 * We can re-enter isolation, but only with the same ASI (we don't
> +	 * support nesting isolation). Also, if isolation is still active,
> +	 * then we should be re-entering with the same task.
> +	 */
> +	if (state == ASI_SESSION_STATE_ACTIVE) {
> +		current_asi = this_cpu_read(cpu_asi_session.asi);
> +		if (current_asi != asi) {
> +			WARN_ON(1);
> +			return -EBUSY;
> +		}
> +		WARN_ON(this_cpu_read(cpu_asi_session.task) != current);
> +		return 0;
> +	}
> +
> +	/* isolation is not active so we can safely access the percpu pointer */
> +	asi_session = &get_cpu_var(cpu_asi_session);

get_cpu_var()?? Where is the matching put_cpu_var() ? get_cpu_var()
contains a preempt_disable ...

What's wrong with a simple this_cpu_ptr() here?

> +void asi_exit(struct asi *asi)
> +{
> +	struct asi_session *asi_session;
> +	enum asi_session_state asi_state;
> +	unsigned long original_cr3;
> +
> +	asi_state = this_cpu_read(cpu_asi_session.state);
> +	if (asi_state == ASI_SESSION_STATE_INACTIVE)
> +		return;
> +
> +	/* TODO: Kick sibling hyperthread before switching to kernel cr3 */
> +	original_cr3 = this_cpu_read(cpu_asi_session.original_cr3);
> +	if (original_cr3)

Why would this be 0 if the session is active?

> +		write_cr3(original_cr3);
> +
> +	/* page-table was switched, we can now access the percpu pointer */
> +	asi_session = &get_cpu_var(cpu_asi_session);

See above.

> +	WARN_ON(asi_session->task != current);
> +	asi_session->state = ASI_SESSION_STATE_INACTIVE;
> +	asi_session->asi = NULL;
> +	asi_session->task = NULL;
> +	asi_session->original_cr3 = 0;
> +}

Thanks,

	tglx