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

[Alexandre Chartre]

On 7/11/19 11:33 PM, Thomas Gleixner wrote:
> 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?

Oups, my mistake, I should be using this_cpu_ptr(). I will replace all get_cpu_var()
with this_cpu_ptr().


> > +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?

Correct, original_cr3 won't be 0. I think this is a remain from a previous version
where original_cr3 was handled differently.


> > +		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.

Will fix that.


Thanks,

alex.

> > +	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