Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called in unsafe region

[Peter Zijlstra]

On Wed, Feb 13, 2019 at 02:00:26PM +0000, Will Deacon wrote:
> > This; how is getting preempted fundamentally different from scheduling
> > ourselves?
> 
> The difference is because getting preempted in the sequence above is
> triggered off the back of an interrupt. On arm64, and I think also on x86,
> the user access state (SMAP or PAN) is saved and restored across exceptions
> but not across context switch. Consequently, taking an irq in a
> user_access_{begin,end} section and then scheduling is fine, but calling
> schedule directly within such a section is not.

So how's this then:

        if (user_access_begin()) {

                preempt_disable();

                <IRQ>
                        set_need_resched();
                </IRQ no preempt>

                preempt_enable()
                  __schedule();

                user_access_end();
        }

That _should_ work just fine but explodes with the proposed nonsense.