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

On Wed, Feb 13, 2019 at 02:24:34PM +0000, Julien Thierry wrote:

On 13/02/2019 14:17, Peter Zijlstra wrote:

quoted

On Wed, Feb 13, 2019 at 02:00:26PM +0000, Will Deacon wrote:

quoted

quoted

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.

AFAICT, This does not work properly because when you schedule out this
task, you won't be saving the EFLAGS.AF/PSTATE.PAN bit on the stack, and

EFLAGS.AC, but yes.

next time you schedule the task back in, it might no longer have the
correct flag value (so an unsafe_get/put_user() will fail even though
you haven't reached user_access_end()).

/me looks at __switch_to_asm() and there is indeed a distinct lack of
pushing and popping EFLAGS :/

One solution is to deal with this in task switching code, but so far
I've been told that calling schedule() in such a context is not expected
to be supported.

Well, per the above it breaks the preemption model. And I hates that.

And the added WARN doesn't even begin to cover it, since you'd have to
actually hit the preempt_enable() reschedule for it trigger.

So far, all 6 in-tree users are indeed free of dodgy code, but *groan*.

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