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

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Message-ID: <20190213141712.GV32494@hirez.programming.kicks-ass.net>
Date:   Wed, 13 Feb 2019 15:17:12 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Will Deacon <will.deacon@....com>
Cc:     Julien Thierry <julien.thierry@....com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, mingo@...hat.com,
        catalin.marinas@....com, james.morse@....com, hpa@...or.com,
        valentin.schneider@....com
Subject: Re: [PATCH v3 3/4] uaccess: Check no rescheduling function is called
 in unsafe region

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.