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

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Date:   Wed, 13 Feb 2019 14:24:34 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Cc:     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 13/02/2019 14:17, Peter Zijlstra wrote:
> 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.

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
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()).

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.

Cheers,

-- 
Julien Thierry