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 14:17:20 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Julien Thierry <julien.thierry@....com>
Cc:     Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, mingo@...hat.com,
        catalin.marinas@....com, will.deacon@....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 10:50:21AM +0000, Julien Thierry wrote:
> On 13/02/2019 10:35, Peter Zijlstra wrote:
> > On Tue, Feb 12, 2019 at 09:15:13AM +0000, Julien Thierry wrote:
> > 
> >>>>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> >>>>> index a674c7db..b1bb7e9 100644
> >>>>> --- a/kernel/sched/core.c
> >>>>> +++ b/kernel/sched/core.c
> >>>>> @@ -3289,6 +3289,14 @@ static inline void schedule_debug(struct task_struct *prev)
> >>>>>  		__schedule_bug(prev);
> >>>>>  		preempt_count_set(PREEMPT_DISABLED);
> >>>>>  	}
> >>>>> +
> >>>>> +	if (IS_ENABLED(CONFIG_DEBUG_UACCESS_SLEEP) &&
> >>>>> +	    unlikely(unsafe_user_region_active())) {
> >>>>> +		printk(KERN_ERR "BUG: scheduling while user_access enabled: %s/%d/0x%08x\n",
> >>>>> +		       prev->comm, prev->pid, preempt_count());
> >>>>> +		dump_stack();
> >>>>> +	}
> >>>>> +
> >>>>>  	rcu_sleep_check();
> >>>>>
> >>>>>  	profile_hit(SCHED_PROFILING, __builtin_return_address(0));
> > 
> >> I guess I'll drop the might_resched() part of this patch if that sounds
> >> alright.
> > 
> > I'm still confused by the schedule_debug() part. How is that not broken?
> 
> Hmmm, I am not exactly sure which part you expect to be broken, I guess
> it's because of the nature of the uaccess unsafe accessor usage.
> 
> Basically, the following is a definite no:
> 	if (user_access_begin(ptr, size)) {
> 
> 		[...]
> 
> 		//something that calls schedule
> 
> 		[...]
> 
> 		user_access_end();
> 	}
> 	
> 
> However the following is fine:
> 
> - user_access_begin(ptr, size)
> - taking irq/exception
> - get preempted

This; how is getting preempted fundamentally different from scheduling
ourselves?

> - get resumed at some point in time
> - restore state + eret
> - user_access_end()
> 
> That's because exceptions/irq implicitly "suspend" the user access
> region. (That's what I'm trying to clarify with the comment)
> So, unsafe_user_region_active() should return false in a irq/exception
> context.
> 
> Is this what you were concerned about? Or there still something that
> might be broken?

I really hate the asymetry introduced between preemptible and being able
to schedule. Both end up calling __schedule() and there really should
not be a difference.