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

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Message-ID: <1c2429a4-9df9-40a3-98e0-51577de4bd6a@arm.com>
Date:   Wed, 13 Feb 2019 10:50:21 +0000
From:   Julien Thierry <julien.thierry@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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 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
- 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?

Thanks,

-- 
Julien Thierry