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

[Julien Thierry]

On 13/02/2019 14:00, Will Deacon wrote:
> Hi Peter,
> 
> On Wed, Feb 13, 2019 at 02:17:20PM +0100, Peter Zijlstra wrote:
>> 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?
> 
> 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.
> 
> Julien -- please yell if I've missed some crucial detail, but I think that's
> the gist of what we're trying to describe here.
> 

Yes, this summarizes things correctly. Thanks!

I might also stress out that this limitation is already existing for x86
(and is in the arm64 patches picked by Catalin for 5.1), as was
discussed in here:

https://lkml.org/lkml/2018/11/23/430

So this patch is not introducing new semantics, it is only making
existing ones explicit.

If the current state is not good, we need to re-discuss the semantics of
user_access regions.

Thanks,

-- 
Julien Thierry