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

Julien Thierry
2019-02-13T10:50:30Z

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