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

Will Deacon
2019-02-13T14:00:34Z

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.

Will