Re: [bug] kernel: bpf: syscall: a possible sleep-in-atomic bug in __bpf_prog_put()

[Yonghong Song]

On 5/24/23 12:44 PM, Alexei Starovoitov wrote:
> On Wed, May 24, 2023 at 12:34 PM Yonghong Song <yhs@xxxxxxxx> wrote:
> > On 5/24/23 5:42 AM, Teng Qi wrote:
> > > Thank you.
> > >
> > > > We cannot use rcu_read_lock_held() in the 'if' statement. The return
> > > > value rcu_read_lock_held() could be 1 for some configurations regardless
> > > > whether rcu_read_lock() is really held or not. In most cases,
> > > > rcu_read_lock_held() is used in issuing potential warnings.
> > > > Maybe there are other ways to record whether rcu_read_lock() is held or not?
> > >
> > > Sorry. I was not aware of the dependency of configurations of
> > > rcu_read_lock_held().
> > >
> > > > If we cannot resolve rcu_read_lock() presence issue, maybe the condition
> > > > can be !in_interrupt(), so any process-context will go to a workqueue.
> > >
> > > I agree that using !in_interrupt() as a condition is an acceptable solution.
> >
> > This should work although it could be conservative.
> >
> > > > Alternatively, we could have another solution. We could add another
> > > > function e.g., bpf_prog_put_rcu(), which indicates that bpf_prog_put()
> > > > will be done in rcu context.
> > >
> > > Implementing a new function like bpf_prog_put_rcu() is a solution that involves
> > > more significant changes.
> >
> > Maybe we can change signature of bpf_prog_put instead? Like
> >      void bpf_prog_put(struct bpf_prog *prog, bool in_rcu)
> > and inside bpf_prog_put we can add
> >      WARN_ON_ONCE(in_rcu && !bpf_rcu_lock_held());
>
> bpf_rcu_lock_held() is used for different cases.

Sorry, I actually mean rcu_read_lock_held() ...

> Here s/in_irq/in_interrupt/ inside bpf_prog_put() is enough
> to address this theoretical issue.

Maybe

                if (!in_interrupt()) {
                        INIT_WORK(&aux->work, bpf_prog_put_deferred);
                        schedule_work(&aux->work);
                } else {
                        bpf_prog_put_deferred(&aux->work);
                }
?

Basically for any process context, use a work queue since
we have no idea whether rcu_read_lock() is held or not.
In process context, is_atmoc() and irqs_disabled() should
already use the work queue.
As we discussed in the above, if in_interrupt() is true,
kvfree seems okay, so can directly call
bpf_prog_put_deferred().
Does this sound reasonable?