Re: [PATCH bpf-next v5 1/6] bpf/helpers: introduce sleepable bpf_timers

[Alexei Starovoitov]

On Wed, Mar 27, 2024 at 10:02 AM Benjamin Tissoires
<benjamin.tissoires@xxxxxxxxxx> wrote:
> > >                 goto out;
> > >         }
> > > +       spin_lock(&t->sleepable_lock);
> > >         drop_prog_refcnt(t);
> > > +       spin_unlock(&t->sleepable_lock);
> >
> > this also looks odd.
>
> I basically need to protect "t->prog = NULL;" from happening while
> bpf_timer_work_cb is setting up the bpf program to be run.

Ok. I think I understand the race you're trying to fix.
The bpf_timer_cancel_and_free() is doing
cancel_work()
and proceeds with
kfree_rcu(t, rcu);

That's the only race and these extra locks don't help.

The t->prog = NULL is nothing to worry about.
The bpf_timer_work_cb() might still see callback_fn == NULL
"when it's being setup" and it's ok.
These locks don't help that.

I suggest to drop sleepable_lock everywhere.
READ_ONCE of callback_fn in bpf_timer_work_cb() is enough.
Add rcu_read_lock_trace() before calling bpf prog.

The race to fix is above 'cancel_work + kfree_rcu'
since kfree_rcu might free 'struct bpf_hrtimer *t'
while the work is pending and work_queue internal
logic might UAF struct work_struct work.
By the time it may luckily enter bpf_timer_work_cb() it's too late.
The argument 'struct work_struct *work' might already be freed.

To fix this problem, how about the following:
don't call kfree_rcu and instead queue the work to free it.
After cancel_work(&t->work); the work_struct can be reused.
So set it up to call "freeing callback" and do
schedule_work(&t->work);

There is a big assumption here that new work won't be
executed before cancelled work completes.
Need to check with wq experts.

Another approach is to do something smart with
cancel_work() return code.
If it returns true set a flag inside bpf_hrtimer and
make bpf_timer_work_cb() free(t) after bpf prog finishes.

> Also, side note: if anyone feels like it would go faster to fix those
> races by themself instead of teaching me how to properly do it, this
> is definitely fine from me :)

Most of the time goes into analyzing and thinking :)
Whoever codes it doesn't speed things much.
Pls do another respin if you still have cycles to work on it.