On Fri, Dec 6, 2024 at 9:09 AM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> On Wed, Oct 23, 2024 at 09:01:02AM -0700, Andrii Nakryiko wrote:
> > On Wed, Oct 23, 2024 at 3:01 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> > >
> > > Peter reported that perf_event_detach_bpf_prog might skip to release
> > > the bpf program for -ENOENT error from bpf_prog_array_copy.
> > >
> > > This can't happen because bpf program is stored in perf event and is
> > > detached and released only when perf event is freed.
> > >
> > > Let's make it obvious and add WARN_ON_ONCE on the -ENOENT check and
> > > make sure the bpf program is released in any case.
> > >
> > > Cc: Sean Young <sean@xxxxxxxx>
> > > Fixes: 170a7e3ea070 ("bpf: bpf_prog_array_copy() should return -ENOENT if exclude_prog not found")
> > > Closes: https://lore.kernel.org/lkml/20241022111638.GC16066@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> > > Reported-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > Signed-off-by: Jiri Olsa <jolsa@xxxxxxxxxx>
> > > ---
> > > kernel/trace/bpf_trace.c | 5 +++--
> > > 1 file changed, 3 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > > index 95b6b3b16bac..2c064ba7b0bd 100644
> > > --- a/kernel/trace/bpf_trace.c
> > > +++ b/kernel/trace/bpf_trace.c
> > > @@ -2216,8 +2216,8 @@ void perf_event_detach_bpf_prog(struct perf_event *event)
> > >
> > > old_array = bpf_event_rcu_dereference(event->tp_event->prog_array);
> > > ret = bpf_prog_array_copy(old_array, event->prog, NULL, 0, &new_array);
> > > - if (ret == -ENOENT)
> > > - goto unlock;
> > > + if (WARN_ON_ONCE(ret == -ENOENT))
> > > + goto put;
> > > if (ret < 0) {
> > > bpf_prog_array_delete_safe(old_array, event->prog);
> >
> > seeing
> >
> > if (ret < 0)
> > bpf_prog_array_delete_safe(old_array, event->prog);
> >
> > I think neither ret == -ENOENT nor WARN_ON_ONCE is necessary, tbh. So
> > now I feel like just dropping WARN_ON_ONCE() is better.
>
> hi,
> there's syzbot report [1] where we could end up with following
>
> - create perf event and set bpf program to it
> - clone process -> create inherited event
> - exit -> release both events
> - first perf_event_detach_bpf_prog call will release tp_event->prog_array
> and second perf_event_detach_bpf_prog will crash because
> tp_event->prog_array is NULL
>
> we can fix that quicly with change below, I guess we could add refcount
> to bpf_prog_array_item and allow one of the parent/inherited events to
> work while the other is gone.. but that might be too much, will check
>
> jirka
>
>
> [1] https://lore.kernel.org/bpf/Z1MR6dCIKajNS6nU@krava/T/#m91dbf0688221ec7a7fc95e896a7ef9ff93b0b8ad
> ---
> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> index fe57dfbf2a86..d4b45543ebc2 100644
> --- a/kernel/trace/bpf_trace.c
> +++ b/kernel/trace/bpf_trace.c
> @@ -2251,6 +2251,8 @@ void perf_event_detach_bpf_prog(struct perf_event *event)
> goto unlock;
>
> old_array = bpf_event_rcu_dereference(event->tp_event->prog_array);
> + if (!old_array)
> + goto put;
How does this inherited event stuff work? You can have two separate
events sharing the same prog_array? What if we attach different
programs to each of those events, will both of them be called for
either of two events? That sounds broken, if that's true.
> ret = bpf_prog_array_copy(old_array, event->prog, NULL, 0, &new_array);
> if (ret < 0) {
> bpf_prog_array_delete_safe(old_array, event->prog);
> @@ -2259,6 +2261,7 @@ void perf_event_detach_bpf_prog(struct perf_event *event)
> bpf_prog_array_free_sleepable(old_array);
> }
>
> +put:
> bpf_prog_put(event->prog);
> event->prog = NULL;
>
