bpf,perf: Fix invalid prog_array access in perf_event_detach_bpf_prog

Syzbot reported [1] crash that happens for following tracing scenario:

  - create tracepoint perf event with attr.inherit=1, attach it to the
    process and set bpf program to it
  - attached process forks -> chid creates inherited event

    the new child event shares the parent's bpf program and tp_event
    (hence prog_array) which is global for tracepoint

  - exit both process and its child -> 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

The fix makes sure the perf_event_detach_bpf_prog checks prog_array
is valid before it tries to remove the bpf program from it.

[1] https://lore.kernel.org/bpf/Z1MR6dCIKajNS6nU@krava/T/#m91dbf0688221ec7a7fc95e896a7ef9ff93b0b8ad

Reported-by: [email protected]
Fixes: 0ee288e ("bpf,perf: Fix perf_event_detach_bpf_prog error handling")
Signed-off-by: Jiri Olsa <[email protected]>

kernel/trace/bpf_trace.c

@@ -2250,6 +2250,8 @@ 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;
 	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);
@@ -2258,6 +2260,7 @@ 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;