Re: [PATCH bpf] bpf/tracing: fix a deadlock in perf_event_detach_bpf_prog

[Alexei Starovoitov]

On 4/9/18 9:18 AM, Yonghong Song wrote:

syzbot reported a possible deadlock in perf_event_detach_bpf_prog.

...

@@ -985,16 +986,31 @@ int perf_event_query_prog_array(struct perf_event *event, 
void __user *info)
                return -EINVAL;
        if (copy_from_user(&query, uquery, sizeof(query)))
                return -EFAULT;
-       if (query.ids_len > BPF_TRACE_MAX_PROGS)
+
+       ids_len = query.ids_len;
+       if (ids_len > BPF_TRACE_MAX_PROGS)
                return -E2BIG;
+       ids = kcalloc(ids_len, sizeof(u32), GFP_USER | __GFP_NOWARN);
+       if (!ids)
+               return -ENOMEM;

        mutex_lock(&bpf_event_mutex);
        ret = bpf_prog_array_copy_info(event->tp_event->prog_array,
-                                      uquery->ids,
-                                      query.ids_len,
-                                      &uquery->prog_cnt);
+                                      ids,
+                                      ids_len,
+                                      &prog_cnt);
        mutex_unlock(&bpf_event_mutex);

+       if (!ret || ret == -ENOSPC) {
+               if (copy_to_user(&uquery->prog_cnt, &prog_cnt, 
sizeof(prog_cnt)) ||
+                   copy_to_user(uquery->ids, ids, ids_len * sizeof(u32))) {
+                       ret = -EFAULT;
+                       goto out;
+               }
+       }
+
+out:
+       kfree(ids);

alloc/free just to avoid this locking dependency feels suboptimal.

may be we can get rid of bpf_event_mutex in some cases?
the perf event itself is locked via perf_event_ctx_lock() when we're
calling perf_event_query_prog_array, perf_event_attach|detach_bpf_prog.
I forgot what was the motivation for us to introduce it in the
first place.