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

[Alexei Starovoitov <ast@fb.com>]

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.