From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Miller <davem@davemloft.net>
Subject: bpf_test_finish()
Date: Mon, 01 May 2017 23:56:10 -0400 (EDT)
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It dereferences a user pointer:

static int bpf_test_finish(union bpf_attr __user *uattr, const void *data,
			   u32 size, u32 retval, u32 duration)
{
	void __user *data_out = u64_to_user_ptr(uattr->test.data_out);
                                                ^^^^^^^^^^^^^^^^^^^^
Which of course doesn't work so well :-)

I really wish that didn't silently work on x86/x86_64.

You're going to have to do a "get_user(&uattr->test.data_out)"