Because the CMB BAR has a min_access_size of 2, if you read the last
byte it will try to memcpy *2* bytes from n->cmbuf, causing an off-by-one
error. This is CVE-2018-16847.
Another way to fix this might be to register the CMB as a RAM memory
region, which would also be more efficient. However, that might be a
change for big-endian machines; I didn't think this through and I don't
know how real hardware works. Add a basic testcase for the CMB in case
somebody does this change later on.