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.

Cc: Keith Busch <keith.busch@intel.com>
Cc: qemu-block@nongnu.org
Reported-by: NLi Qiang <liq3ea@gmail.com>
Reviewed-by: NLi Qiang <liq3ea@gmail.com>
Tested-by: NLi Qiang <liq3ea@gmail.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: NPhilippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>