commit 645881139b3d2c86acf9d644c3a1471520bc9e57
Author: Peter Krempa <pkrempa>
Date:   Tue Dec 1 14:08:37 2015 +0100

    qemu: domain: Prevent overflows in memory alignment code
    
    Since libvirt for dubious historical reasons stores memory size as
    kibibytes, it's possible that the alignments done in the qemu code
    overflow the the maximum representable size in bytes. The XML parser
    code handles them in bytes in some stages. Prevent this by doing
    overflow checks when alinging the size and add a test case.

Verify this bug with libvirt-1.3.4-1.el7.x86_64:

1. prepare a inactive guest:

# virsh list --all
 Id    Name                           State
----------------------------------------------------
 -     rhel72-test                    shut off

2. change guest maxmemory to 9007199254740991KiB:

# virsh dumpxml rhel72-test |grep max
  <maxMemory slots='16' unit='KiB'>9007199254740991</maxMemory>

3. start guest:

# virsh start rhel72-test
error: Failed to start domain rhel72-test
error: unsupported configuration: maximum memory size overflowed after alignment

4. change guest memory to 9007199254740991KiB:

# virsh dumpxml rhel72 |grep emory
  <memory unit='KiB'>9007199254740991</memory>
  <currentMemory unit='KiB'>9007199254740991</currentMemory>

5. start guest

# virsh start rhel72
error: Failed to start domain rhel72
error: unsupported configuration: initial memory size overflowed after alignment

6. test guest used a maxmemory which won't cause overflowed problem:

# virsh dumpxml rhel72-test |grep max
  <maxMemory slots='16' unit='KiB'>9007199254739968</maxMemory>

# virsh start rhel72-test
Domain rhel72-test started

# service libvirtd restart
Redirecting to /bin/systemctl restart  libvirtd.service

# virsh list --all
 Id    Name                           State
----------------------------------------------------
 51    rhel72-test                    running

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory, and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://rhn.redhat.com/errata/RHSA-2016-2577.html