The C+ mode offload emulation in the RTL8139 network card device model in QEMU, as used in Xen 4.5.x and earlier, allows remote attackers to read process heap memory via unspecified vectors.

Due converting PIO to the new memory read/write api we no longer provide separate I/O region lenghts for read and write operations. As a result, reading from PIT Mode/Command register will end with accessing pit->channels with invalid index and potentially cause memory corruption and/or minor information leak.

A privileged guest user in a guest with QEMU PIT emulation enabled could potentially (tough unlikely) use this flaw to execute arbitrary code on the host with the privileges of the hosting QEMU process. (QEMU part of the vulnerability)

A privileged guest user in a guest could potentially (tough unlikely) use this flaw to execute arbitrary code on the host. (KVM part of the vulnerability)

Acknowledgements:

Red Hat would like to thank Matt Tait of Google's Project Zero security team for reporting this issue.