qemu and xen-tools -- denial of service vulnerabilities in AMD PC-Net II NIC support

Prasad J Pandit, Red Hat Product Security Team, reports:

Qemu emulator built with the AMD PC-Net II Ethernet Controller
support is vulnerable to a heap buffer overflow flaw. While
receiving packets in the loopback mode, it appends CRC code to the
receive buffer. If the data size given is same as the receive buffer
size, the appended CRC code overwrites 4 bytes beyond this
‘s->buffer’ array.
A privileged(CAP_SYS_RAWIO) user inside guest could use this flaw
to crash the Qemu instance resulting in DoS or potentially execute
arbitrary code with privileges of the Qemu process on the host.

The AMD PC-Net II emulator(hw/net/pcnet.c), while receiving packets
from a remote host(non-loopback mode), fails to validate the
received data size, thus resulting in a buffer overflow issue. It
could potentially lead to arbitrary code execution on the host, with
privileges of the Qemu process. It requires the guest NIC to have
larger MTU limit.
A remote user could use this flaw to crash the guest instance
resulting in DoS or potentially execute arbitrary code on a remote
host with privileges of the Qemu process.