xen - security update

Multiple vulnerabilities have been discovered in the Xen hypervisor. The
Common Vulnerabilities and Exposures project identifies the following
problems:

• CVE-2016-7092 (XSA-185)

Jeremie Boutoille of Quarkslab and Shangcong Luan of Alibaba
discovered a flaw in the handling of L3 pagetable entries, allowing
a malicious 32-bit PV guest administrator can escalate their
privilege to that of the host.

• CVE-2016-7094 (XSA-187)

x86 HVM guests running with shadow paging use a subset of the x86
emulator to handle the guest writing to its own pagetables. Andrew
Cooper of Citrix discovered that there are situations a guest can
provoke which result in exceeding the space allocated for internal
state. A malicious HVM guest administrator can cause Xen to fail a
bug check, causing a denial of service to the host.

• CVE-2016-7154 (XSA-188)

Mikhail Gorobets of Advanced Threat Research, Intel Security
discovered a use after free flaw in the FIFO event channel code. A
malicious guest administrator can crash the host, leading to a
denial of service. Arbitrary code execution (and therefore privilege
escalation), and information leaks, cannot be excluded.

For the stable distribution (jessie), these problems have been fixed in
version 4.4.1-9+deb8u7.

We recommend that you upgrade your xen packages.