Xen SYSCALL singlestep Handling Privilege Escalation (XSA-204)

This script is Copyright (C) 2017 Tenable Network Security, Inc.


Synopsis :

The remote Xen hypervisor installation is missing a security update.

Description :

According to its self-reported version number, the Xen hypervisor
installed on the remote host is missing a security update. It is,
therefore, affected by a privilege elevation vulnerability in the
instruction emulator when handling SYSCALL by single-stepping
applications. This is due to incorrect exceptions being raised based
on the flags at the start of an instruction. A local attacker can
exploit this issue to gain elevated privileges on the guest operating
system.

Please note the following items :

- The vulnerability is only exposed to 64-bit x86 HVM
guests.

- On Xen version 4.6 and earlier, the vulnerability is
exposed to all guest user processes, including
unprivileged processes in such guests.

- On Xen version 4.7 and later, the vulnerability is
exposed only to guest user processes granted a degree of
privilege (e.g., direct hardware access) by the guest
administrator, or else to all user processes when the
VM has been explicitly configured with a non-default CPU
vendor string (in xm/xl, this would be done with a
'cpuid=' domain config option).

- A 64-bit guest kernel which uses an IST for #DB handling
will most likely mitigate the issue but will have a
single unexpected #DB exception frame to deal with. In
practice, this means that Linux is not vulnerable.

- The vulnerability is not exposed to 32-bit HVM guests.
This is because the emulation bug also matches real
hardware behavior, and a 32-bit guest kernel using
SYSCALL will already have to be using a Task Gate for
handling #DB to avoid being susceptible to an escalation
of privilege.

- The vulnerability is not exposed to PV guests.

- ARM systems are not vulnerable.

Note that Nessus has not tested for this vulnerability but has instead
relied only on the changeset versions based on the xen.git change log.
Nessus did not check guest hardware configurations or if patches were
applied manually to the source code before a recompile and reinstall.

See also :

https://xenbits.xen.org/xsa/advisory-204.html
https://xenbits.xen.org/gitweb/?p=xen.git;a=summary

Solution :

Apply the appropriate patch according to the vendor advisory.

Risk factor :

High / CVSS Base Score : 7.2
(CVSS2#AV:L/AC:L/Au:N/C:C/I:C/A:C)
CVSS Temporal Score : 5.3
(CVSS2#E:U/RL:OF/RC:C)
Public Exploit Available : false

Family: Misc.

Nessus Plugin ID: 96960 ()

Bugtraq ID: 94963

CVE ID: CVE-2016-10013

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