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Xen Security Advisory XSA-106
version 2

Missing privilege level checks in x86 emulation of software interrupts

UPDATES IN VERSION 2
====================

Public Release.

ISSUE DESCRIPTION
=================

The emulation of instructions which generate software interrupts fails
to perform supervisor mode permission checks.

However these instructions are not usually handled by the emulator.
Exceptions to this are
- - when a memory operand (implicit for the affected instructions) lives
in (emulated or passed through) memory mapped IO space,
- - in the case of guests running in 32-bit PAE mode, when such an
instruction is (in execution flow) within four instructions of one
doing a page table update,
- - when an Invalid Opcode exception gets raised by a guest instruction,
and the guest then (likely maliciously) alters the instruction to
become one of the affected ones,
- - when the guest is in real mode (in which case there are no privilege
checks anyway).

IMPACT
======

Malicious HVM guest user mode code may be able to crash the guest.

VULNERABLE SYSTEMS
==================

Xen versions from 3.3 onwards are vulnerable.

Only user processes in HVM guests can take advantage of this
vulnerability.

MITIGATION
==========

Running only PV guests will avoid this issue.

There is no mitigation available for HVM guests.

CREDITS
=======

This issue was discovered Andrei Lutas at BitDefender and analyzed by
Andrew Cooper at Citrix.

RESOLUTION
==========

Applying the attached patch resolves this issue.

xsa106.patch xen-unstable, Xen 4.4.x, Xen 4.3.x, Xen 4.2.x

$ sha256sum xsa106*.patch
301060f801ab39c15ac773e1bcc250f0e6bf30d748007a96173459b83afc9270 xsa106.patch
$
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