FreeBSD : xen-kernel -- x86 null segments not always treated as unusable (50ac2e96-ba4d-11e6-ae1b-002590263bf5)

This script is Copyright (C) 2016-2017 Tenable Network Security, Inc.

Synopsis :

The remote FreeBSD host is missing a security-related update.

Description :

The Xen Project reports :

The Xen x86 emulator erroneously failed to consider the unusability of
segments when performing memory accesses.

The intended behaviour is as follows: The user data segment (%ds, %es,
%fs and %gs) selectors may be NULL in 32-bit to prevent access. In
64-bit, NULL has a special meaning for user segments, and there is no
way of preventing access. However, in both 32-bit and 64-bit, a NULL
LDT system segment is intended to prevent access.

On Intel hardware, loading a NULL selector zeros the base as well as
most attributes, but sets the limit field to its largest possible
value. On AMD hardware, loading a NULL selector zeros the attributes,
leaving the stale base and limit intact.

Xen may erroneously permit the access using unexpected base/limit

Ability to exploit this vulnerability on Intel is easy, but on AMD
depends in a complicated way on how the guest kernel manages LDTs.

An unprivileged guest user program may be able to elevate its
privilege to that of the guest operating system.

See also :

Solution :

Update the affected package.

Risk factor :

Medium / CVSS Base Score : 4.6

Family: FreeBSD Local Security Checks

Nessus Plugin ID: 95505 ()

Bugtraq ID:

CVE ID: CVE-2016-9386

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