FreeBSD VuXML: Documenting security issues in FreeBSD and the FreeBSD Ports Collection

xen-kernel -- x86 null segments not always treated as unusable

Affected packages
xen-kernel < 4.7.1


VuXML ID 50ac2e96-ba4d-11e6-ae1b-002590263bf5
Discovery 2016-11-22
Entry 2016-12-04

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 values.

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.


CVE Name CVE-2016-9386
FreeBSD PR ports/214936