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 Xen Security Advisory CVE-2022-23036,CVE-2022-23037,CVE-2022-23038,CVE-2022-23039,CVE-2022-23040,CVE-2022-23041,CVE-2022-23042 / XSA-396
                                                                 version 3

      Linux PV device frontends vulnerable to attacks by backends

UPDATES IN VERSION 3
====================

Public release.

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

Several Linux PV device frontends are using the grant table interfaces
for removing access rights of the backends in ways being subject to
race conditions, resulting in potential data leaks, data corruption
by malicious backends, and denial of service triggered by malicious
backends:

blkfront, netfront, scsifront and the gntalloc driver are testing
whether a grant reference is still in use. If this is not the case,
they assume that a following removal of the granted access will always
succeed, which is not true in case the backend has mapped the granted
page between those two operations. As a result the backend can keep
access to the memory page of the guest no matter how the page will be
used after the frontend I/O has finished. The xenbus driver has a
similar problem, as it doesn't check the success of removing the
granted access of a shared ring buffer.
blkfront: CVE-2022-23036
netfront: CVE-2022-23037
scsifront: CVE-2022-23038
gntalloc: CVE-2022-23039
xenbus: CVE-2022-23040

blkfront, netfront, scsifront, usbfront, dmabuf, xenbus, 9p, kbdfront,
and pvcalls are using a functionality to delay freeing a grant reference
until it is no longer in use, but the freeing of the related data page
is not synchronized with dropping the granted access. As a result the
backend can keep access to the memory page even after it has been freed
and then re-used for a different purpose.
CVE-2022-23041


netfront will fail a BUG_ON() assertion if it fails to revoke access in
the rx path. This will result in a Denial of Service (DoS) situation of
the guest which can be triggered by the backend.
CVE-2022-23042

IMPACT
======

Due to race conditions and missing tests of return codes in the Linux
PV device frontend drivers a malicious backend could gain access (read
and write) to memory pages it shouldn't have, or it could directly
trigger Denial of Service (DoS) in the guest.

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

All Linux guests using PV devices are vulnerable in case potentially
malicious PV device backends are being used.

MITIGATION
==========

There is no mitigation available other than not using PV devices in case
a backend is suspected to be potentially malicious.

RESOLUTION
==========

Applying the attached patches resolves this issue.


References:

https://xenbits.xen.org/xsa/advisory-396.html