Xen Security Advisory 329 v3 (CVE-2020-15852) - Linux ioperm bitmap context switching issues

[Xen.org security team <security@xxxxxxx>]

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            Xen Security Advisory CVE-2020-15852 / XSA-329
                              version 3

             Linux ioperm bitmap context switching issues

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

CVE assigned.

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

Linux 5.5 overhauled the internal state handling for the iopl() and ioperm()
system calls.  Unfortunately, one aspect on context switch wasn't wired up
correctly for the Xen PVOps case.

IMPACT
======

IO port permissions don't get rescinded when context switching to an
unprivileged task.  Therefore, all userspace can use the IO ports granted to
the most recently scheduled task with IO port permissions.

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

Only x86 guests are vulnerable.

All versions of Linux from 5.5 are potentially vulnerable.

Linux is only vulnerable when running as x86 PV guest.  Linux is not
vulnerable when running as an x86 HVM/PVH guests.

The vulnerability can only be exploited in domains which have been granted
access to IO ports by Xen.  This is typically only the hardware domain, and
guests configured with PCI Passthrough.

MITIGATION
==========

Running only HVM/PVH guests avoids the vulnerability.

CREDITS
=======

This issue was discovered by Andy Lutomirski.

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

xsa329.patch           Linux 5.5 and later

$ sha256sum xsa329*
cdb5ac9bfd21192b5965e8ec0a1c4fcf12d0a94a962a8158cd27810e6aa362f0  xsa329.patch
$

DEPLOYMENT DURING EMBARGO
=========================

Deployment of the patches and/or mitigations described above (or
others which are substantially similar) is permitted during the
embargo, even on public-facing systems with untrusted guest users and
administrators.

But: Distribution of updated software is prohibited (except to other
members of the predisclosure list).

Predisclosure list members who wish to deploy significantly different
patches and/or mitigations, please contact the Xen Project Security
Team.


(Note: this during-embargo deployment notice is retained in
post-embargo publicly released Xen Project advisories, even though it
is then no longer applicable.  This is to enable the community to have
oversight of the Xen Project Security Team's decisionmaking.)

For more information about permissible uses of embargoed information,
consult the Xen Project community's agreed Security Policy:
  http://www.xenproject.org/security-policy.html
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Attachment: xsa329.patch
Description: Binary data