Xen Security Advisory 386 v1 (CVE-2021-28702) - PCI devices with RMRRs not deassigned correctly

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

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            Xen Security Advisory CVE-2021-28702 / XSA-386

            PCI devices with RMRRs not deassigned correctly

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

Certain PCI devices in a system might be assigned Reserved Memory
Regions (specified via Reserved Memory Region Reporting, "RMRR").
These are typically used for platform tasks such as legacy USB
emulation.

If such a device is passed through to a guest, then on guest shutdown
the device is not properly deassigned.  The IOMMU configuration for
these devices which are not properly deassigned ends up pointing to a
freed data structure, including the IO Pagetables.

Subsequent DMA or interrupts from the device will have unpredictable
behaviour, ranging from IOMMU faults to memory corruption.

IMPACT
======

Administrators of guests which have been assigned RMRR-using PCI
devices can cause denial of service and other problems, possibly
including escalation of privilege.

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

All versions of Xen from at least 4.4 onwards are vulnerable.

Only Intel x86 systems are affected.  AMD x86 systems, and Arm
systems, are all unaffected.

Only systems using PCI passthrough are affected.  (And then, only if
the assigned devices have RMRRs, but whether a device advertises RMRRs
is not easy to discern.)

MITIGATION
==========

There is no mitigation (other than not passing through PCI devices
with RMRRs to guests).

RESOLUTION
==========

Applying the appropriate attached patch resolves this issue.

Note that patches for released versions are generally prepared to
apply to the stable branches, and may not apply cleanly to the most
recent release tarball.  Downstreams are encouraged to update to the
tip of the stable branch before applying these patches.

xsa386.patch           xen-unstable - Xen 4.12.x

$ sha256sum xsa386*
f2f83c825e249bba9454437b48bbd8307fe7a224f56484388a67af124dfd279b  xsa386.patch
$

NOTE CONCERNING LACK OF EMBARGO
===============================

This issue was reported and debugged in public before the security nature
became apparent.
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Attachment: xsa386.patch
Description: Binary data