[kernel-xen] Xen Security Advisory 90 - Linux netback crash trying to disable due to malformed packet

[Steven Haigh]

                    Xen Security Advisory XSA-90

      Linux netback crash trying to disable due to malformed packet

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

When Linux's netback sees a malformed packet, it tries to disable the
interface which serves the misbehaving frontend.

This involves taking a mutex, which might sleep.  But in recent
versions of Linux the guest transmit path is handled by NAPI in
softirq context, where sleeping is not allowed.  The end result is
that the backend domain (often, Dom0) crashes with "scheduling while
atomic".

IMPACT
======

Malicious guest administrators can cause denial of service.  If driver
domains are not in use, the impact is a host crash.

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

This bug affects systems using Linux as the driver domain, including
non-disaggregated systems using Linux as dom0.

Only versions of Linux whose netback uses NAPI are affected.  In Linux
mainline this is all versions of Linux containing git changeset
b3f980bd82, which was introduced between Linux 3.11 and 3.12-rc1.

Systems using a different OS as dom0 (eg, NetBSD, Solaris) are not
vulnerable.

Both x86 and ARM systems are affected.

MITIGATION
==========

Using driver domains may limit the scope of the denial of service, and
may make it possible to resume service without restarting guests (by
restarting the driver domain).  Advice on reconfiguring a system to
use driver domains is beyond the reasonable scope of this advisory.

In the case of an x86 HVM guest, the exploit can be prevented by
disabling the PV IO paths; normally this would come with a substantial
performance cost, and it may involve reconfiguring the guest as well
as the host.  This is not recommended.

NOTE REGARDING LACK OF EMBARGO
==============================

This bug was publicly reported on xen-devel, before it was appreciated
that there was a security problem.  The public mailing list thread
nevertheless contains information strongly suggestive of a security
bug, and a different security bug (with CVE) is suggested as seeming
"similar".

For these reasons we (the Xen Project Security Team) have concluded
that the presence of this bug, as a security problem, is not (any
longer) a secret.

CREDITS
=======

This issue was discovered as a bug by Török Edwin and analysed by
Wei Liu of Citrix.

RESOLUTION
==========

Fixed in kernel-xen-3.13.7-1 and higher.

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