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            Xen Security Advisory CVE-2023-46836 / XSA-446
                               version 2

                x86: BTC/SRSO fixes not fully effective

UPDATES IN VERSION 2
====================

Grammar fixes.

Public release.

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

The fixes for XSA-422 (Branch Type Confusion) and XSA-434 (Speculative
Return Stack Overflow) are not IRQ-safe.  It was believed that the
mitigations always operated in contexts with IRQs disabled.

However, the original XSA-254 fix for Meltdown (XPTI) deliberately left
interrupts enabled on two entry paths; one unconditionally, and one
conditionally on whether XPTI was active.

As BTC/SRSO and Meltdown affect different CPU vendors, the mitigations
are not active together by default.  Therefore, there is a race
condition whereby a malicious PV guest can bypass BTC/SRSO protections
and launch a BTC/SRSO attack against Xen.

IMPACT
======

An attacker in a PV guest might be able to infer the contents of memory
belonging to other guests.

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

All versions of Xen are vulnerable.

Xen is only vulnerable in default configurations on AMD and Hygon CPUs.

Xen is not believed to be vulnerable in default configurations on CPUs
from other hardware vendors.

Only PV guests can leverage the vulnerability.

MITIGATION
==========

Running only HVM or PVH VMs will avoid the vulnerability.

CREDITS
=======

This issue was discovered by Andrew Cooper of XenServer.

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.

xsa446.patch           xen-unstable - Xen 4.15.x

$ sha256sum xsa446*
ed27ad5f36af31233e25c80daefb8b0078eeb18cacbc1923fdd6f10f0b394201  xsa446.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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