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FreeBSD-SA-20:29.bhyve_svm Security Advisory
The FreeBSD Project

Topic: bhyve SVM guest escape

Category: core
Module: bhyve
Announced: 2020-09-15
Credits: Maxime Villard
Affects: All supported versions of FreeBSD.
Corrected: 2020-09-15 20:25:30 UTC (stable/12, 12.2-STABLE)
2020-09-15 21:46:39 UTC (releng/12.2, 12.2-BETA1-p1)
2020-09-15 21:46:39 UTC (releng/12.1, 12.1-RELEASE-p10)
2020-09-15 20:26:31 UTC (stable/11, 11.4-STABLE)
2020-09-15 21:46:39 UTC (releng/11.4, 11.4-RELEASE-p4)
2020-09-15 21:46:39 UTC (releng/11.3, 11.3-RELEASE-p14)
CVE Name: CVE-2020-7467

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit <URL:https://security.FreeBSD.org/>.

I. Background

bhyve(8) is a hypervisor that supports running a variety of guest operating
systems in virtual machines on AMD and Intel CPUs. AMD and Intel provide
broadly similar virtualization interfaces, but each provides its own specific
instructions for manipulating virtual machine state.

II. Problem Description

A number of AMD virtualization instructions operate on host physical
addresses, are not subject to nested page table translation, and guest use of
these instructions was not trapped.

III. Impact

– From kernel mode a malicious guest can write to arbitrary host memory (with
some constraints), affording the guest full control of the host.

IV. Workaround

No workaround is available. Systems not using bhyve, and systems that
use bhyve with an Intel CPU, are not vulnerable.

V. Solution

Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date,
and reboot.

Perform one of the following:

1) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD on the i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min “Rebooting for a security update”

2) To update your vulnerable system via a source code patch:

The following patches have been verified to apply to the applicable
FreeBSD release branches.

a) Download the relevant patch from the location below, and verify the
detached PGP signature using your PGP utility.

# fetch https://security.FreeBSD.org/patches/SA-20:29/bhyve_svm.patch
# fetch https://security.FreeBSD.org/patches/SA-20:29/bhyve_svm.patch.asc
# gpg –verify bhyve_svm.patch.asc

b) Apply the patch. Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile your kernel as described in
<URL:https://www.FreeBSD.org/handbook/kernelconfig.html> and reboot the
system.

VI. Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path Revision
– ————————————————————————-
stable/12/ r365767
releng/12.2/ r365780
releng/12.1/ r365780
stable/11/ r365769
releng/11.4/ r365780
releng/11.3/ r365780
– ————————————————————————-

To see which files were modified by a particular revision, run the
following command, replacing NNNNNN with the revision number, on a
machine with Subversion installed:

# svn diff -cNNNNNN –summarize svn://svn.freebsd.org/base

Or visit the following URL, replacing NNNNNN with the revision number:

<URL:https://svnweb.freebsd.org/base?view=revision&revision=NNNNNN>

VII. References

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2020-7467>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-20:29.bhyve_svm.asc>
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