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FreeBSD-SA-23:07.bhyve                                      Security Advisory
                                                          The FreeBSD Project

Topic:          bhyve privileged guest escape via fwctl

Category:       core
Module:         bhyve
Announced:      2023-08-01
Credits:        Omri Ben Bassat and Vladimir Eli Tokarev from Microsoft
Affects:        FreeBSD 13.1 and 13.2
Corrected:      2023-08-01 19:48:53 UTC (stable/13, 13.2-STABLE)
                2023-08-01 19:50:47 UTC (releng/13.2, 13.2-RELEASE-p2)
                2023-08-01 19:48:26 UTC (releng/13.1, 13.1-RELEASE-p9)
CVE Name:       CVE-2023-3494

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)'s fwctl interface provides a mechanism through which guest
firmware can query the hypervisor for information about the virtual
machine.  The fwctl interface is available to guests when bhyve is run
with the "-l bootrom" option, used for example when booting guests in
UEFI mode.

bhyve is currently only supported on the amd64 platform.

II.  Problem Description

The fwctl driver implements a state machine which is executed when the
guest accesses certain x86 I/O ports.  The interface lets the guest copy
a string into a buffer resident in the bhyve process' memory.  A bug in
the state machine implementation can result in a buffer overflowing when
copying this string.

III. Impact

A malicious, privileged software running in a guest VM can exploit the
buffer overflow to achieve code execution on the host in the bhyve
userspace process, which typically runs as root.  Note that bhyve runs
in a Capsicum sandbox, so malicious code is constrained by the
capabilities available to the bhyve process.

IV.  Workaround

No workaround is available.  bhyve guests that are executed without the
"-l bootrom" option are unaffected.

V.   Solution

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

Perform one of the following:

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

Systems running a RELEASE version of FreeBSD on the amd64, i386, or
(on FreeBSD 13 and later) arm64 platforms can be updated via the
freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

Restart all affected virtual machines.

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.

[FreeBSD 13.2]
# fetch https://security.FreeBSD.org/patches/SA-23:07/bhyve.13.2.patch
# fetch https://security.FreeBSD.org/patches/SA-23:07/bhyve.13.2.patch.asc
# gpg --verify bhyve.13.2.patch.asc

[FreeBSD 13.1]
# fetch https://security.FreeBSD.org/patches/SA-23:07/bhyve.13.1.patch
# fetch https://security.FreeBSD.org/patches/SA-23:07/bhyve.13.1.patch.asc
# gpg --verify bhyve.13.1.patch.asc

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

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

c) Recompile the operating system using buildworld and installworld as
described in <URL:https://www.FreeBSD.org/handbook/makeworld.html>.

Restart all affected virtual machines.

VI.  Correction details

This issue is corrected by the corresponding Git commit hash or Subversion
revision number in the following stable and release branches:

Branch/path                             Hash                     Revision
- -------------------------------------------------------------------------
stable/13/                              9fe302d78109    stable/13-n255918
releng/13.2/                            2bae613e0da3  releng/13.2-n254625
releng/13.1/                            87702e38a4b4  releng/13.1-n250190
- -------------------------------------------------------------------------

Run the following command to see which files were modified by a
particular commit:

# git show --stat <commit hash>

Or visit the following URL, replacing NNNNNN with the hash:

<URL:https://cgit.freebsd.org/src/commit/?id=NNNNNN>

To determine the commit count in a working tree (for comparison against
nNNNNNN in the table above), run:

# git rev-list --count --first-parent HEAD

VII. References

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2023-3494>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-23:07.bhyve.asc>
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