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FreeBSD-SA-19:23.midi Security Advisory
The FreeBSD Project

Topic: kernel memory disclosure from /dev/midistat

Category: core
Module: sound
Announced: 2019-08-20
Credits: Peter Holm, Mark Johnston
Affects: All supported versions of FreeBSD.
Corrected: 2019-08-20 17:53:16 UTC (stable/12, 12.0-STABLE)
2019-08-20 17:50:33 UTC (releng/12.0, 12.0-RELEASE-p10)
2019-08-20 17:54:18 UTC (stable/11, 11.3-STABLE)
2019-08-20 17:50:33 UTC (releng/11.3, 11.3-RELEASE-p3)
2019-08-20 17:50:33 UTC (releng/11.2, 11.2-RELEASE-p14)
CVE Name: CVE-2019-5612

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/>.

0. Revision history

v1.0 2019-08-20 Initial release.
v1.1 2019-08-21 Updated workaround.

I. Background

/dev/midistat is a device file which can be read to obtain a
human-readable list of the available MIDI-capable devices in the system.

II. Problem Description

The kernel driver for /dev/midistat implements a handler for read(2).
This handler is not thread-safe, and a multi-threaded program can
exploit races in the handler to cause it to copy out kernel memory
outside the boundaries of midistat’s data buffer.

III. Impact

The races allow a program to read kernel memory within a 4GB window
centered at midistat’s data buffer. The buffer is allocated each
time the device is opened, so an attacker is not limited to a static
4GB region of memory.

On 32-bit platforms, an attempt to trigger the race may cause a page
fault in kernel mode, leading to a panic.

IV. Workaround

Restrict permissions on /dev/midistat by adding an entry to
/etc/devfs.conf and restarting the service:

# echo “perm midistat 0600” >> /etc/devfs.conf
# service devfs restart

Custom kernels without “device sound” 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.

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 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-19:23/midi.patch
# fetch https://security.FreeBSD.org/patches/SA-19:23/midi.patch.asc
# gpg –verify midi.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/ r351264
releng/12.0/ r351260
stable/11/ r351265
releng/11.3/ r351260
releng/11.2/ r351260
– ————————————————————————-

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

<other info on vulnerability>

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-5612>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-19:23.midi.asc>
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