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FreeBSD-SA-19:11.cd_ioctl Security Advisory
The FreeBSD Project

Topic: Privilege escalation in cd(4) driver

Category: core
Module: kernel
Announced: 2019-07-02
Credits: Alex Fortune
Affects: All supported versions of FreeBSD.
Corrected: 2019-07-03 00:11:31 UTC (stable/12, 12.0-STABLE)
2019-07-02 00:03:55 UTC (releng/12.0, 12.0-RELEASE-p7)
2019-07-03 00:12:50 UTC (stable/11, 11.3-PRERELEASE)
2019-07-02 00:03:55 UTC (releng/11.3, 11.3-RC3-p1)
2019-07-02 00:03:55 UTC (releng/11.2, 11.2-RELEASE-p11)
CVE Name: CVE-2019-5602

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

The cd(4) driver implements a number of ioctls to permit low-level access to
the media in the CD-ROM device. The Linux emulation layer provides a
corresponding set of ioctls, some of which are implemented as wrappers of
native cd(4) ioctls.

These ioctls are available to users in the operator group, which gets
read-only access to cd(4) devices by default.

II. Problem Description

To implement one particular ioctl, the Linux emulation code used a special
interface present in the cd(4) driver which allows it to copy subchannel
information directly to a kernel address. This interface was erroneously
made accessible to userland, allowing users with read access to a cd(4)
device to arbitrarily overwrite kernel memory when some media is present in
the device.

III. Impact

A user in the operator group can make use of this interface to gain root
privileges on a system with a cd(4) device when some media is present in the
device.

IV. Workaround

devfs.conf(5) and devfs.rules(5) can be used to remove read permissions from
cd(4) devices.

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 i386 or amd64
platforms can be updated via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

Afterwards, reboot the system.

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 12.x]
# fetch https://security.FreeBSD.org/patches/SA-19:11/cd_ioctl.12.patch
# fetch https://security.FreeBSD.org/patches/SA-19:11/cd_ioctl.12.patch.asc
# gpg –verify cd_ioctl.12.patch.asc

[FreeBSD 11.x]
# fetch https://security.FreeBSD.org/patches/SA-19:11/cd_ioctl.11.patch
# fetch https://security.FreeBSD.org/patches/SA-19:11/cd_ioctl.11.patch.asc
# gpg –verify cd_ioctl.11.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/ r349628
releng/12.0/ r349625
stable/11/ r349629
releng/11.3/ r349625
releng/11.2/ r349625
– ————————————————————————-

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-2019-5602>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-19:11.cd_ioctl.asc>
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