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FreeBSD-SA-20:17.usb Security Advisory
The FreeBSD Project

Topic: USB HID descriptor parsing error

Category: core
Module: kernel
Announced: 2020-06-09
Credits: Andy Nguyen, Google
Affects: All supported versions of FreeBSD.
Corrected: 2020-06-08 09:32:57 UTC (stable/12, 12.1-STABLE)
2020-06-09 16:13:54 UTC (releng/12.1, 12.1-RELEASE-p6)
2020-06-08 09:33:37 UTC (stable/11, 11.4-STABLE)
2020-06-09 16:13:54 UTC (releng/11.4, 11.4-RC2-p1)
2020-06-09 16:13:54 UTC (releng/11.3, 11.3-RELEASE-p10)
CVE Name: CVE-2020-7456

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

USB Human Interface Device (HID) descriptors may push/pop the current state
to allow description of items residing in a so-called union. FreeBSD
supports 4 such pop levels.

II. Problem Description

If the push/pop level is not restored within the processing of the same HID
item, an invalid memory location may be used for subsequent HID item
processing.

III. Impact

An attacker with physical access to a USB port may be able to use a specially
crafted USB device to gain kernel or user-space code execution.

IV. Workaround

Setting “sysctl hw.usb.disable_enumeration=1” disables USB device
enumeration preventing the error from occurring.

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-20:17/usb.patch
# fetch https://security.FreeBSD.org/patches/SA-20:17/usb.patch.asc
# gpg –verify usb.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/ r361918
releng/12.1/ r361972
stable/11/ r361919
releng/11.4/ r361972
releng/11.3/ r361972
– ————————————————————————-

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-2020-7456>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-20:17.usb.asc>
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