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FreeBSD-SA-20:31.icmp6 Security Advisory
The FreeBSD Project

Topic: ICMPv6 use-after-free in error message handling

Category: core
Module: icmp6
Announced: 2020-12-01
Credits: Maxime Villard
Affects: All supported versions of FreeBSD.
Corrected: 2020-11-05 22:41:54 UTC (stable/12, 12.2-STABLE)
2020-12-01 19:38:52 UTC (releng/12.2, 12.2-RELEASE-p1)
2020-12-01 19:38:52 UTC (releng/12.1, 12.1-RELEASE-p11)
2020-12-01 03:07:26 UTC (stable/11, 11.4-STABLE)
2020-12-01 19:38:52 UTC (releng/11.4, 11.4-RELEASE-p5)
CVE Name: CVE-2020-7469

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

ICMPv6 is the ICMP protocol for IPv6. It is used to transmit informational
and error messages between IPv6 hosts.

II. Problem Description

When an ICMPv6 error message is received, the FreeBSD ICMPv6 stack may
extract information from the message to hand to upper-layer protocols. As a
part of this operation, it may parse IPv6 header options from a packet
embedded in the ICMPv6 message.

The handler for a routing option caches a pointer into the packet buffer
holding the ICMPv6 message. However, when processing subsequent options the
packet buffer may be freed, rendering the cached pointer invalid. The
network stack may later dereference the pointer, potentially triggering a
use-after-free.

III. Impact

A remote host may be able to trigger a read of freed kernel memory. This may
trigger a kernel panic if the address had been unmapped.

IV. Workaround

Systems with IPv6 disabled are not affected. No workaround is available
except to disable IPv6 on the system’s network interfaces.

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.

[FreeBSD 12.2]
# fetch https://security.FreeBSD.org/patches/SA-20:31/icmp6.12.2.patch
# fetch https://security.FreeBSD.org/patches/SA-20:31/icmp6.12.2.patch.asc
# gpg –verify icmp6.12.2.patch.asc

[FreeBSD 12.1]
# fetch https://security.FreeBSD.org/patches/SA-20:31/icmp6.12.1.patch
# fetch https://security.FreeBSD.org/patches/SA-20:31/icmp6.12.1.patch.asc
# gpg –verify icmp6.12.1.patch.asc

[FreeBSD 11.4]
# fetch https://security.FreeBSD.org/patches/SA-20:31/icmp6.11.4.patch
# fetch https://security.FreeBSD.org/patches/SA-20:31/icmp6.11.4.patch.asc
# gpg –verify icmp6.11.4.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/ r367402
releng/12.2/ r368255
releng/12.1/ r368255
stable/11/ r368202
releng/11.4/ r368255
– ————————————————————————-

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-2020-7469>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-20:31.icmp6.asc>
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