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FreeBSD-SA-18:08.tcp Security Advisory
The FreeBSD Project
Topic: Resource exhaustion in TCP reassembly
Category: core
Module: inet
Announced: 2018-08-06
Credits: Juha-Matti Tilli <juha-matti.tilli@iki.fi> from
Aalto University, Department of Communications and Networking
and Nokia Bell Labs
Affects: All supported versions of FreeBSD.
Corrected: 2018-08-06 18:46:09 UTC (stable/11, 11.1-STABLE)
2018-08-06 17:47:47 UTC (releng/11.2, 11.2-RELEASE-p1)
2018-08-06 17:48:46 UTC (releng/11.1, 11.1-RELEASE-p12)
2018-08-06 18:47:03 UTC (stable/10, 10.4-STABLE)
2018-08-06 17:50:40 UTC (releng/10.4, 10.4-RELEASE-p10)
CVE Name: CVE-2018-6922
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 Transmission Control Protocol (TCP) of the TCP/IP protocol suite
provides a connection-oriented, reliable, sequence-preserving data
stream service.
To transmit a stream of data, TCP breaks the data stream into segments
for transmission through the Internet, and reassembles the segments at
the receiving side to recreate the data stream.
II. Problem Description
One of the data structures that holds TCP segments uses an inefficient
algorithm to reassemble the data. This causes the CPU time spent on
segment processing to grow linearly with the number of segments in the
reassembly queue.
III. Impact
An attacker who has the ability to send TCP traffic to a victim system
can degrade the victim system’s network performance and/or consume
excessive CPU by exploiting the inefficiency of TCP reassembly
handling, with relatively small bandwidth cost.
IV. Workaround
As a workaround, system administrators should configure their systems
to only accept TCP connections from trusted end-stations, if it is
possible to do so.
For systems which must accept TCP connections from untrusted
end-stations, the workaround is to limit the size of each reassembly
queue. The capability to do that is added by the patches noted in the
“Solution” section below.
V. Solution
As a temporary solution to this problem, these patches limit the size
of each TCP connection’s reassembly queue. The value is controlled by
a sysctl (net.inet.tcp.reass.maxqueuelen), which sets the maximum
number of TCP segments that can be outstanding on a session’s
reassembly queue. This value defaults to 100.
Note that setting this value too low could impact the throughput of
TCP connections which experience significant loss or
reordering. However, the higher this number is set, the more resources
can be consumed on TCP reassembly processing.
Perform one of the following:
1) Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.
Afterward, reboot the system.
2) 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
Afterward, reboot the system.
3) 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 10.4]
# fetch https://security.FreeBSD.org/patches/SA-18:08/tcp-10.patch
# fetch https://security.FreeBSD.org/patches/SA-18:08/tcp-10.patch.asc
# gpg –verify tcp-10.patch.asc
[FreeBSD 11.x]
# fetch https://security.FreeBSD.org/patches/SA-18:08/tcp-11.patch
# fetch https://security.FreeBSD.org/patches/SA-18:08/tcp-11.patch.asc
# gpg –verify tcp-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/10/ r337392
releng/10.4/ r337389
stable/11/ r337391
releng/11.1/ r337388
releng/11.2/ r337387
– ————————————————————————-
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-2018-6922>
<URL:https://www.kb.cert.org/vuls/id/962459>
The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-18:08.tcp.asc>
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