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FreeBSD-SA-17:04.ipfilter                                   Security Advisory
                                                          The FreeBSD Project

Topic:          ipfilter(4) fragment handling panic

Category:       contrib
Module:         ipfilter
Announced:      2017-04-27
Credits:        Cy Schubert
Affects:        All supported versions of FreeBSD.
Corrected:      2017-04-21 01:51:49 UTC (stable/11, 11.0-STABLE)
                2017-04-27 06:52:30 UTC (releng/11.0, 11.0-RELEASE-p10)
                2017-04-21 01:51:49 UTC (stable/10, 10.3-STABLE)
                2017-04-27 06:52:30 UTC (releng/10.3, 10.3-RELEASE-p19)
CVE Name:       CVE-2017-1081

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

IP Filter, also known as ipfilter(4), is a cross-platform, open source packet
filter (firewall) originally written for BSD operating systems, including
FreeBSD, NetBSD, and OpenBSD, and for Solaris.  ipfilter(4) is one of three
firewalls included in FreeBSD (the others being ipfw(4) and pf(4)).  It
performs firewall and NAT functions using the pfil(9) framework as do the
other firewalls in FreeBSD in the kernel.

II.  Problem Description

ipfilter(4), capable of stateful packet inspection, using the "keep state"
or "keep frags" rule options, will not only maintain the state of
connections, such as TCP streams or UDP communication, it also maintains
the state of fragmented packets.  When a packet fragments are received they
are cached in a hash table (and linked list).  When a fragment is received it
is compared with fragments already cached in the hash table for a match.  If
it does not match the new entry is used to create a new entry in the hash
table.  If on the other hand it does match, unfortunately the wrong entry is
freed, the entry in the hash table.  This results in use after free panic
(and for a brief moment prior to the panic a memory leak due to the wrong
entry being freed).

III. Impact

Carefully feeding fragments that are allowed to pass by an ipfilter(4)
firewall can be used to cause a panic followed by reboot loop denial of
service attack.

IV.  Workaround

No workaround is available, but systems not using ipfilter(4) are not
vulnerable.  A default installation doesn't enable ipfilter(4).
ipfilter(4) configurations not using "keep state" pr "keep frags" are not
vulnerable.  Users may be able to temporarily replace stateful inspection
with stateless rules however this is not as secure as stateful inspection.

V.   Solution

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.

Reload the ipl.ko kernel module or 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

Reload the ipl.ko kernel module or 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.

# fetch https://security.FreeBSD.org/patches/SA-17:04/ipfilter.patch
# fetch https://security.FreeBSD.org/patches/SA-17:04/ipfilter.patch.asc
# gpg --verify ipfilter.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 or reload the ipl.ko kernel module.

VI.  Correction details

The following list contains the correction revision numbers for each
affected branch.

Branch/path                                                      Revision
- -------------------------------------------------------------------------
stable/10/                                                        r317241
releng/10.3/                                                      r317487
stable/11/                                                        r317241
releng/11.0/                                                      r317487
- -------------------------------------------------------------------------

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-2017-1081>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-17:04.ipfilter.asc>
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