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FreeBSD-EN-21:12.divert                                         Errata Notice
                                                          The FreeBSD Project

Topic:          Kernel double free when transmitting on a divert socket

Category:       core
Module:         divert(4)
Announced:      2021-05-26
Affects:        FreeBSD 13.0
Corrected:      2021-05-10 13:36:08 UTC (stable/13, 13.0-STABLE)
                2021-05-26 19:30:51 UTC (releng/13.0, 13.0-RELEASE-p1)

For general information regarding FreeBSD Errata Notices and Security
Advisories, including descriptions of the fields above, security
branches, and the following sections, please visit
<URL:https://security.FreeBSD.org/>.

I.   Background

divert(4) sockets are a facility that permit firewalls to transmit a
copy of a packet to a userspace process.  They may also be used by
userspace programs to inject packets into the IP packet processing
stack.  In the FreeBSD base system, the only user of divert(4) sockets
is natd(8).

II.  Problem Description

A bug in the error handling of transmission on a divert(4) socket could
result in a double free of an mbuf.

III. Impact

Systems making use of divert(4) may misbehave or panic in a
non-deterministic manner.

IV.  Workaround

No workaround is available.  Systems not making use of divert(4) sockets
are unaffected.  divert(4) sockets appear in sockstat(8) output as using
protocol "div".

V.   Solution

Upgrade your 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 system via a binary patch:

Systems running a RELEASE version of FreeBSD on the amd64, i386, or
(on FreeBSD 13 and later) arm64 platforms can be updated via the
freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install
# shutdown -r +10min "Rebooting for an erratum update"

2) To update your 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/EN-21:12/divert.patch
# fetch https://security.FreeBSD.org/patches/EN-21:12/divert.patch.asc
# gpg --verify divert.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

This issue is corrected by the corresponding Git commit hash or Subversion
revision number in the following stable and release branches:

Branch/path                             Hash                     Revision
- -------------------------------------------------------------------------
stable/13/                              eafeee082c50    stable/13-n245578
releng/13.0/                            22b58630d6ba  releng/13.0-n244737
- -------------------------------------------------------------------------

Run the following command to see which files were modified by a
particular commit:

# git show --stat <commit hash>

Or visit the following URL, replacing NNNNNN with the hash:

<URL:https://cgit.freebsd.org/src/commit/?id=NNNNNN>

To determine the commit count in a working tree (for comparison against
nNNNNNN in the table above), run:

# git rev-list --count --first-parent HEAD

VII. References

<URL:https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=255104>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-EN-21:12.divert.asc>
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