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FreeBSD-SA-19:13.pts Security Advisory
The FreeBSD Project

Topic: pts(4) write-after-free

Category: core
Module: kernel
Announced: 2019-07-24
Credits: syzkaller
Affects: All supported versions of FreeBSD.
Corrected: 2019-07-07 14:19:46 UTC (stable/12, 12.0-STABLE)
2019-07-24 12:53:06 UTC (releng/12.0, 12.0-RELEASE-p8)
2019-07-07 14:20:14 UTC (stable/11, 11.2-STABLE)
2019-07-24 12:53:06 UTC (releng/11.2, 11.2-RELEASE-p12)
2019-07-24 12:53:06 UTC (releng/11.3, 11.3-RELEASE-p1)
CVE Name: CVE-2019-5606

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 posix_openpt(2) system call allocates a pseudo-terminal device and
returns a descriptor referencing that device. Such a descriptor may be
configured such that a SIGIO signal will be sent to a designated process
or process group when the device is ready to perform I/O.

II. Problem Description

The code which handles a close(2) of a descriptor created by
posix_openpt(2) fails to undo the configuration which causes SIGIO to be
raised. This bug can lead to a write-after-free of kernel memory.

III. Impact

The bug permits malicious code to trigger a write-after-free, which may
be used to gain root privileges or escape a jail.

IV. Workaround

No workaround is available.

V. Solution

Upgrade your vulnerable system to a supported FreeBSD stable or
release / security branch (releng) dated after the correction date.

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 “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-19:13/pts.patch
# fetch https://security.FreeBSD.org/patches/SA-19:13/pts.patch.asc
# gpg –verify pts.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/ r349805
releng/12.0/ r350282
stable/11/ r349806
releng/11.2/ r350282
releng/11.3/ r350282
– ————————————————————————-

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-2019-5606>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-19:13.pts.asc>
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