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FreeBSD-SA-16:15.sysarch                                    Security Advisory
                                                          The FreeBSD Project

Topic:          Incorrect argument validation in sysarch(2)

Category:       core
Module:         kernel
Announced:      2016-03-16
Credits:        Core Security
Affects:        All supported versions of FreeBSD.
Corrected:      2016-03-16 22:35:55 UTC (stable/10, 10.2-STABLE)
                2016-03-16 22:31:04 UTC (releng/10.2, 10.2-RELEASE-p14)
                2016-03-16 22:30:56 UTC (releng/10.1, 10.1-RELEASE-p31)
                2016-03-16 22:36:02 UTC (stable/9, 9.3-STABLE)
                2016-03-16 22:30:03 UTC (releng/9.3, 9.3-RELEASE-p39)
CVE Name:       CVE-2016-1885

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 IA-32 architecture allows programs to define segments, which provides
based and size-limited view into the program address space.  The
memory-resident processor structure, called Local Descriptor Table,
usually abbreviated LDT, contains definitions of the segments.  Since
incorrect or malicious segments would breach system integrity, operating
systems do not provide processes direct access to the LDT, instead
they provide system calls which allow controlled installation and removal 
of segments.

II.  Problem Description

A special combination of sysarch(2) arguments, specify a request to
uninstall a set of descriptors from the LDT.  The start descriptor
is cleared and the number of descriptors are provided.  Due to invalid
use of a signed intermediate value in the bounds checking during argument
validity verification, unbound zero'ing of the process LDT and adjacent
memory can be initiated from usermode.

III. Impact

This vulnerability could cause the kernel to panic. In addition it is
possible to perform a local Denial of Service against the system by
unprivileged processes. 

IV.  Workaround

No workaround is available, but only the amd64 architecture is affected.

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.

Reboot is required.

2) To update your vulnerable system via a binary patch:

Systems running a RELEASE version of FreeBSD platforms can be updated
via the freebsd-update(8) utility:

# freebsd-update fetch
# freebsd-update install

Reboot is required.

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-16:15/sysarch.patch
# fetch https://security.FreeBSD.org/patches/SA-16:15/sysarch.patch.asc
# gpg --verify sysarch.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/9/                                                         r296958
releng/9.3/                                                       r296953
stable/10/                                                        r296957
releng/10.1/                                                      r296954
releng/10.2/                                                      r296955
- -------------------------------------------------------------------------

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-2016-1885>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-16:15.sysarch.asc>
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