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FreeBSD-SA-18:06.debugreg Security Advisory
The FreeBSD Project

Topic: Mishandling of x86 debug exceptions

Category: core
Module: kernel
Announced: 2018-05-08
Credits: Nick Peterson, Everdox Tech LLC
https://www.linkedin.com/in/everdox
Andy Lutomirski
Affects: All supported versions of FreeBSD.
Corrected: 2018-05-08 17:03:33 UTC (stable/11, 11.2-PRERELEASE)
2018-05-08 17:12:10 UTC (releng/11.1, 11.1-RELEASE-p10)
2018-05-08 17:05:39 UTC (stable/10, 10.4-STABLE)
2018-05-08 17:12:10 UTC (releng/10.4, 10.4-RELEASE-p9)
CVE Name: CVE-2018-8897

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

On x86 architecture systems, the stack is represented by the combination of
a stack segment and a stack pointer, which must remain in sync for proper
operation. Instructions related to manipulating the stack segment have
special handling to facilitate consistency with changes to the stack pointer.

II. Problem Description

The MOV SS and POP SS instructions inhibit debug exceptions until the
instruction boundary following the next instruction. If that instruction is
a system call or similar instruction that transfers control to the operating
system, the debug exception will be handled in the kernel context instead of
the user context.

III. Impact

An authenticated local attacker may be able to read sensitive data in kernel
memory, control low-level operating system functions, or may panic the
system.

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,
using either a binary or source code patch, and then reboot.

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

And reboot.

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.

[FreeBSD 11.1]
# fetch https://security.FreeBSD.org/patches/SA-18:06/debugreg.11.1.patch
# fetch https://security.FreeBSD.org/patches/SA-18:06/debugreg.11.1.patch.asc
# gpg –verify debugreg.11.1.patch.asc

[FreeBSD 10.4]
# fetch https://security.FreeBSD.org/patches/SA-18:06/debugreg.10.4.patch
# fetch https://security.FreeBSD.org/patches/SA-18:06/debugreg.10.4.patch.asc
# gpg –verify debugreg.10.4.patch.asc

b) Apply the patch. Execute the following commands as root:

# cd /usr/src
# patch < /path/to/patch

c) Recompile and install 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/ r333370
releng/10.4/ r333371
stable/11/ r333369
releng/11.1/ r333371
– ————————————————————————-

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-2018-8897>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-18:06.debugreg.asc>
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