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FreeBSD-SA-18:07.lazyfpu Security Advisory
The FreeBSD Project

Topic: Lazy FPU State Restore Information Disclosure

Category: core
Module: kernel
Announced: 2018-06-21
Credits: Julian Stecklina from Amazon Germany
Thomas Prescher from Cyberus Technology GmbH
Zdenek Sojka from SYSGO AG
Colin Percival
Affects: All supported version of FreeBSD.
Corrected: 2018-06-14 18:50:49 UTC (stable/11, 11.2-PRERELEASE)
2018-06-15 13:21:37 UTC (releng/11.2, 11.2-RC3)
2018-06-21 05:17:13 UTC (releng/11.1, 11.1-RELEASE-p11)
CVE Name: CVE-2018-3665

Special Note: This advisory only addresses this issue for FreeBSD 11.x on
i386 and amd64. We expect to update this advisory to include
10.x in the near future.

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

Modern CPUs have a floating point unit (FPU) which needs to maintain state
per thread. One technique is to only save and to only restore the FPU state
for a thread when a thread attempts to utilize the FPU. This technique is
called Lazy FPU state restore.

II. Problem Description

A subset of Intel processors can allow a local thread to infer data from
another thread through a speculative execution side channel when Lazy FPU
state restore is used.

III. Impact

Any local thread can potentially read FPU state information from other
threads running on the host. This could include cryptographic keys when the
AES-NI CPU feature is present.

IV. Workaround

No workaround is available, but non-Intel branded CPUs are not believed
to be vulnerable.

V. Solution

The patch changes from Lazy FPU state restore to Eager FPU state restore.
This new technique is the recommended practice from Intel and in some cases
can actually increase performance, depending on workload.

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.

Afterward, 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

Afterward, 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.

[FreeBSD 11.1]
# fetch https://security.FreeBSD.org/patches/SA-18:07/lazyfpu-11.patch
# fetch https://security.FreeBSD.org/patches/SA-18:07/lazyfpu-11.patch.asc
# gpg –verify lazyfpu-11.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/11/ r335169
releng/11.2/ r335196
releng/11.1/ r335465
– ————————————————————————-

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://www.intel.com/content/www/us/en/security-center/advisory/intel-sa-00145.html>

<URL:https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2018-3665>

The latest revision of this advisory is available at
<URL:https://security.FreeBSD.org/advisories/FreeBSD-SA-18:07.lazyfpu.asc>
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