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Category:   OS (UNIX)  >   FreeBSD Kernel Vendors:   FreeBSD
(FreeBSD Issues Fix) Intel Core-based Processors 'Lazy FPU Restore' Lets Local Users Obtain Potentially Sensitive FPU State Information on the Target System
SecurityTracker Alert ID:  1041164
SecurityTracker URL:
CVE Reference:   CVE-2018-3665   (Links to External Site)
Date:  Jun 21 2018
Impact:   Disclosure of system information
Fix Available:  Yes  Vendor Confirmed:  Yes  
Version(s): 11.x
Description:   A vulnerability was reported in Intel Core-based processors. A local user can obtain potentially sensitive information. FreeBSD Kernel is affected.

A local user can conduct cache side-channel attacks to exploit a flaw in the 'Lazy Floating Point Unit (FPU)' scheme where the kernel saves and restores FPU state during task switching to potentially determine FPU state bits.

System software that utilize the Lazy FPU state restore method to delay the restoring of state until an instruction operating on that state is actually executed by the new process is affected.

Julian Stecklina from Amazon Germany, Thomas Prescher from Cyberus Technology GmbH (, Zdenek Sojka from SYSGO AG (, and Colin Percival reported this vulnerability.

Impact:   A local user can obtain potentially sensitive FPU state information on the target system.
Solution:   FreeBSD has issued a fix for FreeBSD 11.x on i386 and amd64.

A fix is planned for 10.x.

The FreeBSD advisory is available at:

Vendor URL: (Links to External Site)
Cause:   Access control error, State error

Message History:   This archive entry is a follow-up to the message listed below.
Jun 14 2018 Intel Core-based Processors 'Lazy FPU Restore' Lets Local Users Obtain Potentially Sensitive FPU State Information on the Target System

 Source Message Contents

Subject:  FreeBSD Security Advisory FreeBSD-SA-18:07.lazyfpu

Hash: SHA512

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:>.

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
# fetch
# 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:> and reboot the

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://

Or visit the following URL, replacing NNNNNN with the revision number:


VII. References



The latest revision of this advisory is available at


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