CVE Name:       CAN-2005-0109

For general information regarding FreeBSD Security Advisories,
including descriptions of the fields above, security branches, and the
following sections, please visit
<URL:http://www.freebsd.org/security/>.

I.   Background

Sharing the execution resources of a superscalar processor between
multiple execution threads is referred to as "simultaneous
multithreading".  "Hyper-Threading Technology" or HTT is the name used
for the implementation of simultaneous multithreading on Intel Pentium
4, Mobile Pentium 4, and Xeon processors.  HTT involves sharing
certain CPU resources between multiple threads, including memory
caches.  FreeBSD supports HTT when using a kernel compiled with
the SMP option.

II.  Problem Description

When running on processors supporting Hyper-Threading Technology, it is
possible for a malicious thread to monitor the execution of another
thread.

NOTE:  Similar problems may exist in other simultaneous multithreading
implementations, or even some systems in the absence of simultaneous
multithreading.  However, current research has only demonstrated this
flaw in Hyper-Threading Technology, where shared memory caches are used.

III. Impact

Information may be disclosed to local users, allowing in many cases for
privilege escalation.  For example, on a multi-user system, it may be
possible to steal cryptographic keys used in applications such as OpenSSH
or SSL-enabled web servers.

IV.  Workaround

Systems not using processors with Hyper-Threading Technology support are
not affected by this issue.  On systems which are affected, the security
flaw can be eliminated by setting the "machdep.hlt_logical_cpus" tunable:

# echo "machdep.hlt_logical_cpus=1" >> /boot/loader.conf

The system must be rebooted in order for tunables to take effect.

Use of this workaround is not recommended on "dual-core" systems, as
this workaround will also disable one of the processor cores.

V.   Solution

Disable Hyper-Threading Technology on processors that support it.

NOTE:  It is expected that future work in cryptographic libraries and
operating system schedulers may remedy this problem for many or most
users, without necessitating the disabling of Hyper-Threading
Technology.  Future advisories will address individual cases.

Perform one of the following:

1) Upgrade your vulnerable system to 4-STABLE or 5-STABLE, or to the
RELENG_5_4, RELENG_5_3, RELENG_4_11, or RELENG_4_10 security branch
dated after the correction date.

2) To patch your present system:

The following patches have been verified to apply to FreeBSD 4.10,
4.11, 5.3, and 5.4 systems.

a) Download the relevant patch from the location below and verify the
detached PGP signature using your PGP utility.

[FreeBSD 4.10]
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-05:09/htt410.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-05:09/htt410.patch.asc

[FreeBSD 4.11]
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-05:09/htt411.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-05:09/htt411.patch.asc

[FreeBSD 5.x]
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-05:09/htt5.patch
# fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-05:09/htt5.patch.asc

b) Apply the patch.

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

c) Recompile your kernel as described in
<URL:http://www.freebsd.org/handbook/kernelconfig.html> and reboot the
system.

NOTE:  For users that are certain that their environment is not affected
by this vulnerability, such as single-user systems, Hyper-Threading
Technology may be re-enabled by setting the tunable
"machdep.hyperthreading_allowed".

VI.  Correction details

The following list contains the revision numbers of each file that was
corrected in FreeBSD.

Branch                                                                                                    
                         Path                                                                            Revision
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VII. References

http://www.daemonology.net/hyperthreading-considered-harmful/
 

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