From security-officer@netbsd.org Thu Jun 30 18:22:34 2005 From: NetBSD Security-Officer To: bugtraq@securityfocus.com Date: Thu, 30 Jun 2005 18:12:26 -0400 Subject: NetBSD Security Advisory 2005-001: Crypto leaks across HyperThreaded CPUs (i386, P4, HTT+SMP only) -----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 NetBSD Security Advisory 2005-001 ================================= Topic: Crypto leaks across HyperThreaded CPUs (i386, P4, HTT+SMP only) Version: NetBSD-current: affected, i386 on P4 with HTT and SMP kernels NetBSD 2.0: affected, i386 on P4 with HTT and SMP kernels NetBSD 1.6 and earlier: not affected, i386 SMP was not in these releases Severity: Possible exposure of cryptographic key information to local users Fixed: Best Practices are under discussion. See below. Abstract ======== The Pentium CPU shares caches between HyperThreads. This permits a local process to gain a side-channel against cryptographic processes running on the other HyperThread. Testing for cached data can be accomplished by timing reads. Under some circumstances, this permits the spying process to extract bits of the key. This has been demonstrated against OpenSSL. Technical Details ================= The full explanation of the issue can be found here: http://www.daemonology.net/papers/htt.pdf This issue affects only a subset of i386 systems. Your system is not affected if you are running a CPU without HyperThreading. Your system is not affected if you are running a non-SMP kernel. Your system is not affected if you have disabled HyperThreading in your BIOS, and confirmed that the virtual CPUs are not detected by the kernel during boot. Your system is affected, but probably not at risk, if you do not permit shell access by untrusted users. Additional resources: http://www.daemonology.net/hyperthreading-considered-harmful/ ftp://ftp.freebsd.org/pub/FreeBSD/CERT/advisories/FreeBSD-SA-05:09.htt.asc Solutions and Workarounds ========================= This issue is fundamental to the design and implementation of HyperThreading in Intel processors. Avoiding the problem is possible, and two workarounds are available now. Others which may appear later are also discussed. Option 1. Disable HyperThreading in the BIOS. Disabling HyperThreading is a complete workaround for the issue. Option 2. Run a non-SMP kernel. On single physical CPU systems, running a non-SMP kernel effectively disables HyperThreading. On a multi-CPU system, this option is not the best solution, since it will disable additional physical CPUs as well. Later potential workarounds: 1. Reimplement all cryptographic code to use constant time, and constant cache-access execution patterns. There is some interest along these lines from various groups, as a result of this issue. NetBSD's Security Officers will monitor the availability of such code. 2. Implement some form of locking which prevents the sibling HyperThread from running while cryptographic or other sensitive code is active on a processor. Additionally, FreeBSD has chosen to offer a tunable bootloader option to disable spinup of HyperThread Virtual CPUs, without requiring a change to BIOS settings. The NetBSD Security Officer has not seen a demand from the NetBSD community for a similar workaround, but would discuss the possibility in the appropriate forums, if asked to. Thanks To ========= Colin Percival Jacques A. Vidrine Revision History ================ 2005-06-30 Initial release More Information ================ Advisories may be updated as new information becomes available. The most recent version of this advisory (PGP signed) can be found at ftp://ftp.NetBSD.org/pub/NetBSD/security/advisories/NetBSD-SA2005-001.txt.asc Information about NetBSD and NetBSD security can be found at http://www.NetBSD.org/ and http://www.NetBSD.org/Security/. Copyright 2005, The NetBSD Foundation, Inc. All Rights Reserved. Redistribution permitted only in full, unmodified form. $NetBSD: NetBSD-SA2005-001.txt,v 1.3 2005/06/22 05:44:50 wiz Exp $ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.1 (NetBSD) iQCVAwUBQsQHlj5Ru2/4N2IFAQI6egQAoUSxClcXFCSwtO1e5DFRu+QhKVR3cczl TBh4i7nGYKiZqJiV5N1EnfGMFgwRgR1pvaVpt+0f7/BaibqYd0LOE4QOJx2usIuZ vjxcP3IT8MUXpi3jDnzsamst1hIc14i35nquGGKmBV1L71qBqv6bDp5QWj2U5XUw pnsE1AJBPeY= =JklU -----END PGP SIGNATURE-----