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Intel Chips' 'Lazy FP' Vulnerability Could Leak Secure Data

A group of security researchers have found a new vulnerability with Intel's chips that can theoretically allow an attack to utilize the 'Lazy FP' state of the process and gain access to sensitive data.

Larry Loeb

June 15, 2018

3 Min Read

Intel is warning that the Spectre and Meltdown variant 3A exploit will allow a program to obtain scraps of the Floating Point Unit (FPU) context of another app on Intel Core and Xeon processors.

The tracking number for this is CVE-2018-3665.

This "moderate" security problem happens because system software can utilize what is called "the Lazy FP state restore technique" to delay the restoring of state until an instruction operating on that state is actually executed by the new process. This may allow a local process to infer data utilizing Lazy FP state restore from another process through a speculative execution side channel.

Intel recommends system software developers utilize the "Eager FP state restore" in lieu of Lazy FP state restore. This includes:

  • Modern Linux: Going from kernel version 4.9, which was released in 2016

  • Windows: Including Windows Server 2016

  • Other Kernels: Including OpenSD and DragonflyBSD, which already use this technique routinely

(Source: Flickr)

(Source: Flickr)

Windows Server 2008 will need a patch, and the Linux kernel team is said to be back-porting mitigations to pre-4.9 kernels.

These techniques do not carry a performance hit. It has been found that eager state switching can even increase performance.

This kind of vulnerability -- though there are no known exploits in the wild at this time --could compromise cryptographic keys that are used to secure data in the system. Crypto programs routinely utilize the FPU to carry out the mathematical operations needed for cryptography.

Other external programs, such as malware, might be able to obtain or guess parts of these cryptographic keys by examining the state of the FPU.

Intel credited Julian Stecklina from Amazon Germany; Thomas Prescher from Cyberus Technology GmbH; Zdenek Sojka from SYSGO AG; as well as researcher Colin Perceiver, for bringing this latest side-channel vulnerability to light.

It has also been reported that Google found that its systems are secured against the lazy FPU state restore.

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Red Hat Linux also addressed the problem. The open source company notes, "RHEL-7 will automatically default to (safe) 'eager' floating point register restore on Sandy Bridge and newer Intel processors. AMD processors are not affected."

The company also say that users can mitigate this issue on older processors in RHEL 7 by booting the kernel with the "eagerfpu=on" parameter to enable eager FPU restore mode.

RHEL 6 and earlier versions are affected by the problem, but Red Hat says it will be releasing updates that will fix it.

It's also been reported by the Register that the announcement of the vulnerability was supposed to occur on June 27, but the BSD teams issued an early mitigation fix after Intel declined to work with them under embargo. This forced Intel to go public earlier than it had planned.

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— Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek.

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About the Author(s)

Larry Loeb

Blogger, Informationweek

Larry Loeb has written for many of the last century's major "dead tree" computer magazines, having been, among other things, a consulting editor for BYTE magazine and senior editor for the launch of WebWeek. He has written a book on the Secure Electronic Transaction Internet protocol. His latest book has the commercially obligatory title of Hack Proofing XML. He's been online since uucp "bang" addressing (where the world existed relative to !decvax), serving as editor of the Macintosh Exchange on BIX and the VARBusiness Exchange. His first Mac had 128 KB of memory, which was a big step up from his first 1130, which had 4 KB, as did his first 1401. You can e-mail him at [email protected].

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