An engineer holding a semiconductor about to be integrated in a circuit.
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Google researcher Daniel Moghimi discovered a new vulnerability affecting millions of Intel chip models. The vulnerability, dubbed Downfall by Moghimi, enables an attacker to steal sensitive data from victims if exploited successfully. Mitigation is provided by Intel for affected hardware, which includes computing devices based on Intel Core processors from the 6th Skylake to the 11th Tiger Lake generation.

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What is the Downfall vulnerability?

The Intel advisory reports that CVE-2022-40982/Downfall is a hardware information disclosure vulnerability with medium severity.

According to Moghimi, the vulnerability is located in memory optimization features in Intel’s processors. Successful exploitation reveals internal hardware registers to software. Untrusted software could therefore access data stored by other software, which shouldn’t be possible.

More specifically, the researcher ” … discovered that the Gather instruction, meant to speed up accessing scattered data in memory, leaks the content of the internal vector register file during speculative execution.” He also stated that “The Gather instruction appears to use a temporal buffer shared across sibling CPU threads, and it transiently forwards data to later dependent instructions, and the data belongs to a different process and gather execution running on the same core.”

Moghimi published a detailed account of his research about the vulnerability, as well as the full Downfall source code. Intel released technical documentation on Gather Data Sampling, the name used by the company to refer to Downfall.

The Downfall vulnerability was first reported to Intel in August 2022 and kept under embargo until it was fixed. This reporting is inline with the coordinated vulnerability disclosure practice in which a vulnerability is publicly disclosed only after mitigations are available.

Downfall vulnerability exploitation scenarios

A few Downfall vulnerability exploitation scenarios have been tested successfully and provided by Moghimi in his research paper, in addition to showing videos of it on his website. The scenarios enable different types of data theft.

Stealing cryptographic keys

Moghimi has shown an attack aimed at the Advanced Encryption Standard executed by the OpenSSL command line tool. The tool is being executed on one virtual machine while the attack is run from another virtual machine on a sibling thread of the same CPU core.

He did his tests on 100 different AES keys; the success rate was 100% for AES-128 keys and 86% for AES-256 keys. This drop in the success rate can be bypassed by rerunning the attack multiple times to recover the complete key.

Stealing arbitrary data

Arbitrary data at rest can also be stolen, as long as the attack runs on the same physical processor core as the victim.

As an example, Moghimi showed a video where he extracts data from a Linux kernel, but the attack could be used for extracting other data. In another video example, Moghimi showed it’s possible to spy on printable characters.

More attack possibilities

Moghimi wrote that a hacker can target high-value credentials such as passwords and encryption keys, which might lead to other attacks that violate the availability and integrity of computers.

Intel wrote that “Malicious software may be able to infer data previously stored in vector registers used by either the same thread, or the sibling thread on the same physical core. These registers may have been used by other security domains such as other virtual machine (VM) guests, the operating system (OS) kernel, or Intel® Software Guard Extensions (Intel® SGX) enclaves.”

How to mitigate this cybersecurity threat

Intel has released firmware updates and recommends that users of affected Intel processors update to the latest version firmware that addresses these issues.

For Intel SGX customers, the company advises updating the microcode located in platform flash designated by firmware interface table entry point 1.

Other mitigations are offered by Moghimi, although most have severe disadvantages:

  • Disabling Simultaneous Multithreading would partially mitigate the risk, yet with a performance cost — you would lose about 30% of computing speed. It would also not prevent data leaks across context switching.
  • Disallowing affected instructions that leaks secrets to Gather, yet this could not mitigate the attack fully.
  • Disabling Gather slows down applications and could crash applications that rely on this feature.

Moghimi also recommends preventing transient forwarding of data after the Gather instruction, which can mitigate the Downfall attacks without the disadvantages of the previous mitigation propositions. This mitigation is the one that Intel implemented in its latest microcode update.

Disclosure: I work for Trend Micro, but the views expressed in this article are mine.

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