A long-standing vulnerability known as Sinkclose has been identified in AMD processors, posing a serious threat by granting hackers access to highly restricted areas of a computer system. This flaw allows malware to penetrate deeply into a system’s memory, often making it more practical to replace the affected device than to attempt a cleanup.
The Sinkclose vulnerability enables attackers to execute code in System Management Mode (SMM), a secure operational mode intended for a protected segment of firmware. This mode is supposed to be shielded from unauthorized access, but the flaw compromises this security.
IOActive researchers have highlighted that nearly all AMD processors manufactured since 2006, and possibly even before, are vulnerable to this issue. The vulnerability is assigned a CVSS base score of 7.5 and is identified as CVE-2023-31315.
AMD’s security advisory indicates that improper validation in a model-specific register (MSR) could allow a malicious program with ring0 access to alter SMM configuration while the SMI lock is active. This could lead to the execution of arbitrary code.
While the memory controller typically blocks access to SMRAM—the specific area of physical memory used by SMM when the CPU is not in SMM—IOActive researchers discovered that certain MSR registers, accessible from ring 0, are not read-only even when the SmmLock flag is set. This flaw was reported by Krzysztof Okupski and Enrique Nissim from IOActive.
The researchers note that exploiting this vulnerability requires significant access to an AMD-based system. However, once exploited, it allows attackers to deploy malware such as a “bootkit,” which can evade antivirus software and remain hidden from the operating system while providing full control over the system.
Moreover, if AMD’s Platform Secure Boot feature is improperly implemented, detecting or removing such malware becomes even more challenging. This issue affects the majority of systems tested, with the potential for malware persistence even through operating system reinstallation.
Okupski emphasizes the severity of the problem: “Imagine a sophisticated attacker intent on remaining undetected. Even if you attempt a clean drive wipe, the malware could persist.”
To eradicate this malware, Okupski suggests that physical intervention is necessary—specifically, opening the computer case, connecting to the memory chips with an SPI Flash programmer, and conducting a thorough inspection of the memory.
AMD has acknowledged the issue and has issued mitigation measures for its Ryzen PC and data center products. Mitigations for AMD’s embedded products will follow shortly. While updates have been provided for most recent processors, including all EPYC data center models, the latest Threadripper versions, and Ryzen processors, AMD has not extended these updates to Ryzen 1000, 2000, and 3000 series processors or Threadripper 1000 and 2000 models.
