CVE-2023-1998: Spectre v2 SMT mitigations problem in Linux kernel

Vulnerability summary

CVE-2023-1998 is a medium (CVSS 5.6) vulnerability in debian debian_linux. EPSS estimates a 1.38% chance of exploitation in the next 30 days, higher than 72.1% of scored vulnerabilities. 2 public exploits are recorded.

Timeline

1. 21-04-2023
   PublishedVulnerability first disclosed.
2. 13-02-2025
   Last modifiedVulnerability information updated.

Description

The Linux kernel allows userspace processes to enable mitigations by calling prctl with PR_SET_SPECULATION_CTRL which disables the speculation feature as well as by using seccomp. We had noticed that on VMs of at least one major cloud provider, the kernel still left the victim process exposed to attacks in some cases even after enabling the spectre-BTI mitigation with prctl. The same behavior can be observed on a bare-metal machine when forcing the mitigation to IBRS on boot command line. This happened because when plain IBRS was enabled (not enhanced IBRS), the kernel had some logic that determined that STIBP was not needed. The IBRS bit implicitly protects against cross-thread branch target injection. However, with legacy IBRS, the IBRS bit was cleared on returning to userspace, due to performance reasons, which disabled the implicit STIBP and left userspace threads vulnerable to cross-thread branch target injection against which STIBP protects.

• CWE-1303Non-Transparent Sharing of Microarchitectural ResourcesHardware structures shared across execution contexts (e.g., caches and branch predictors) can violate the expected architecture isolation between contexts.
• CWE-203Observable DiscrepancyThe product behaves differently or sends different responses under different circumstances in a way that is observable to an unauthorized actor.