CVE-2023-1998

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.

Weakness

Hardware structures shared across execution contexts (e.g., caches and branch predictors) can violate the expected architecture isolation between contexts.

Affected Software

Extended Description

Modern processors use techniques such as out-of-order execution, speculation, prefetching, data forwarding, and caching to increase performance. Details about the implementation of these techniques are hidden from the programmer’s view. This is problematic when the hardware implementation of these techniques results in resources being shared across supposedly isolated contexts. Contention for shared resources between different contexts opens covert channels that allow malicious programs executing in one context to recover information from another context. Some examples of shared micro-architectural resources that have been used to leak information between contexts are caches, branch prediction logic, and load or store buffers. Speculative and out-of-order execution provides an attacker with increased control over which data is leaked through the covert channel. If the extent of resource sharing between contexts in the design microarchitecture is undocumented, it is extremely difficult to ensure system assets are protected against disclosure.

Potential Mitigations

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/663.html

References

• https://github.com/google/security-research/security/advisories/GHSA-mj4w-6495-6crx
• https://github.com/torvalds/linux/commit/6921ed9049bc7457f66c1596c5b78aec0dae4a9d
• https://kernel.dance/#6921ed9049bc7457f66c1596c5b78aec0dae4a9d
• https://lists.debian.org/debian-lts-announce/2023/05/msg00005.html
• https://lists.debian.org/debian-lts-announce/2023/05/msg00006.html
• https://github.com/google/security-research/security/advisories/GHSA-mj4w-6495-6crx
• https://github.com/torvalds/linux/commit/6921ed9049bc7457f66c1596c5b78aec0dae4a9d
• https://kernel.dance/#6921ed9049bc7457f66c1596c5b78aec0dae4a9d
• https://lists.debian.org/debian-lts-announce/2023/05/msg00005.html
• https://lists.debian.org/debian-lts-announce/2023/05/msg00006.html