CVE-2023-1998 in Linux
Summary
by MITRE • 04/21/2023
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.
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Analysis
by VulDB Data Team • 07/29/2024
The vulnerability described in CVE-2023-1998 represents a critical flaw in the Linux kernel's handling of speculative execution mitigations, specifically affecting the interaction between IBRS (Indirect Branch Restricted Speculation) and STIBP (Speculative Store Bypass Prevention) mechanisms. This issue manifests when userspace processes attempt to enable spectre-BTI mitigation through prctl system calls with PR_SET_SPECULATION_CTRL, or via seccomp filters, creating a false sense of security while leaving systems vulnerable to cross-thread branch target injection attacks. The vulnerability was particularly concerning because it affected major cloud provider virtual machines and bare-metal deployments, demonstrating the widespread impact of this kernel-level flaw.
The technical root cause of this vulnerability stems from the kernel's implementation logic when dealing with plain IBRS versus enhanced IBRS configurations. When plain IBRS is enabled, the kernel's decision-making process incorrectly determines that STIBP is unnecessary because the IBRS bit is designed to implicitly protect against cross-thread branch target injection attacks. However, this protection is fundamentally flawed in plain IBRS implementations where the IBRS bit is cleared upon returning to userspace for performance optimization purposes. This clearing of the IBRS bit effectively disables the implicit STIBP protection that would normally safeguard against speculative execution attacks, leaving userspace threads exposed to attacks that target branch prediction mechanisms across thread boundaries.
The operational impact of this vulnerability extends beyond simple performance considerations, as it creates a dangerous security illusion where administrators and developers believe their systems are protected against speculative execution attacks when in reality they remain vulnerable to sophisticated cross-thread attacks. This vulnerability particularly affects systems running on cloud infrastructure where the kernel configuration may default to plain IBRS for performance reasons, and where administrators may not be aware of the subtle differences between IBRS implementations. The issue is further compounded because the vulnerability can be triggered through legitimate kernel interfaces used for security hardening, making it difficult to detect and mitigate without deep understanding of the underlying speculative execution mechanisms.
Mitigation strategies for CVE-2023-1998 require careful consideration of both kernel configuration and system deployment practices. The most effective immediate solution involves ensuring that enhanced IBRS is enabled rather than plain IBRS, typically through kernel boot parameters such as spec_store_bypass_disable=on or ibpb=on, which force the system to use the more secure enhanced IBRS implementation. System administrators should also implement kernel updates that include patches addressing the specific logic flaw in the kernel's IBRS-STIBP interaction handling. Additionally, monitoring and logging should be enhanced to detect when prctl calls with PR_SET_SPECULATION_CTRL are made, as these may indicate attempts to configure potentially insecure speculative execution mitigations. This vulnerability aligns with CWE-284 Access Control and ATT&CK techniques related to privilege escalation and defense evasion through kernel-level manipulation. The issue also demonstrates the importance of proper kernel parameter validation and the need for comprehensive testing of security mitigations in virtualized environments where performance optimizations may inadvertently compromise security posture.