CVE-2023-0045 in Linux
Summary
by MITRE • 04/26/2023
The current implementation of the prctl syscall does not issue an IBPB immediately during the syscall. The ib_prctl_set function updates the Thread Information Flags (TIFs) for the task and updates the SPEC_CTRL MSR on the function __speculation_ctrl_update, but the IBPB is only issued on the next schedule, when the TIF bits are checked. This leaves the victim vulnerable to values already injected on the BTB, prior to the prctl syscall. The patch that added the support for the conditional mitigation via prctl (ib_prctl_set) dates back to the kernel 4.9.176.
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Analysis
by VulDB Data Team • 05/26/2026
The vulnerability in question relates to the improper implementation of the prctl system call within the Linux kernel, specifically concerning indirect branch prediction barrier (IBPB) execution timing. This issue affects the kernel's handling of speculative execution mitigations, particularly those related to the Intel Software Guard Extensions and broader microarchitectural side-channel defenses. The problem manifests when the ib_prctl_set function modifies Thread Information Flags (TIFs) and updates the SPEC_CTRL model-specific register through the __speculation_ctrl_update function, yet fails to immediately execute the IBPB instruction during the syscall execution. This temporal gap creates a window where branch target buffers may contain maliciously injected values that persist beyond the prctl call, leaving systems vulnerable to speculative execution attacks. The vulnerability stems from a design flaw where the IBPB execution is deferred until the next scheduling operation when TIF bits are checked, rather than being executed immediately upon prctl system call completion. This delay in IBPB execution creates a critical timing window where attackers could potentially exploit the speculative execution behavior to inject values into the branch target buffer before the mitigation takes effect. The vulnerability has existed since kernel version 4.9.176 and represents a fundamental flaw in the implementation of conditional mitigations for speculative execution vulnerabilities, particularly those related to the Speculative Store Bypass (SSB) and similar side-channel threats. According to the CWE taxonomy, this represents a weakness in the design of system-level protections, specifically categorized as CWE-119: Improper Access of Resource During Expected Lifetime, as the system fails to properly secure resources during the expected lifetime of the system call execution. The issue directly relates to the ATT&CK technique T1059.001: Command and Scripting Interpreter - PowerShell, where attackers could potentially exploit the timing gap to perform indirect branch prediction attacks, though the specific attack vectors are more related to microarchitectural side-channel exploitation patterns.
The operational impact of this vulnerability extends beyond simple performance degradation to represent a serious security risk for systems running affected kernel versions. Systems utilizing the prctl system call to configure speculative execution mitigations become temporarily vulnerable during the window between the prctl call execution and the subsequent IBPB execution. This vulnerability affects all systems that rely on prctl for configuring indirect branch prediction barriers, particularly those deployed in environments where microarchitectural side-channel attacks are a concern. The risk is compounded by the fact that the vulnerability exists in a core kernel subsystem that handles privilege escalation and system configuration, making it particularly dangerous in multi-tenant or security-sensitive environments. The timing of the vulnerability's introduction in kernel 4.9.176 indicates that systems running this or later kernel versions are potentially exposed, with the vulnerability being present for several years without proper detection or remediation. Organizations using virtualized environments or cloud services may be particularly vulnerable, as the speculative execution mitigations are often configured through prctl calls during system initialization or runtime configuration. The vulnerability also affects systems that utilize security frameworks or applications that depend on prctl for configuring speculative execution behavior, creating a broad impact across various security implementations.
The recommended mitigation involves upgrading the kernel past the specific commit a664ec9158eeddd75121d39c9a0758016097fa96 which addresses the timing issue in the prctl implementation. This upgrade ensures that the IBPB instruction is executed immediately upon prctl system call completion rather than being deferred to the next scheduling operation. The fix represents a fundamental correction to the timing behavior of speculative execution mitigations, ensuring that the system maintains proper security boundaries during the critical window of system call execution. Organizations should prioritize this kernel upgrade as part of their security maintenance procedures, particularly those systems that frequently utilize prctl for configuring speculative execution mitigations. The patch addresses the root cause by ensuring immediate IBPB execution, thereby eliminating the temporal gap that previously left systems vulnerable to branch target buffer injection attacks. This mitigation aligns with industry best practices for maintaining kernel security and ensuring proper implementation of microarchitectural side-channel defenses. The upgrade also helps organizations comply with security standards that require timely patching of known vulnerabilities, particularly those affecting system-level security controls. Additionally, administrators should review their system configurations to ensure that prctl calls are properly monitored and that the updated kernel behavior is functioning as expected, particularly in environments where speculative execution mitigations are dynamically configured. The vulnerability's resolution through this specific commit ensures that the kernel's approach to speculative execution control is properly synchronized with the security requirements of modern microarchitectures.