CVE-2021-47255 in Linux
Summary
by MITRE • 05/21/2024
In the Linux kernel, the following vulnerability has been resolved:
kvm: LAPIC: Restore guard to prevent illegal APIC register access
Per the SDM, "any access that touches bytes 4 through 15 of an APIC register may cause undefined behavior and must not be executed." Worse, such an access in kvm_lapic_reg_read can result in a leak of kernel stack contents. Prior to commit 01402cf81051 ("kvm: LAPIC: write down valid APIC registers"), such an access was explicitly disallowed. Restore the guard that was removed in that commit.
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Analysis
by VulDB Data Team • 05/21/2024
The vulnerability CVE-2021-47255 represents a critical security flaw in the Linux kernel's KVM (Kernel-based Virtual Machine) subsystem, specifically within the Local APIC (Advanced Programmable Interrupt Controller) implementation. This issue arises from the improper handling of APIC register access operations that violate Intel's Software Developer's Manual (SDM) specifications. The Local APIC is a crucial component in x86 virtualization environments, responsible for managing interrupt delivery and processor communication within virtual machine environments. When a guest operating system attempts to access APIC registers in a manner that touches bytes 4 through 15 of these registers, the system should prevent such operations to maintain system stability and security.
The technical flaw stems from a regression in the KVM subsystem where a protective guard mechanism was inadvertently removed during the commit 01402cf81051 titled "kvm: LAPIC: write down valid APIC registers." This commit, while intended to improve APIC register handling, accidentally eliminated a critical safety check that previously prevented access to the problematic byte ranges. The vulnerability allows for illegal APIC register access patterns that can result in kernel stack information leakage, providing attackers with sensitive data that could aid in further exploitation attempts. The specific function kvm_lapic_reg_read becomes vulnerable when processing accesses that touch bytes 4 through 15 of APIC registers, creating a potential information disclosure channel that could expose kernel memory contents to unauthorized parties.
The operational impact of this vulnerability is significant within virtualized environments where KVM is actively utilized. Attackers with access to a guest operating system could potentially exploit this weakness to leak kernel stack information, which might contain sensitive data such as memory addresses, kernel function pointers, or other confidential information. This information disclosure could enable more sophisticated attacks including privilege escalation or bypass of security mechanisms. The vulnerability particularly affects systems running Linux kernels that incorporate the affected KVM implementation, making it a widespread concern for cloud providers, virtualization platforms, and any organization relying on KVM-based virtualization technologies. The issue represents a direct violation of the principle of least privilege and could undermine the security isolation that virtualization environments are designed to provide.
Mitigation strategies for CVE-2021-47255 primarily involve applying the official kernel patches that restore the removed guard mechanism for APIC register access. System administrators should update their Linux kernels to versions that include the fix for commit 01402cf81051, which re-enables the necessary protection against illegal byte range access in APIC registers. Organizations should also implement monitoring for unusual APIC register access patterns that might indicate exploitation attempts. The vulnerability aligns with CWE-125: Out-of-bounds Read and CWE-200: Information Exposure, demonstrating how improper bounds checking in virtualization components can create security risks. From an ATT&CK perspective, this vulnerability could be leveraged in techniques such as T1059: Command and Scripting Interpreter and T1068: Exploitation for Privilege Escalation, as attackers might use the leaked information to craft more effective attacks against the virtualized environment. Regular kernel updates and proper virtualization security hardening practices remain essential defenses against this and similar vulnerabilities in virtualized computing environments.