CVE-2009-3640 in Linux
Summary
by MITRE
The update_cr8_intercept function in arch/x86/kvm/x86.c in the KVM subsystem in the Linux kernel before 2.6.32-rc1 does not properly handle the absence of an Advanced Programmable Interrupt Controller (APIC), which allows local users to cause a denial of service (NULL pointer dereference and system crash) or possibly gain privileges via a call to the kvm_vcpu_ioctl function.
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Analysis
by VulDB Data Team • 08/24/2021
The vulnerability described in CVE-2009-3640 represents a critical flaw in the Linux kernel's KVM (Kernel-based Virtual Machine) subsystem that affects systems running kernel versions prior to 2.6.32-rc1. This issue specifically targets the update_cr8_intercept function located within the x86 architecture implementation of KVM virtualization components. The vulnerability arises from improper handling of scenarios where an Advanced Programmable Interrupt Controller (APIC) is absent from the system configuration, creating a dangerous condition that can be exploited by local attackers to compromise system stability and potentially escalate privileges.
The technical root cause of this vulnerability stems from a NULL pointer dereference condition within the update_cr8_intercept function. When the KVM subsystem attempts to manage virtual machine control structures and encounters a system configuration lacking proper APIC support, the function fails to validate the presence of required hardware components before attempting to access APIC-related data structures. This flaw directly maps to CWE-476 which identifies NULL pointer dereference as a common vulnerability pattern where software attempts to access memory through a pointer that has not been properly initialized or validated. The absence of proper null checks in the kernel's virtualization code path creates a predictable crash scenario that can be reliably triggered by local users.
From an operational perspective, this vulnerability presents a significant risk to virtualized environments as it allows local attackers with minimal privileges to either cause system crashes or potentially achieve privilege escalation. The denial of service aspect manifests as a system crash due to the NULL pointer dereference, which can result in complete system instability and require manual intervention to restore normal operation. The privilege escalation possibility, while more complex to exploit, represents a severe concern as it could allow attackers to gain elevated system privileges within the virtualized environment. This vulnerability affects systems that rely on KVM for virtualization and particularly impacts environments where virtual machines are configured to operate without proper APIC support or in configurations where APIC detection fails.
The exploitation of this vulnerability typically requires local access to the system and involves invoking the kvm_vcpu_ioctl function, which is part of the KVM virtualization interface. Attackers can leverage this to trigger the problematic code path in the update_cr8_intercept function, causing the kernel to attempt operations on uninitialized or null APIC references. This attack vector aligns with ATT&CK technique T1068 which covers "Local Port/Service Discovery" and T1059 which covers "Command and Scripting Interpreter" as attackers would need to execute specific kernel interface calls to trigger the vulnerability. The impact extends beyond simple service disruption to potentially compromising the integrity of the virtualization layer, which could affect multiple virtual machines running on the same host system.
Mitigation strategies for this vulnerability primarily involve upgrading to kernel versions 2.6.32-rc1 or later where the issue has been addressed through proper null pointer validation and APIC presence checking. System administrators should prioritize patching affected systems, particularly in virtualized environments where KVM is actively used. Additional protective measures include implementing proper access controls to limit local user privileges, monitoring for unusual system crashes or kernel oops messages that might indicate exploitation attempts, and ensuring that virtual machine configurations properly account for hardware capabilities. The fix implemented in the patched kernel versions typically involves adding comprehensive null checks before accessing APIC-related structures and ensuring that the virtualization subsystem gracefully handles configurations lacking APIC support, thereby preventing the NULL pointer dereference condition that led to system instability and potential privilege escalation opportunities.