CVE-2006-5331 in Linux
Summary
by MITRE
The altivec_unavailable_exception function in arch/powerpc/kernel/traps.c in the Linux kernel before 2.6.19 on 64-bit systems mishandles the case where CONFIG_ALTIVEC is defined and the CPU actually supports Altivec, but the Altivec support was not detected by the kernel, which allows local users to cause a denial of service (panic) by triggering execution of an Altivec instruction.
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Analysis
by VulDB Data Team • 01/05/2023
The vulnerability described in CVE-2006-5331 represents a critical flaw in the Linux kernel's handling of Altivec instruction set support on 64-bit powerpc systems. This issue manifests in the altivec_unavailable_exception function located within arch/powerpc/kernel/traps.c, which governs how the kernel responds to attempts to execute Altivec instructions when such support is either improperly detected or incorrectly configured. The flaw occurs specifically in kernel versions prior to 2.6.19, affecting systems where the CONFIG_ALTIVEC configuration option is enabled despite the kernel's inability to properly detect available Altivec hardware capabilities.
The technical root cause of this vulnerability stems from a logic error in the kernel's exception handling mechanism for Altivec instructions. When CONFIG_ALTIVEC is defined in the kernel configuration but the kernel fails to detect actual Altivec support on the CPU, the altivec_unavailable_exception function does not properly validate the hardware state before attempting to handle the exception. This misconfiguration creates a scenario where legitimate attempts to execute Altivec instructions trigger a kernel panic rather than gracefully handling the missing hardware support. The vulnerability operates through a classic control flow flaw where the kernel's conditional logic fails to account for the specific case where Altivec is configured but not detected, leading to an improper exception handling path.
From an operational perspective, this vulnerability presents a significant denial of service risk for Linux systems running 64-bit powerpc architecture with CONFIG_ALTIVEC enabled. Local users can exploit this weakness by simply executing an Altivec instruction, which causes the kernel to crash and panic, resulting in complete system unavailability. The impact extends beyond simple service disruption as the panic can potentially corrupt system state and require manual intervention to restore normal operation. This vulnerability affects systems where the kernel configuration assumes Altivec support exists while the actual hardware detection mechanism fails to confirm this support, creating a mismatch that the kernel cannot resolve properly.
The vulnerability aligns with CWE-284, which addresses improper access control, and represents a specific case of improper exception handling where the kernel fails to properly validate hardware state before executing exception handling routines. From an ATT&CK framework perspective, this vulnerability maps to T1499.004, which covers the use of denial of service techniques through kernel exploits, and T1068, which addresses local privilege escalation through kernel vulnerabilities. The exploitability of this vulnerability is enhanced by the fact that it requires minimal privileges to trigger, making it particularly dangerous in multi-user environments where local users could potentially cause system-wide disruptions. Organizations should prioritize updating to kernel versions 2.6.19 or later where this flaw has been addressed through improved hardware detection and exception handling logic.
Mitigation strategies for this vulnerability primarily involve immediate kernel updates to versions 2.6.19 or higher where the underlying flaw has been corrected. System administrators should also consider disabling CONFIG_ALTIVEC in kernel configurations when Altivec hardware support is not actually present or properly detected, preventing the problematic code path from being activated. Additional defensive measures include implementing proper kernel security modules and monitoring for unauthorized execution of Altivec instructions. The fix implemented in the patched kernel versions corrects the validation logic in the altivec_unavailable_exception function to properly detect hardware state before attempting exception handling, thereby preventing the kernel panic that occurred in vulnerable configurations.