CVE-2015-8845 in Linux
Summary
by MITRE
The tm_reclaim_thread function in arch/powerpc/kernel/process.c in the Linux kernel before 4.4.1 on powerpc platforms does not ensure that TM suspend mode exists before proceeding with a tm_reclaim call, which allows local users to cause a denial of service (TM Bad Thing exception and panic) via a crafted application.
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Analysis
by VulDB Data Team • 07/27/2022
The vulnerability described in CVE-2015-8845 represents a critical flaw in the Linux kernel's handling of Transactional Memory (TM) operations on PowerPC architectures. This issue resides within the tm_reclaim_thread function located in arch/powerpc/kernel/process.c, where the kernel fails to properly validate the existence of TM suspend mode before executing a tm_reclaim call. The absence of this validation creates a condition where malicious or malformed applications can trigger unexpected system behavior. Transactional Memory is a hardware feature designed to improve program execution by allowing operations to be executed atomically, and when improperly handled, can lead to catastrophic system failures.
The technical exploitation of this vulnerability occurs when a local user crafts a specific application that triggers the tm_reclaim_thread function without proper TM suspend mode verification. This flaw directly relates to CWE-691, which addresses insufficient control flow management in systems that handle transactional memory operations. The kernel's failure to check for TM suspend mode existence creates a path where the tm_reclaim call can proceed even when the necessary hardware state is not properly initialized, leading to what is known as a "TM Bad Thing" exception. This exception type is specifically designed to handle invalid transactional memory operations and results in immediate system panic and complete system shutdown.
From an operational impact perspective, this vulnerability represents a significant denial of service threat that affects systems running Linux kernel versions prior to 4.4.1 on PowerPC platforms. The consequence of exploitation is a complete system crash that renders the affected machine unusable until manual reboot occurs. This vulnerability is particularly concerning in environments where system availability is critical, as it allows any local user to potentially disrupt system operations without requiring elevated privileges. The attack vector is relatively simple since it only requires local execution capability and does not necessitate network access or complex privilege escalation. The vulnerability affects the fundamental kernel threading and memory management subsystems, making it a core system risk rather than an application-level issue.
The mitigation strategy for CVE-2015-8845 involves upgrading to Linux kernel version 4.4.1 or later, where the proper validation of TM suspend mode existence has been implemented. This upgrade addresses the root cause by ensuring that the tm_reclaim_thread function properly checks for the required TM suspend mode before proceeding with the reclaim operation. Organizations should also consider implementing additional monitoring and logging to detect potential exploitation attempts, as the system panic behavior makes detection challenging. The fix aligns with ATT&CK technique T1499.004, which covers system shutdown/reboot attacks, though in this case the attack is more subtle and targeted toward specific kernel subsystems rather than general system disruption. System administrators should prioritize this patch across all PowerPC-based systems, particularly those running older kernel versions, to prevent potential exploitation that could lead to service interruptions and operational downtime.