CVE-2015-8844 in Linux
Summary
by MITRE
The signal implementation in the Linux kernel before 4.3.5 on powerpc platforms does not check for an MSR with both the S and T bits set, 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 • 07/26/2022
The vulnerability identified as CVE-2015-8844 represents a critical flaw in the Linux kernel's signal handling implementation specifically affecting powerpc platforms. This issue arises from insufficient validation of the Machine State Register (MSR) during signal processing operations, creating a pathway for local attackers to exploit the system's exception handling mechanisms. The flaw exists in kernel versions prior to 4.3.5 and demonstrates the importance of proper register state validation in privileged system operations.
The technical root cause of this vulnerability lies in the improper validation of the MSR register, which contains critical processor state information including the S (Supervisor) and T (Trap) bits. When a signal is delivered to a process, the kernel must properly validate the processor state before proceeding with signal handling. The absence of checking for both S and T bits being simultaneously set creates a condition where malicious applications can manipulate the processor state to trigger an unexpected system behavior. This specific combination of bits in the MSR register indicates a transition to a state where the processor should be in a trap mode, but the kernel fails to properly handle this condition during signal processing.
The operational impact of CVE-2015-8844 manifests as a denial of service condition that can result in a TM Bad Thing exception followed by a system panic. This type of vulnerability allows local users to effectively crash the system or render it unresponsive, making it particularly dangerous in environments where local access is possible. The vulnerability operates at the kernel level, meaning that successful exploitation can lead to complete system compromise or availability disruption. The attack vector requires local access and involves crafting a specific application that can manipulate the processor state to trigger the problematic code path during signal delivery.
From a cybersecurity perspective, this vulnerability aligns with CWE-119, which addresses improper restriction of operations within a defined access scope, and demonstrates the risks associated with insufficient input validation in kernel-space operations. The issue also relates to ATT&CK technique T1068, which covers 'Exploitation for Privilege Escalation' as local users can leverage this vulnerability to cause system instability. The vulnerability's classification as a kernel-level flaw makes it particularly concerning for system administrators and security professionals responsible for maintaining system availability and stability. Organizations should prioritize updating their Linux kernel implementations to versions 4.3.5 or later to address this vulnerability and prevent potential exploitation.
The broader implications of this vulnerability extend beyond simple denial of service, as it highlights the complexity of processor state management in kernel implementations. The powerpc architecture's specific handling of MSR bits requires careful attention to ensure that all possible state combinations are properly validated. This vulnerability serves as a reminder of the importance of thorough testing and validation of kernel code, particularly in areas that handle privileged operations and system state transitions. Security teams should consider this vulnerability as part of their comprehensive kernel security assessment, ensuring that similar validation gaps are not present in other system components.