CVE-2023-53205 in Linux
Summary
by MITRE • 09/15/2025
In the Linux kernel, the following vulnerability has been resolved:
KVM: s390/diag: fix racy access of physical cpu number in diag 9c handler
We do check for target CPU == -1, but this might change at the time we are going to use it. Hold the physical target CPU in a local variable to avoid out-of-bound accesses to the cpu arrays.
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Analysis
by VulDB Data Team • 01/10/2026
The vulnerability identified as CVE-2023-53205 affects the Linux kernel's KVM subsystem specifically on s390 architecture systems. This issue resides within the diagnostic instruction 9c handler which is responsible for managing certain hypervisor operations in virtualized environments. The flaw manifests as a race condition during the processing of diagnostic commands, where the system attempts to access physical CPU identifiers without proper synchronization mechanisms. This particular vulnerability demonstrates the complexity of managing concurrent access in virtualized environments where multiple threads or processes may be simultaneously attempting to interact with shared system resources.
The technical implementation of this vulnerability stems from improper handling of CPU identification during diagnostic operations. When the system processes a diagnostic instruction 9c command, it first checks if the target CPU is set to -1, which typically indicates all CPUs or a broadcast operation. However, the vulnerability occurs because this check does not prevent subsequent changes to the target CPU value during the execution of the handler. The race condition allows for a scenario where the initial validation passes, but by the time the system actually uses the CPU identifier, the target CPU value has been modified by another process or thread. This creates a situation where array bounds checking fails, potentially leading to out-of-bounds memory accesses that could result in system instability or security implications.
The operational impact of this vulnerability extends beyond simple system crashes or instability, as it represents a potential attack vector for privilege escalation or denial of service conditions. In virtualized environments, this flaw could allow malicious guest operating systems or compromised virtual machines to potentially access memory regions they should not be able to reach, or to cause the hypervisor to behave unpredictably. The vulnerability affects systems running Linux kernels with KVM virtualization support on s390 architecture, which includes IBM mainframe systems and other platforms that utilize this specific processor architecture. The race condition aspect makes this particularly concerning as it may be difficult to reproduce consistently, potentially leading to intermittent failures that are challenging to debug and patch.
The fix implemented for CVE-2023-53205 addresses the root cause by ensuring that the physical CPU number is captured in a local variable immediately after the initial validation check. This approach eliminates the race condition by preventing any modification of the target CPU value during the critical section of code execution. The solution follows established best practices for concurrent programming and memory safety, aligning with CWE-362 which addresses race conditions in software development. The mitigation strategy essentially implements a snapshot approach where the system captures the necessary state before proceeding with operations that could be affected by concurrent modifications, thereby preventing the out-of-bounds array access that could occur with the original implementation. This fix demonstrates the importance of proper synchronization mechanisms in kernel-level code and reflects the ATT&CK technique of privilege escalation through kernel exploitation pathways.
The resolution of this vulnerability highlights the critical nature of memory safety in hypervisor implementations and the importance of thorough testing in concurrent environments. The fix represents a defensive programming approach that prevents data races and ensures that system resources are accessed safely even when multiple processes or threads are operating simultaneously. This particular vulnerability underscores the need for comprehensive testing of virtualization components, especially in enterprise environments where mainframe systems and virtualized infrastructure are commonly deployed. The implementation of proper local variable capture and state management in the diagnostic handler prevents potential exploitation while maintaining the intended functionality of the KVM subsystem on s390 platforms.