Linux Kernel CVE-2014-1438 vm86 FPU Exception Local Privilege Escalation Analysis

Key Information Summary Vulnerability Description Issue Description: The vm86 system call in Linux (used to emulate virtual 8086 mode in user space for running legacy 8086 software) is susceptible to triggering Floating-Point Unit (FPU) errors. These typically occur during handling of the FPU control register, especially causing unhandled exceptions on AMD K7/K8 architecture kernels, leading to CPU exceptions including kernel panic. Root Cause: Lack of proper handling of FPU exceptions during task switching, specifically when the instruction is executed. Exploitation Method 1. Proof-of-Concept (PoC): The Virtual86SwitchToEmmsFault.c program can trigger a kernel panic. Its effectiveness can be enhanced by combining it with a random code execution tool (Virtual86RandomCode.c), forming a reliable test case. 2. Advanced Exploitation: - Exploiting the instruction within the vm86 system call, which allows the FPU to enter the same state, leading to a NULL pointer dereference error. - Modifying shared memory extended attributes to manipulate the processor list, enabling the kernel to execute arbitrary code (e.g., altering executable file paths to create a SUID backdoor). 3. Fix: - A patch is provided to address the issue, confirmed to affect only specific AMD processor architectures. Impact and Exploitation Outcomes Can cause random tasks to be terminated, resulting in denial of service. On non-SMP 32-bit architectures, it may even trigger NULL pointer errors for privilege escalation. Due to hardware architecture limitations and the difficulty of local exploitation, the overall security impact on the system is considered low. Timeline and Related Materials Report Date: December 28, 2013; CVE assigned: CVE-2014-1438 Resource Links: - Patch - git - CVE - OSVDB - SCIP - Secunia Related Vulnerability Reports: Refer to discussions on the Linux Kernel Mailing List and other security channels.

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Linux Kernel CVE-2014-1438 vm86 FPU Exception Local Privilege Escalation Analysis