CVE-2017-1000255 in Linux
Summary
by MITRE
On Linux running on PowerPC hardware (Power8 or later) a user process can craft a signal frame and then do a sigreturn so that the kernel will take an exception (interrupt), and use the r1 value *from the signal frame* as the kernel stack pointer. As part of the exception entry the content of the signal frame is written to the kernel stack, allowing an attacker to overwrite arbitrary locations with arbitrary values. The exception handling does produce an oops, and a panic if panic_on_oops=1, but only after kernel memory has been over written. This flaw was introduced in commit: "5d176f751ee3 (powerpc: tm: Enable transactional memory (TM) lazily for userspace)" which was merged upstream into v4.9-rc1. Please note that kernels built with CONFIG_PPC_TRANSACTIONAL_MEM=n are not vulnerable.
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Analysis
by VulDB Data Team • 01/05/2023
This vulnerability represents a critical kernel-level privilege escalation flaw affecting Linux systems running on PowerPC hardware, specifically Power8 and later processors. The vulnerability stems from improper handling of signal frame data during exception processing, creating a pathway for user-space processes to manipulate kernel execution flow through crafted signal frames. The flaw exploits the interaction between signal handling mechanisms and the kernel's exception entry routines, allowing attackers to manipulate the kernel stack pointer through the r1 register value stored in the signal frame. This creates a fundamental breakdown in kernel memory protection boundaries, enabling arbitrary memory overwrite capabilities that can be leveraged for privilege escalation.
The technical implementation of this vulnerability involves the exploitation of the signal return mechanism within the PowerPC architecture's kernel. When a user process crafts a malicious signal frame and executes a sigreturn operation, the kernel's exception handling code retrieves the r1 register value from the signal frame and uses it as the kernel stack pointer. During the exception entry process, the signal frame contents are written to kernel stack memory locations, effectively allowing an attacker to control what data gets written where. This memory corruption occurs before the kernel can detect and respond to the violation, as the system only produces an oops message and potential panic after the memory has already been compromised. The vulnerability was introduced in kernel commit 5d176f751ee3 which enabled transactional memory support for userspace in version 4.9-rc1, fundamentally altering how kernel stacks are managed during signal handling operations.
The operational impact of this vulnerability is severe and potentially catastrophic for affected systems. An attacker with user-level privileges can leverage this flaw to achieve full kernel compromise, effectively bypassing all kernel security mechanisms including privilege separation, memory protection, and access controls. The vulnerability allows for arbitrary code execution within kernel space, enabling attackers to modify kernel data structures, escalate privileges to root access, and potentially gain complete system control. The fact that this vulnerability only triggers a panic after memory corruption has already occurred means that attackers can perform extensive damage before the system recognizes the compromise. This makes detection and prevention particularly challenging as the system continues to operate normally until the panic occurs, potentially allowing for extended periods of unauthorized access.
Mitigation strategies for this vulnerability focus on both immediate system hardening and long-term architectural solutions. The most effective immediate mitigation is to disable transactional memory support in kernel configuration by setting CONFIG_PPC_TRANSACTIONAL_MEM=n, which completely eliminates the vulnerability as transactional memory support is not available. System administrators should also implement strict kernel parameter controls including setting panic_on_oops=1 to ensure immediate system response to memory corruption events, though this does not prevent the initial compromise. Additionally, kernel updates to versions that contain the specific fix for commit 5d176f751ee3 are essential for permanent remediation. Organizations should also consider implementing runtime monitoring solutions that can detect anomalous kernel stack behavior and signal frame manipulation patterns. The vulnerability aligns with CWE-122 (Heap-based Buffer Overflow) and CWE-787 (Out-of-bounds Write) categories, and represents a technique categorized under ATT&CK tactic T1068 (Exploitation for Privilege Escalation) with potential use of T1059 (Command and Scripting Interpreter) for maintaining access post-exploitation.