CVE-2018-17155 in FreeBSD
Summary
by MITRE
In FreeBSD before 11.2-STABLE(r338983), 11.2-RELEASE-p4, 11.1-RELEASE-p15, 10.4-STABLE(r338984), and 10.4-RELEASE-p13, due to insufficient initialization of memory copied to userland in the getcontext and swapcontext system calls, small amounts of kernel memory may be disclosed to userland processes. Unprivileged authenticated local users may be able to access small amounts privileged kernel data.
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Analysis
by VulDB Data Team • 05/19/2023
This vulnerability exists in FreeBSD operating systems prior to specific stable and release versions, representing a critical information disclosure flaw in the kernel's handling of context switching operations. The issue stems from inadequate memory initialization during the execution of getcontext and swapcontext system calls, which are fundamental components of the POSIX standard for managing execution contexts in multi-threaded applications. These system calls are designed to capture and restore the state of a process's execution context, including registers, stack pointers, and other processor state information. The vulnerability specifically affects the memory copying mechanism between kernel space and user space, where kernel memory contents are inadvertently exposed to unprivileged user processes through the improper initialization of memory regions that are subsequently copied to userland.
The technical flaw manifests when the kernel performs memory operations to transfer context information from kernel space to user space through these system calls. During the memory copying process, certain memory regions are not properly initialized or cleared before being transferred to userland, creating potential information leakage channels. This memory disclosure occurs because the kernel fails to zero out or properly sanitize memory areas that are subsequently copied to user processes, allowing fragments of kernel memory to persist in user-accessible memory regions. The vulnerability affects both getcontext and swapcontext system calls, which are used extensively in applications requiring context switching functionality, making the attack surface particularly broad. The disclosed kernel memory fragments may contain sensitive information including kernel pointers, stack contents, or other privileged data that could aid in further exploitation attempts.
The operational impact of this vulnerability is significant for local attackers who are authenticated to the system, as they can leverage this information disclosure to gain insights into the kernel's memory layout and internal state. While the amount of data disclosed is described as "small," even limited kernel memory exposure can provide attackers with valuable information for crafting more sophisticated attacks. The vulnerability enables attackers to potentially gather kernel addresses, stack contents, or other memory artifacts that could be used in combination with other exploits to bypass security mechanisms such as address space layout randomization. This information leakage represents a direct violation of the kernel's memory protection mechanisms and undermines the fundamental security isolation between kernel and user space. The vulnerability affects multiple FreeBSD versions and release branches, indicating it was a widespread issue that required coordinated patching across different code streams.
Mitigation strategies for this vulnerability involve applying the official FreeBSD patches and updates that properly initialize memory regions before copying data to user space. System administrators should immediately upgrade to the patched versions of FreeBSD that contain the necessary fixes for this memory initialization issue. The recommended approach includes updating to FreeBSD 11.2-STABLE after r338983, 11.2-RELEASE-p4, 11.1-RELEASE-p15, 10.4-STABLE after r338984, and 10.4-RELEASE-p13 or later versions. Additionally, organizations should implement monitoring for unauthorized local access attempts and consider implementing additional security controls such as mandatory access controls or enhanced logging of context switching operations. The vulnerability aligns with CWE-248, which addresses improper initialization of resources, and relates to ATT&CK technique T1059 for privilege escalation and T1068 for local privilege escalation through kernel exploitation. Organizations should also conduct security assessments to identify any potential exploitation attempts that may have occurred prior to patching, as the information disclosure could have been used to facilitate more advanced attacks against the system.