CVE-2018-12633 in Linux
Summary
by MITRE
An issue was discovered in the Linux kernel through 4.17.2. vbg_misc_device_ioctl() in drivers/virt/vboxguest/vboxguest_linux.c reads the same user data twice with copy_from_user. The header part of the user data is double-fetched, and a malicious user thread can tamper with the critical variables (hdr.size_in and hdr.size_out) in the header between the two fetches because of a race condition, leading to severe kernel errors, such as buffer over-accesses. This bug can cause a local denial of service and information leakage.
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Analysis
by VulDB Data Team • 03/28/2023
The vulnerability identified as CVE-2018-12633 represents a critical race condition flaw within the Linux kernel version 4.17.2 and earlier, specifically affecting the VirtualBox guest driver component. This issue resides in the vbg_misc_device_ioctl function located in drivers/virt/vboxguest/vboxguest_linux.c, where the kernel performs duplicate data retrieval operations that create exploitable timing windows for malicious actors. The vulnerability manifests when the kernel reads user-space data twice using the copy_from_user function, with the first read occurring for header information and the second for the actual data payload. The fundamental flaw lies in the kernel's failure to properly synchronize access to critical variables during this double-fetch operation, creating a window where malicious user threads can manipulate header parameters between the two data fetches.
The technical implementation of this vulnerability stems from a classic race condition pattern where the kernel's ioctl handler processes input data without proper serialization between multiple data access points. During the first copy_from_user operation, the kernel reads the header structure containing hdr.size_in and hdr.size_out fields that define buffer sizes for subsequent operations. However, the second copy_from_user call occurs without any mechanism to prevent modification of these critical parameters by concurrent user-space threads. This race condition allows an attacker to modify hdr.size_in and hdr.size_out values between the two fetch operations, potentially causing the kernel to perform buffer overflows or underflows when processing the subsequent data operations. The vulnerability is classified under CWE-362, which specifically addresses race conditions in kernel space operations, and aligns with ATT&CK technique T1068 which covers local privilege escalation through kernel exploits.
The operational impact of this vulnerability extends beyond simple denial of service to include potential information leakage and system instability. Local attackers with access to the affected system can exploit this race condition to trigger kernel memory corruption, leading to system crashes, data corruption, or in some cases, privilege escalation opportunities. The buffer over-accesses resulting from manipulated size parameters can cause the kernel to access memory outside of intended boundaries, potentially exposing sensitive kernel memory contents or corrupting critical data structures. This vulnerability affects systems running VirtualBox guest additions where the vboxguest driver is active, making it particularly relevant for virtualized environments and cloud computing platforms that utilize VirtualBox virtualization technologies. The exploitability of this vulnerability requires local access and the ability to manipulate kernel data structures through the ioctl interface, making it a local privilege escalation vector that could be leveraged by malicious users with user-level access to cause system-wide disruptions.
Mitigation strategies for CVE-2018-12633 involve both immediate patching and defensive programming approaches. The primary solution is applying the kernel patch released by the Linux kernel development team that addresses the race condition by ensuring proper synchronization between the two copy_from_user operations. This patch typically involves either eliminating the duplicate data fetches or implementing proper locking mechanisms to prevent concurrent modification of header parameters. Organizations should prioritize updating their kernel versions to 4.17.3 or later where this vulnerability has been resolved. Additionally, system administrators can implement runtime protections such as kernel lockdown modes and enhanced memory protection mechanisms to limit the impact of potential exploitation attempts. The vulnerability also highlights the importance of proper input validation and defensive programming practices in kernel space code, particularly when handling user-supplied data through ioctl interfaces. Security teams should monitor for potential exploitation attempts and implement network-based detection measures to identify suspicious ioctl operations targeting the affected driver component. Regular security audits of kernel modules and driver code should include specific checks for similar race condition patterns to prevent future vulnerabilities of this nature from being introduced into the kernel codebase.