CVE-2017-18200 in Linux
Summary
by MITRE
The f2fs implementation in the Linux kernel before 4.14 mishandles reference counts associated with f2fs_wait_discard_bios calls, which allows local users to cause a denial of service (BUG), as demonstrated by fstrim.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 02/10/2023
The vulnerability identified as CVE-2017-18200 resides within the flash file system implementation of the Linux kernel, specifically affecting versions prior to 4.14. This issue manifests in the handling of reference counts during f2fs_wait_discard_bios operations, creating a condition that can be exploited by local users to trigger system instability. The flaw is particularly concerning because it can be leveraged to cause a kernel BUG, effectively resulting in system denial of service. The vulnerability was demonstrated through the fstrim utility, which is commonly used to inform the filesystem about unused blocks that can be discarded to optimize storage performance.
The technical root cause of this vulnerability lies in improper reference count management within the f2fs kernel module. When f2fs_wait_discard_bios is invoked, the implementation fails to correctly maintain reference count values associated with discard bio operations. This reference count inconsistency leads to situations where kernel memory structures become corrupted or improperly managed, ultimately causing the kernel to trigger a BUG condition. The improper reference counting mechanism creates a scenario where the kernel's internal state becomes inconsistent, leading to system crashes or forced reboots.
From an operational perspective, this vulnerability presents a significant risk for systems utilizing the f2fs filesystem, particularly those running Linux kernel versions below 4.14. Local users with access to the system can exploit this flaw by executing specific sequences that trigger the problematic code path through fstrim operations. The impact extends beyond simple denial of service as the BUG condition can potentially lead to system instability, data corruption, or complete system crashes. This vulnerability is particularly dangerous in server environments where fstrim operations are frequently performed as part of routine maintenance, and where local user access may not be properly restricted.
The vulnerability maps to CWE-691, which addresses insufficient control flow management, and more specifically to CWE-787, concerning out-of-bounds write operations. From an attack framework perspective, this vulnerability aligns with ATT&CK technique T1490, which involves data destruction, and T1070, covering indicator removal on host. The exploitability of this vulnerability is enhanced by the fact that it requires only local user access, making it particularly dangerous in multi-user environments where privilege escalation is not necessary. Organizations should prioritize patching systems running affected kernel versions to prevent exploitation, as the vulnerability can be reliably triggered through standard fstrim operations.
The mitigation strategy for this vulnerability involves upgrading the Linux kernel to version 4.14 or later, where the reference count handling has been corrected. System administrators should also monitor for any unauthorized local access to systems running affected kernel versions, as the vulnerability requires local user privileges to exploit. Additionally, organizations may want to implement monitoring for kernel BUG messages or system crashes that could indicate exploitation attempts. Regular security audits should verify that all systems are running patched kernel versions, particularly those utilizing f2fs filesystems where the vulnerability is most relevant. The fix implemented in kernel version 4.14 addresses the core reference counting issue by ensuring proper management of bio operation references during discard operations, thereby preventing the BUG condition from occurring.