CVE-2017-18202 in Linux
Summary
by MITRE
The __oom_reap_task_mm function in mm/oom_kill.c in the Linux kernel before 4.14.4 mishandles gather operations, which allows attackers to cause a denial of service (TLB entry leak or use-after-free) or possibly have unspecified other impact by triggering a copy_to_user call within a certain time window.
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Analysis
by VulDB Data Team • 02/16/2023
The vulnerability identified as CVE-2017-18202 resides within the Linux kernel's memory management subsystem, specifically in the __oom_reap_task_mm function located in mm/oom_kill.c. This flaw represents a critical security issue that affects Linux kernel versions prior to 4.14.4, where the kernel's Out-Of-Memory (OOM) killer mechanism contains a fundamental flaw in how it handles memory management operations during the reaping process of tasks that have triggered OOM conditions. The vulnerability manifests when the kernel attempts to gather memory management information from processes that are being terminated due to memory exhaustion, creating a window where malicious actors can exploit the improper handling of these gather operations.
The technical implementation of this vulnerability stems from a race condition and improper memory management within the kernel's OOM handling code path. When the kernel encounters an OOM condition and attempts to reap memory management structures from processes, the __oom_reap_task_mm function fails to properly synchronize access to memory management data structures. This improper handling leads to situations where copy_to_user operations can be executed against memory that has already been freed or is in an inconsistent state, resulting in either a use-after-free condition or a TLB entry leak. The race condition occurs because the function does not adequately protect against concurrent access patterns that can cause memory management structures to be modified or deallocated while the gather operation is in progress.
The operational impact of this vulnerability extends beyond simple denial of service conditions to potentially enable more sophisticated attacks. An attacker who can trigger an OOM condition and then exploit the timing window for the copy_to_user call can cause the kernel to access freed memory locations, leading to unpredictable behavior that may result in system crashes, memory corruption, or potentially privilege escalation. The TLB entry leak aspect of this vulnerability can cause performance degradation and memory management instability, while the use-after-free condition creates opportunities for attackers to manipulate kernel memory structures. This vulnerability aligns with CWE-416, which describes the use of freed memory issue, and can be categorized under ATT&CK technique T1068, which covers "Exploitation for Privilege Escalation" through kernel vulnerabilities.
Mitigation strategies for CVE-2017-18202 primarily focus on upgrading to Linux kernel version 4.14.4 or later, where the vulnerability has been addressed through proper synchronization mechanisms and memory management handling in the OOM killer code path. System administrators should prioritize patching affected systems, particularly those running older kernel versions in production environments where memory pressure scenarios are common. Additionally, monitoring for OOM conditions and implementing proper memory limits for processes can help reduce the likelihood of triggering this vulnerability. The fix implemented in kernel 4.14.4 involves strengthening the synchronization primitives used during the memory management structure gathering process and ensuring that copy_to_user operations are only performed against valid memory regions. Organizations should also consider implementing kernel hardening measures such as kernel address space layout randomization and stack canaries to further reduce the attack surface and potential impact of similar vulnerabilities.