CVE-2015-8952 in Linux
Summary
by MITRE
The mbcache feature in the ext2 and ext4 filesystem implementations in the Linux kernel before 4.6 mishandles xattr block caching, which allows local users to cause a denial of service (soft lockup) via filesystem operations in environments that use many attributes, as demonstrated by Ceph and Samba.
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Analysis
by VulDB Data Team • 09/26/2022
The vulnerability identified as CVE-2015-8952 represents a critical flaw in the Linux kernel's handling of extended attributes within the ext2 and ext4 filesystem implementations. This issue specifically affects kernel versions prior to 4.6 and manifests through improper management of the mbcache feature, which is responsible for caching metadata blocks during filesystem operations. The flaw becomes particularly pronounced when systems operate with numerous extended attributes, creating a scenario where the caching mechanism fails to properly manage memory resources and metadata structures. This vulnerability impacts widely deployed filesystem technologies including those used in Ceph distributed storage systems and Samba file sharing services, making it relevant across enterprise and cloud computing environments where extensive attribute usage is common.
The technical root cause of this vulnerability lies in the mbcache implementation's failure to correctly handle xattr block caching operations within the ext2 and ext4 filesystem drivers. When extended attributes are heavily utilized, the caching mechanism becomes overwhelmed with metadata blocks that are not properly released or managed, leading to a progressive degradation of system performance. The flaw creates a condition where the kernel's metadata cache becomes saturated with stale or improperly managed entries, causing the system to enter a soft lockup state where the kernel becomes unresponsive to further operations. This occurs because the mbcache subsystem fails to properly invalidate or update cached blocks when extended attribute operations modify the underlying filesystem structures, resulting in a cascade of failures that ultimately leads to system instability and denial of service conditions.
The operational impact of CVE-2015-8952 extends beyond simple system unresponsiveness to encompass complete service disruption in environments that rely heavily on extended attributes. Systems running Ceph storage clusters experience significant performance degradation and potential service outages when the mbcache mechanism becomes overwhelmed with attribute metadata, as these distributed systems frequently utilize extended attributes for storing metadata about objects and their properties. Similarly, Samba file servers that employ extended attributes for sharing permissions, user information, and other metadata characteristics become vulnerable to soft lockup conditions that can render entire file sharing services unavailable. The vulnerability particularly affects high-traffic environments where multiple concurrent filesystem operations occur simultaneously, as the cumulative effect of attribute operations rapidly exhausts the caching resources, leading to system-wide lockups that require manual intervention or system reboot to resolve.
Mitigation strategies for CVE-2015-8952 primarily focus on kernel version upgrades to 4.6 or later, which contain the necessary patches to properly handle extended attribute caching operations. System administrators should prioritize updating their kernel versions across all affected systems, particularly those running Ceph or Samba services that are likely to experience high extended attribute usage. Additional protective measures include monitoring filesystem attribute usage patterns and implementing resource limits on extended attribute operations to prevent the caching mechanism from becoming overwhelmed. Organizations should also consider implementing proactive system monitoring to detect early signs of mbcache saturation, such as increased filesystem operation times or memory usage patterns that indicate caching resource exhaustion. The vulnerability aligns with CWE-129 and CWE-476 as it involves improper handling of resource management and metadata structures, while also mapping to ATT&CK techniques related to system resource exhaustion and denial of service attacks that leverage kernel-level vulnerabilities.