CVE-2019-9857 in Linux
Summary
by MITRE
In the Linux kernel through 5.0.2, the function inotify_update_existing_watch() in fs/notify/inotify/inotify_user.c neglects to call fsnotify_put_mark() with IN_MASK_CREATE after fsnotify_find_mark(), which will cause a memory leak (aka refcount leak). Finally, this will cause a denial of service.
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Analysis
by VulDB Data Team • 08/04/2023
The vulnerability identified as CVE-2019-9857 represents a critical memory management flaw within the Linux kernel's inotify subsystem, specifically affecting versions through 5.0.2. This issue resides in the fs/notify/inotify/inotify_user.c source file where the inotify_update_existing_watch() function fails to properly manage reference counts during watch descriptor updates. The flaw occurs when the function performs fsnotify_find_mark() to locate an existing mark but subsequently neglects to invoke fsnotify_put_mark() with the IN_MASK_CREATE flag, creating a scenario where reference count tracking becomes inconsistent.
The technical nature of this vulnerability stems from improper reference counting mechanics within the kernel's filesystem notification framework. When inotify_update_existing_watch() processes watch updates, it correctly identifies existing marks using fsnotify_find_mark() but fails to decrement the reference count appropriately through fsnotify_put_mark(). This oversight creates a memory leak where mark structures remain allocated in memory even when they should be freed, leading to gradual resource exhaustion. The vulnerability manifests as a reference count leak that accumulates over time, particularly when applications frequently update inotify watch descriptors.
The operational impact of this vulnerability extends beyond simple memory consumption to potentially cause system instability and denial of service conditions. As the reference count leak accumulates, the kernel's memory management becomes increasingly strained, eventually leading to resource exhaustion that can prevent new inotify watches from being created or cause the system to become unresponsive. This vulnerability affects any system running affected Linux kernel versions where inotify functionality is utilized, making it particularly dangerous in server environments or systems with high inotify activity. The denial of service aspect becomes more pronounced under sustained load conditions where the memory leak compounds over time.
From a cybersecurity perspective, this vulnerability aligns with CWE-401, which specifically addresses memory leaks in software systems. The flaw represents a classic case of improper resource management where reference counting mechanisms fail to properly account for object lifecycle management. The attack surface for this vulnerability includes any application or service that relies on inotify for file system monitoring, including web servers, database systems, and monitoring tools. The vulnerability's exploitability is moderate since it requires sustained inotify watch updates to manifest effectively, but the impact is severe as it can lead to complete system degradation.
Mitigation strategies for CVE-2019-9857 primarily focus on kernel version updates where the vulnerability has been patched, typically through kernel versions 5.1 and later. System administrators should prioritize upgrading to patched kernel releases to eliminate the risk entirely. Additionally, monitoring inotify usage patterns and implementing resource limits on inotify watches can help reduce the impact if exploitation occurs. The vulnerability also highlights the importance of proper reference counting in kernel subsystems and serves as a reminder of the critical nature of memory management in operating system components. Organizations should conduct vulnerability assessments to identify systems running affected kernel versions and implement remediation strategies accordingly. This vulnerability demonstrates how seemingly minor reference counting errors can result in significant security implications, emphasizing the need for rigorous code review and testing of kernel subsystems.