CVE-2009-0935 in Linux
Summary
by MITRE
The inotify_read function in the Linux kernel 2.6.27 to 2.6.27.13, 2.6.28 to 2.6.28.2, and 2.6.29-rc3 allows local users to cause a denial of service (OOPS) via a read with an invalid address to an inotify instance, which causes the device s event list mutex to be unlocked twice and prevents proper synchronization of a data structure for the inotify instance.
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Analysis
by VulDB Data Team • 04/13/2019
The vulnerability described in CVE-2009-0935 represents a critical race condition and double unlock issue within the Linux kernel's inotify subsystem. This flaw exists in kernel versions ranging from 2.6.27 through 2.6.27.13, 2.6.28 through 2.6.28.2, and up to 2.6.29-rc3, affecting systems that utilize inotify for file system event monitoring. The inotify mechanism is designed to provide efficient file system event notification capabilities, allowing applications to monitor file system activities without polling. When a local user processes a read operation on an inotify instance with an invalid memory address, the kernel's internal synchronization mechanisms become corrupted, leading to a kernel panic or system crash.
The technical root cause of this vulnerability lies in the improper handling of mutex locks within the inotify_read function. Specifically, the kernel's event list mutex gets unlocked twice during the execution flow when processing invalid read operations, which violates fundamental locking principles and creates a race condition scenario. This double unlock operation corrupts the internal data structures that maintain the state of inotify instances, causing the kernel to lose track of proper synchronization mechanisms. The flaw is classified under CWE-1174, which deals with improper locking, and represents a classic example of a resource management error that can lead to system instability and denial of service conditions.
The operational impact of CVE-2009-0935 is significant for any system running affected kernel versions, as local users can exploit this vulnerability to cause system crashes and denial of service attacks. Since the vulnerability requires local access but does not need elevated privileges, it presents a particular risk in multi-user environments where malicious users might attempt to disrupt system services. The vulnerability can be exploited by any user with access to the system who has the ability to open inotify instances and perform read operations with invalid memory addresses. This makes it particularly dangerous in shared hosting environments or systems where multiple users have access to local resources.
From an attack perspective, this vulnerability aligns with ATT&CK technique T1499.004, which covers "Toggle System File Integrity," as it can be used to disrupt system stability and availability. The exploitability of this vulnerability is relatively straightforward, requiring only basic knowledge of inotify system calls and memory management concepts. System administrators and security professionals should prioritize patching affected systems, as the vulnerability can be leveraged to cause persistent denial of service conditions that may require system reboot to resolve. The fix for this issue involved proper mutex handling and ensuring that lock operations are correctly matched with unlock operations to prevent the double unlock scenario that leads to the corruption of inotify instance data structures.
The broader implications of this vulnerability highlight the importance of proper synchronization mechanisms in kernel space operations and demonstrate how seemingly minor resource management errors can lead to catastrophic system failures. This flaw serves as a reminder of the critical nature of kernel-level security and the necessity of thorough code review processes for system call implementations. Organizations should implement regular kernel updates and maintain awareness of security advisories to prevent exploitation of such vulnerabilities that can compromise system availability and integrity. The vulnerability also underscores the need for robust testing of edge cases in kernel code, particularly around memory handling and synchronization primitives that are critical for system stability.