Welcome to our detailed exploration of a recently addressed security vulnerability in the Linux kernel, identified as CVE-2021-46904. With a severity rating of MEDIUM and a score of 5.5, this issue deserves attention, especially among users and administrators of Linux-based systems. Today, we'll break down what the vulnerability is, which parts of the kernel it affects, the risk it poses, and how it can be mitigated.
CVE-2021-46904 is a security flaw in the Linux kernel's handling of TTY (teletype) device unregistrations within the 'net: hso' component. Specifically, the vulnerability occurs when multiple TTY devices attempt to claim the same minor number, leading to potential improper unregistrations and, critically, a null-pointer dereference.
This issue originates in the way minor numbers are managed. The kernel function get_free_serial_index() was designed to return an available minor number but did not assign it immediately. This delay allowed subsequent calls to this function to return the same minor number, causing multiple devices to improperly unregister the same device identifier. The first attempt to unregister a device would succeed while subsequent attempts would point to a null or uninitialized space in memory, creating a critical stability and security risk.
The primary impact of CVE-2021-46904 hinges on potential system instability due to improper TTY device handling. Systems running affected versions of the kernel could experience crashes or erratic behavior due to null-pointer dereferences if multiple TTY devices are dynamically registered or unregistered, particularly in environments utilizing serial communications extensively, such as data centers and networking environments.
The fix involves significant alterations to the functions managing minor numbers. The function formerly known as get_free_serial_index() has been renamed to obtain_minor() and updated to immediately assign a minor number once it finds one available. This direct assignment prevents other processes from claiming the same minor number concurrently. Another function, set_serial_by_index(), has been renamed to release_minor(). This directly corresponds to releasing a minor number once a TTY device is unregistered, ensuring that all resources are correctly freed and no overlaps occur.
For Linux users and administrators, it is crucial to address this vulnerability promptly to maintain system stability and security. Patches for CVE-2021-46904 are available through standard update channels for most Linux distributions. We strongly recommend applying these patches without delay to mitigate any risks posed by this flaw.
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Stay informed, stay secure, and ensure your systems are always running flawlessly by adhering to the latest security practices and patch management protocols. Remember, every obtain_minor() should have a corresponding release_minor() call – it's not just good coding, it's crucial for maintaining the integrity and security of your Linux servers.