CVE-2021-46904: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-46904 is a vulnerability discovered in the Linux kernel's HSO (High Speed Option) USB driver. The issue involves a null pointer dereference during TTY device unregistration. The vulnerability was publicly disclosed on February 26, 2024, and affects multiple versions of the Linux kernel from version 2.6.27 up to versions before 4.4.268, 4.9.268, 4.14.232, 4.19.187, and 5.4.112 (NVD).

The vulnerability occurs when multiple TTY devices attempt to claim the same minor number, causing a double unregistration of the same device. The first unregistration succeeds, but subsequent attempts result in a null pointer dereference. The root cause lies in the get_free_serial_index() function, which returns an available minor number but doesn't assign it immediately, allowing multiple callers to receive the same minor number before assignment (Kernel Patch). The vulnerability has been assigned a CVSS v3.1 base score of 5.5 (Medium) with vector AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H (NVD).

The vulnerability can lead to a denial of service condition through system crashes when the null pointer dereference is triggered during TTY device unregistration. The impact is limited to availability, with no direct effects on confidentiality or integrity of the system (Red Hat).

The vulnerability has been fixed by modifying the get_free_serial_index() function to assign the minor number immediately after finding an available one. The function was renamed to obtain_minor() to better reflect its behavior, and set_serial_by_index() was renamed to release_minor(). The fix ensures proper minor number management and prevents double unregistration issues. Multiple Linux distributions have released kernel updates containing the fix (Kernel Patch).