CVE-2023-4623: 
Linux Kernel vulnerability analysis and mitigation

A use-after-free vulnerability was discovered in the Linux kernel's net/sched: schhfsc (HFSC qdisc traffic control) component that could be exploited to achieve local privilege escalation. The vulnerability was identified in September 2023 and affects Linux kernel versions from 2.6.12 up to versions before the patch commit b3d26c5702c7d6c45456326e56d2ccf3f103e60f. The issue occurs when a class with a link-sharing curve (with HFSCFSC flag set) has a parent without a link-sharing curve, leading to a dangling pointer situation (NVD).

The vulnerability stems from a missing configuration check in the schhfsc network scheduler. When a class with a link-sharing curve (HFSCFSC flag set) has a parent without a link-sharing curve, the initvf() function calls vttreeinsert() on the parent, but vttreeremove() is skipped in updatevf(), resulting in a dangling pointer that can cause a use-after-free condition. The vulnerability has been assigned a CVSS v3.1 base score of 7.8 (HIGH) with vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H (NVD).

The vulnerability allows a local user with CAPNETADMIN capability in any user or network namespace to cause a denial of service (system crash or memory corruption) or potentially achieve local privilege escalation. This could lead to complete system compromise by allowing attackers to execute arbitrary code with elevated privileges (Debian LTS).

The vulnerability has been fixed in Linux kernel commit b3d26c5702c7d6c45456326e56d2ccf3f103e60f, which adds validation to ensure that parent classes must have an FSC curve. The fix has been backported to various Linux distributions including Red Hat Enterprise Linux, Ubuntu, and Debian. Users are recommended to upgrade their kernel to versions containing this fix (Kernel Commit).