CVE-2021-46936: 
Linux Kernel vulnerability analysis and mitigation

CVE-2021-46936 is a use-after-free vulnerability discovered in the Linux kernel's networking subsystem, specifically in the tw_timer_handler function. The vulnerability was first reported in 2017 and affects Linux kernel versions from 2.6.27 through 5.15.13. The issue occurs when a net namespace is destroyed while there are inflight time-wait timers (NVD).

The vulnerability stems from a race condition where ipv4_mib_exit_net is called before tcp_sk_exit_batch during net namespace destruction. This occurs because tcp_sk_ops is registered before ipv4_mib_ops in the pernet_list. The issue results in a use-after-free condition on net->mib.net_statistics in tw_timer_handler. The bug was introduced by commit 61a7e26028b9 which put net statistics on struct net and freed it when the net namespace was destroyed. The vulnerability has 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 can lead to a system crash (denial of service) through a page fault error when attempting to access freed memory. In more severe cases, it could potentially allow for arbitrary code execution. The issue affects system availability, confidentiality, and integrity with high severity ratings across all three aspects (NVD).

The issue has been fixed by moving init_ipv4_mibs() to execute before tcp_init() and replacing pr_crit() with panic() since continuing execution is meaningless when init_ipv4_mibs() fails. Users should update to patched kernel versions that include the fix (Kernel Patch).