---
id: CVE-2021-46936
title: |-
  In the Linux kernel, the following vulnerability has been resolved:

  net: fix use-after-free in tw_timer_handler

  A real world panic issue was found as follow in Linux 5.4.

      BUG: unable to handle page fault for address: ffffde49a863d…
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  net: fix use-after-free in tw_timer_handler

  A real world panic issue was found as follow in Linux 5.4.

      BUG: unable to handle page fault for address: ffffde49a863d…
severity: high
cvss: 7.8
cvssVector: 'CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H'
cwe:
  - CWE-416
vendor: linux
product: linux_kernel
affected:
  - 'linux_kernel >= 2.6.27, < 4.4.298'
  - 'linux_kernel >= 4.5.0, < 4.9.296'
  - 'linux_kernel >= 4.10.0, < 4.14.261'
  - 'linux_kernel >= 4.15.0, < 4.19.224'
  - 'linux_kernel >= 4.20.0, < 5.4.170'
  - 'linux_kernel >= 5.5.0, < 5.10.90'
  - 'linux_kernel >= 5.11.0, < 5.15.13'
patched:
  - linux_kernel 5.15.13
published: '2024-02-27'
updated: '2026-08-04'
source: NVD
sourceUrl: 'https://nvd.nist.gov/vuln/detail/CVE-2021-46936'
references:
  - url: 'https://git.kernel.org/stable/c/08eacbd141e2495d2fcdde84358a06c4f95cbb13'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/15579e1301f856ad9385d720c9267c11032a5022'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/2386e81a1d277f540e1285565c9d41d531bb69d4'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/5c2fe20ad37ff56070ae0acb34152333976929b4'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/a8e1944b44f94f5c5f530e434c5eaee787254566'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/e22e45fc9e41bf9fcc1e92cfb78eb92786728ef0'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/e73164e89d1be561228a4534e1091369ee4ba41a'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/fe5838c22b986c1190f1dce9aa09bf6a491c1a69'
    label: 416baaa9-dc9f-4396-8d5f-8c081fb06d67
  - url: 'https://git.kernel.org/stable/c/08eacbd141e2495d2fcdde84358a06c4f95cbb13'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://git.kernel.org/stable/c/15579e1301f856ad9385d720c9267c11032a5022'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://git.kernel.org/stable/c/2386e81a1d277f540e1285565c9d41d531bb69d4'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://git.kernel.org/stable/c/5c2fe20ad37ff56070ae0acb34152333976929b4'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://git.kernel.org/stable/c/a8e1944b44f94f5c5f530e434c5eaee787254566'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://git.kernel.org/stable/c/e22e45fc9e41bf9fcc1e92cfb78eb92786728ef0'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://git.kernel.org/stable/c/e73164e89d1be561228a4534e1091369ee4ba41a'
    label: af854a3a-2127-422b-91ae-364da2661108
  - url: 'https://git.kernel.org/stable/c/fe5838c22b986c1190f1dce9aa09bf6a491c1a69'
    label: af854a3a-2127-422b-91ae-364da2661108
tags:
  - nvd
epss: 0.00246
epssPercentile: 0.14105
ingestedAt: '2026-08-04T10:39:38.434Z'
---

## Overview

In the Linux kernel, the following vulnerability has been resolved:

net: fix use-after-free in tw_timer_handler

A real world panic issue was found as follow in Linux 5.4.

    BUG: unable to handle page fault for address: ffffde49a863de28
    PGD 7e6fe62067 P4D 7e6fe62067 PUD 7e6fe63067 PMD f51e064067 PTE 0
    RIP: 0010:tw_timer_handler+0x20/0x40
    Call Trace:
     <IRQ>
     call_timer_fn+0x2b/0x120
     run_timer_softirq+0x1ef/0x450
     __do_softirq+0x10d/0x2b8
     irq_exit+0xc7/0xd0
     smp_apic_timer_interrupt+0x68/0x120
     apic_timer_interrupt+0xf/0x20

This issue was also reported since 2017 in the thread [1],
unfortunately, the issue was still can be reproduced after fixing
DCCP.

The ipv4_mib_exit_net is called before tcp_sk_exit_batch when a net
namespace is destroyed since tcp_sk_ops is registered befrore
ipv4_mib_ops, which means tcp_sk_ops is in the front of ipv4_mib_ops
in the list of pernet_list. There will be a use-after-free on
net->mib.net_statistics in tw_timer_handler after ipv4_mib_exit_net
if there are some inflight time-wait timers.

This bug is not introduced by commit f2bf415cfed7 ("mib: add net to
NET_ADD_STATS_BH") since the net_statistics is a global variable
instead of dynamic allocation and freeing. Actually, commit
61a7e26028b9 ("mib: put net statistics on struct net") introduces
the bug since it put net statistics on struct net and free it when
net namespace is destroyed.

Moving init_ipv4_mibs() to the front of tcp_init() to fix this bug
and replace pr_crit() with panic() since continuing is meaningless
when init_ipv4_mibs() fails.

[1] https://groups.google.com/g/syzkaller/c/p1tn-_Kc6l4/m/smuL_FMAAgAJ?pli=1

## Affected

- `linux_kernel >= 2.6.27, < 4.4.298`
- `linux_kernel >= 4.5.0, < 4.9.296`
- `linux_kernel >= 4.10.0, < 4.14.261`
- `linux_kernel >= 4.15.0, < 4.19.224`
- `linux_kernel >= 4.20.0, < 5.4.170`
- `linux_kernel >= 5.5.0, < 5.10.90`
- `linux_kernel >= 5.11.0, < 5.15.13`

## Remediation

Upgrade past the affected range:

- `linux_kernel 5.15.13`