---
id: CVE-2026-64525
title: 'xfrm: move policy_bydst RCU sync from per-netns .exit to .pre_exit'
summary: |-
  In the Linux kernel, the following vulnerability has been resolved:

  xfrm: move policy_bydst RCU sync from per-netns .exit to .pre_exit

  The struct pernet_operations docstring in include/net/net_namespace.h
  explicitly warns against block…
severity: none
vendor: Linux
product: Linux
affected:
  - >-
    Linux >= 438b1f668ad58f46ce699bb48e4698a7839e3f9e <
    bca6386dc08750fc7cdcbc7683473748ba3114b9
  - >-
    Linux >= 3733fce2871c9bca9dd18a1a23b1432ea215a094 <
    91cc13978ab0bc6f669139f53e7e613a860d10e0
  - >-
    Linux >= 069daad4f2ae9c5c108131995529d5f02392c446 <
    d14ae8ef88c2c6590e107db61b6adce148cec7b3
  - >-
    Linux >= 069daad4f2ae9c5c108131995529d5f02392c446 <
    3e52417318473782012b236d0325bf7d2266a597
  - Linux b2faee12538b5b008eaef65a4201eb29cb5cefb3
  - Linux b66920a3348c0f63ba18365248fa21fbf0b3a937
  - Linux 33a3149dd81a1e2f52b80ee1e0fc380b39f3d028
  - Linux >= 6.12.83 < 6.12.93
  - Linux >= 6.18.24 < 6.18.35
  - Linux >= 6.1.191 < 6.2
  - Linux >= 6.6.136 < 6.7
  - Linux >= 6.19.14 < 6.20
  - Linux 7.0
published: '2026-07-25'
updated: '2026-10-30'
sourceUpdated: '2026-10-30T12:27:58.969Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-64525'
references:
  - url: 'https://git.kernel.org/stable/c/bca6386dc08750fc7cdcbc7683473748ba3114b9'
  - url: 'https://git.kernel.org/stable/c/91cc13978ab0bc6f669139f53e7e613a860d10e0'
  - url: 'https://git.kernel.org/stable/c/d14ae8ef88c2c6590e107db61b6adce148cec7b3'
  - url: 'https://git.kernel.org/stable/c/3e52417318473782012b236d0325bf7d2266a597'
tags:
  - cve.org
epss: 0.00156
epssPercentile: 0.04808
ingestedAt: '2026-11-05T15:02:32.435Z'
---

## Overview

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

xfrm: move policy_bydst RCU sync from per-netns .exit to .pre_exit

The struct pernet_operations docstring in include/net/net_namespace.h
explicitly warns against blocking RCU primitives in .exit handlers:

    Exit methods using blocking RCU primitives, such as
    synchronize_rcu(), should be implemented via exit_batch.
    [...]
    Please, avoid synchronize_rcu() at all, where it's possible.

    Note that a combination of pre_exit() and exit() can
    be used, since a synchronize_rcu() is guaranteed between
    the calls.

xfrm_policy_fini() violates this: it calls synchronize_rcu() before
freeing the policy_bydst hash tables (so no RCU reader is mid-
traversal at free time), but runs from xfrm_net_ops.exit -- once per
namespace -- so a cleanup_net() of N namespaces pays N full RCU
grace periods serially.

Use the documented pre_exit/exit split. Move the policy flush (and
the workqueue drains it depends on) into a new .pre_exit handler;
xfrm_policy_fini() then runs in .exit and frees the hash tables
after the synchronize_rcu_expedited() that cleanup_net() guarantees
between the two phases. Providing O(1) RCU grace periods per batch
instead of O(N).

Observed on Linux 6.18 with a workload doing unshare(CLONE_NEWNET)
at ~13/sec sustained: cleanup_net() and the netns_wq rescuer kthread
both stuck in xfrm_policy_fini()'s synchronize_rcu(), >300k struct
net accumulated in the cleanup queue, Percpu in /proc/meminfo climbed
to 130+ GB on 256-CPU hosts, and memcg OOMs followed. setup_net and
__put_net counts were balanced, ruling out a refcount leak.

## Affected

- `Linux >= 438b1f668ad58f46ce699bb48e4698a7839e3f9e < bca6386dc08750fc7cdcbc7683473748ba3114b9`
- `Linux >= 3733fce2871c9bca9dd18a1a23b1432ea215a094 < 91cc13978ab0bc6f669139f53e7e613a860d10e0`
- `Linux >= 069daad4f2ae9c5c108131995529d5f02392c446 < d14ae8ef88c2c6590e107db61b6adce148cec7b3`
- `Linux >= 069daad4f2ae9c5c108131995529d5f02392c446 < 3e52417318473782012b236d0325bf7d2266a597`
- `Linux b2faee12538b5b008eaef65a4201eb29cb5cefb3`
- `Linux b66920a3348c0f63ba18365248fa21fbf0b3a937`
- `Linux 33a3149dd81a1e2f52b80ee1e0fc380b39f3d028`
- `Linux >= 6.12.83 < 6.12.93`
- `Linux >= 6.18.24 < 6.18.35`
- `Linux >= 6.1.191 < 6.2`
- `Linux >= 6.6.136 < 6.7`
- `Linux >= 6.19.14 < 6.20`
- `Linux 7.0`

## Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.