Incorrect Synchronization Affecting kernel6.18-tools-debuginfo package, versions <1:6.18.35-68.127.amzn2023


Severity

Recommended
0.0
high
0
10

Based on Amazon Linux security rating.

Threat Intelligence

EPSS
0.16% (6th percentile)

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  • Snyk IDSNYK-AMZN2023-KERNEL618TOOLSDEBUGINFO-18905084
  • published18 Aug 2026
  • disclosed25 Jul 2026

Introduced: 25 Jul 2026

NewCVE-2026-64525  (opens in a new tab)
CWE-821  (opens in a new tab)

How to fix?

Upgrade Amazon-Linux:2023 kernel6.18-tools-debuginfo to version 1:6.18.35-68.127.amzn2023 or higher.
This issue was patched in ALAS2023-2026-1881.

NVD Description

Note: Versions mentioned in the description apply only to the upstream kernel6.18-tools-debuginfo package and not the kernel6.18-tools-debuginfo package as distributed by Amazon-Linux. See How to fix? for Amazon-Linux:2023 relevant fixed versions and status.

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&#39;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.

CVSS Base Scores

version 3.1