Linux Kernel RCU Grace Period Bottleneck in xfrm_policy_fini()
CVE-2026-64525 Published on July 25, 2026
xfrm: move policy_bydst RCU sync from per-netns .exit to .pre_exit
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.
Products Associated with CVE-2026-64525
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Affected Versions
Linux:- Version 438b1f668ad58f46ce699bb48e4698a7839e3f9e and below bca6386dc08750fc7cdcbc7683473748ba3114b9 is affected.
- Version 3733fce2871c9bca9dd18a1a23b1432ea215a094 and below 91cc13978ab0bc6f669139f53e7e613a860d10e0 is affected.
- Version 069daad4f2ae9c5c108131995529d5f02392c446 and below d14ae8ef88c2c6590e107db61b6adce148cec7b3 is affected.
- Version 069daad4f2ae9c5c108131995529d5f02392c446 and below 3e52417318473782012b236d0325bf7d2266a597 is affected.
- Version b66920a3348c0f63ba18365248fa21fbf0b3a937 is affected.
- Version 33a3149dd81a1e2f52b80ee1e0fc380b39f3d028 is affected.
- Version 6.12.83 and below 6.12.93 is affected.
- Version 6.18.24 and below 6.18.35 is affected.
- Version 6.6.136 and below 6.7 is affected.
- Version 6.19.14 and below 6.20 is affected.
- Version 7.0 is affected.
- Before 7.0 is unaffected.
- Version 6.12.93, <= 6.12.* is unaffected.
- Version 6.18.35, <= 6.18.* is unaffected.
- Version 7.0.12, <= 7.0.* is unaffected.
- Version 7.1, <= * is unaffected.