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

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.