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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.
References
Affected products
- <6.18.35
- <7.0
- =<6.18.*
- <6.7
- <6.20
- ==33a3149dd81a1e2f52b80ee1e0fc380b39f3d028
- =<7.0.*
- <91cc13978ab0bc6f669139f53e7e613a860d10e0
- =<6.12.*
- =<*
- <3e52417318473782012b236d0325bf7d2266a597
- <bca6386dc08750fc7cdcbc7683473748ba3114b9
- ==7.0
- <d14ae8ef88c2c6590e107db61b6adce148cec7b3
- <6.12.93
- ==b66920a3348c0f63ba18365248fa21fbf0b3a937