CVE-2026-64525: xfrm: move policy_bydst RCU sync from per-netns .exit to .pre_exit

From: Greg Kroah-Hartman <[email protected]>

Description
===========

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.

The Linux kernel CVE team has assigned CVE-2026-64525 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 6.12.83 with commit 438b1f668ad58f46ce699bb48e4698a7839e3f9e and fixed in 6.12.93 with commit bca6386dc08750fc7cdcbc7683473748ba3114b9
	Issue introduced in 6.18.24 with commit 3733fce2871c9bca9dd18a1a23b1432ea215a094 and fixed in 6.18.35 with commit 91cc13978ab0bc6f669139f53e7e613a860d10e0
	Issue introduced in 7.0 with commit 069daad4f2ae9c5c108131995529d5f02392c446 and fixed in 7.0.12 with commit d14ae8ef88c2c6590e107db61b6adce148cec7b3
	Issue introduced in 7.0 with commit 069daad4f2ae9c5c108131995529d5f02392c446 and fixed in 7.1 with commit 3e52417318473782012b236d0325bf7d2266a597
	Issue introduced in 6.6.136 with commit b66920a3348c0f63ba18365248fa21fbf0b3a937
	Issue introduced in 6.19.14 with commit 33a3149dd81a1e2f52b80ee1e0fc380b39f3d028

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-64525
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	net/xfrm/xfrm_policy.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/bca6386dc08750fc7cdcbc7683473748ba3114b9
	https://git.kernel.org/stable/c/91cc13978ab0bc6f669139f53e7e613a860d10e0
	https://git.kernel.org/stable/c/d14ae8ef88c2c6590e107db61b6adce148cec7b3
	https://git.kernel.org/stable/c/3e52417318473782012b236d0325bf7d2266a597