[PATCH ipsec-next v8 14/14] xfrm: add documentation for XFRM_MSG_MIGRATE_STATE

Antony Antony
2026-05-05T04:35:11Z

Add documentation for the new XFRM_MSG_MIGRATE_STATE netlink message,
which migrates a single SA identified by SPI and mark without involving
policies.

The document covers the motivation and design differences from the
existing XFRM_MSG_MIGRATE, the SA lookup mechanism, supported attributes
with their omit-to-inherit semantics, and usage examples.

Signed-off-by: Antony Antony <antony.antony@secunet.com>

---
v7->v8: unknown flags ignored
v6->v7: update docs to reflect the flags
v5->v6: added this patch
---
Documentation/networking/xfrm/index.rst | 1 +
.../networking/xfrm/xfrm_migrate_state.rst | 231 +++++++++++++++++++++
2 files changed, 232 insertions(+)

diff --git a/Documentation/networking/xfrm/index.rst b/Documentation/networking/xfrm/index.rst
index 7d866da836fe..90191848f8db 100644
--- a/Documentation/networking/xfrm/index.rst
+++ b/Documentation/networking/xfrm/index.rst
@@ -9,5 +9,6 @@ XFRM Framework

xfrm_device
xfrm_proc
+ xfrm_migrate_state
xfrm_sync
xfrm_sysctl
diff --git a/Documentation/networking/xfrm/xfrm_migrate_state.rst b/Documentation/networking/xfrm/xfrm_migrate_state.rst
new file mode 100644
index 000000000000..66e7f2e729d8
--- /dev/null
+++ b/Documentation/networking/xfrm/xfrm_migrate_state.rst
@@ -0,0 +1,231 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=====================
+XFRM SA Migrate State
+=====================
+
+Overview
+========
+
+``XFRM_MSG_MIGRATE_STATE`` migrates a single SA, looked up using SPI and
+mark, without involving policies. Unlike ``XFRM_MSG_MIGRATE``, which couples
+SA and policy migration and allows migrating multiple SAs in one call, this
+interface identifies the SA unambiguously via SPI and supports changing
+the reqid, addresses, encapsulation, selector, and offload.
+
+Because IKE daemons such as *wan manage policies independently of
+the kernel, this interface allows precise per-SA migration without
+requiring policy involvement. Optional netlink attributes follow an
+omit-to-inherit model: omitting an attribute preserves the value from
+the old SA. The ``flags`` field controls two exceptions: hardware offload
+is inherited by default and can be suppressed with
+``XFRM_MIGRATE_STATE_NO_OFFLOAD`` or overridden with ``XFRMA_OFFLOAD_DEV``;
+the new selector is taken from ``new_sel`` by default and can instead be
+derived from the new addresses with ``XFRM_MIGRATE_STATE_UPDATE_SEL``.
+
+SA Identification
+=================
+
+The struct is defined in ``include/uapi/linux/xfrm.h``. The SA is looked
+up using ``xfrm_state_lookup()`` with ``id.spi``,
+``id.daddr``, ``id.proto``, ``id.family``, and
+``old_mark.v & old_mark.m`` as the mark key::
+
+ struct xfrm_user_migrate_state {
+ struct xfrm_usersa_id id; /* spi, daddr, proto, family */
+ xfrm_address_t new_daddr;
+ xfrm_address_t new_saddr;
+ struct xfrm_mark old_mark; /* SA lookup: key = v & m */
+ struct xfrm_selector new_sel; /* new selector (see Flags) */
+ __u32 new_reqid;
+ __u32 flags; /* XFRM_MIGRATE_STATE_* */
+ __u16 new_family;
+ __u16 reserved;
+ };
+
+Supported Attributes
+====================
+
+The following fields in ``xfrm_user_migrate_state`` are always explicit
+and are not inherited from the existing SA. Passing zero is not equivalent
+to "keep unchanged" — zero is used as-is:
+
+- ``new_daddr`` - new destination address
+- ``new_saddr`` - new source address
+- ``new_family`` - new address family
+- ``new_reqid`` - new reqid (0 = no reqid)
+- ``new_sel`` - new selector; used when ``XFRM_MIGRATE_STATE_UPDATE_SEL`` is
+ not set (see `Flags`_ below)
+- ``flags`` - bitmask of ``XFRM_MIGRATE_STATE_*`` flags (see `Flags`_ below)
+
+The following netlink attributes are also accepted. Omitting an attribute
+inherits the value from the existing SA (omit-to-inherit).
+
+.. list-table::
+ :widths: 30 70
+ :header-rows: 1
+
+ * - Attribute
+ - Description
+ * - ``XFRMA_MARK``
+ - Mark on the migrated SA (``struct xfrm_mark``). Absent inherits
+ ``old_mark``. To use no mark on the new SA, send ``XFRMA_MARK``
+ with ``{0, 0}``.
+ * - ``XFRMA_ENCAP``
+ - UDP encapsulation template; only ``UDP_ENCAP_ESPINUDP`` is supported.
+ Set ``encap_type=0`` to remove encap.
+ * - ``XFRMA_OFFLOAD_DEV``
+ - Hardware offload configuration (``struct xfrm_user_offload``). Absent
+ copies offload from the existing SA. When
+ ``XFRM_MIGRATE_STATE_NO_OFFLOAD`` is set in ``flags``, the new SA has
+ no offload; this flag is mutually exclusive with ``XFRMA_OFFLOAD_DEV``
+ and sending both returns ``-EINVAL``.
+ * - ``XFRMA_SET_MARK``
+ - Output mark on the migrated SA; pair with ``XFRMA_SET_MARK_MASK``.
+ Send 0 to clear.
+ * - ``XFRMA_NAT_KEEPALIVE_INTERVAL``
+ - NAT keepalive interval in seconds. Requires encap. Send 0 to clear.
+ Automatically cleared when encap is removed; setting a non-zero
+ value without encap returns ``-EINVAL``.
+ * - ``XFRMA_MTIMER_THRESH``
+ - Mapping maxage threshold. Requires encap. Send 0 to clear.
+ Automatically cleared when encap is removed; setting a non-zero
+ value without encap returns ``-EINVAL``.
+
+The following SA properties are immutable and cannot be changed via
+``XFRM_MSG_MIGRATE_STATE``: algorithms (``XFRMA_ALG_*``), replay state,
+direction (``XFRMA_SA_DIR``), and security context (``XFRMA_SEC_CTX``).
+
+Flags
+=====
+
+The ``flags`` field in ``xfrm_user_migrate_state`` controls optional
+migration behaviour. Unknown flag bits are ignored.
+
+.. list-table::
+ :widths: 40 60
+ :header-rows: 1
+
+ * - Flag
+ - Description
+ * - ``XFRM_MIGRATE_STATE_NO_OFFLOAD``
+ - When set, the new SA has no hardware offload even when
+ ``XFRMA_OFFLOAD_DEV`` is absent. Without this flag, omitting
+ ``XFRMA_OFFLOAD_DEV`` copies the existing offload to the new SA.
+ Mutually exclusive with ``XFRMA_OFFLOAD_DEV``; sending both
+ returns ``-EINVAL``.
+ * - ``XFRM_MIGRATE_STATE_UPDATE_SEL``
+ - When set, the kernel validates that the existing SA selector is a
+ single-host entry matching the SA addresses (``prefixlen_s ==
+ prefixlen_d`` equal to 32 for IPv4 or 128 for IPv6, and addresses
+ matching ``id.daddr`` and ``props.saddr``). If the check passes,
+ the new selector is derived from ``new_daddr`` and ``new_saddr``
+ with the single-host mask for ``new_family``. A mismatch returns
+ ``-EINVAL``. When this flag is not set, ``new_sel`` is used as-is
+ for the migrated SA.
+
+Migration Steps
+===============
+
+#. Install a block policy to drop traffic on the affected selector.
+#. Remove the old policy.
+#. Call ``XFRM_MSG_MIGRATE_STATE`` for each SA.
+#. Reinstall the policies.
+#. Remove the block policy.
+
+Block Policy and IV Safety
+--------------------------
+
+Installing a block policy before migration is required to prevent
+traffic leaks and IV reuse in counter mode.
+
+AES-GCM IV uniqueness is critical: reusing a (key, IV) pair allows
+an attacker to recover the authentication subkey and forge
+authentication tags, breaking both confidentiality and integrity.
+
+``XFRM_MSG_MIGRATE_STATE`` atomically copies the sequence number and
+replay window from the old SA to the new SA and deletes the old SA.
+The block policy ensures no outgoing packets are sent in the migration
+window, preventing IV reuse under the same key.
+
+Feature Detection
+=================
+
+Userspace can probe for kernel support by sending a minimal
+``XFRM_MSG_MIGRATE_STATE`` message with a non-existent SPI:
+
+- ``-ENOPROTOOPT``: not supported (``CONFIG_XFRM_MIGRATE`` not enabled)
+- any other error: supported
+
+Userspace Notification on Success
+=================================
+
+On successful migration the kernel multicasts an
+``XFRM_MSG_MIGRATE_STATE`` message to the ``XFRMNLGRP_MIGRATE`` group.
+The fixed header is ``struct xfrm_user_migrate_state`` copied from the
+request, followed by the same set of netlink attributes that are
+accepted as input, with the differences noted below.
+
+Differences from the request
+-----------------------------
+
+.. list-table::
+ :widths: 25 75
+ :header-rows: 1
+
+ * - Field / Attribute
+ - Difference
+ * - ``new_sel``
+ - Contains the actual selector of the newly installed SA, not the
+ ``new_sel`` from the request. When
+ ``XFRM_MIGRATE_STATE_UPDATE_SEL`` is set the kernel derives the
+ selector from ``new_daddr`` / ``new_saddr``; the caller's
+ ``new_sel`` field is ignored in that case. The notification
+ always carries the real selector of the new SA.
+ * - ``XFRMA_SA_DIR``
+ - Present in the notification (set from the direction of the new
+ SA) but **not accepted as input** — direction is immutable.
+ * - ``flags``
+ - Echoed back as-is. ``XFRM_MIGRATE_STATE_NO_OFFLOAD`` and
+ ``XFRM_MIGRATE_STATE_UPDATE_SEL`` describe the request that was
+ made, not a property of the resulting SA.
+
+Attributes in the notification
+-------------------------------
+
+.. list-table::
+ :widths: 30 70
+ :header-rows: 1
+
+ * - Attribute
+ - Description
+ * - ``XFRMA_ENCAP``
+ - UDP encapsulation template, if configured on the new SA.
+ * - ``XFRMA_OFFLOAD_DEV``
+ - Hardware offload configuration, if active on the new SA.
+ * - ``XFRMA_MARK``
+ - Mark on the new SA, if set.
+ * - ``XFRMA_SET_MARK``
+ - Output mark on the new SA, if set.
+ * - ``XFRMA_SET_MARK_MASK``
+ - Output mark mask, present together with ``XFRMA_SET_MARK``.
+ * - ``XFRMA_MTIMER_THRESH``
+ - Mapping maxage threshold, if non-zero.
+ * - ``XFRMA_NAT_KEEPALIVE_INTERVAL``
+ - NAT keepalive interval, if non-zero.
+ * - ``XFRMA_SA_DIR``
+ - Direction of the new SA.
+
+Error Handling
+==============
+
+If the target SA tuple (daddr, SPI, proto, family) is occupied by an existing
+unrelated SA, the operation returns ``-EEXIST``. In this case both the old and
+the new SA are gone. The old SA cannot be restored as doing so would risk
+duplicate sequence number and IV reuse, which must not occur. Userspace should
+handle ``-EEXIST``, for example by re-establishing the SA at the IKE level.
+
+If the multicast notification (``XFRMNLGRP_MIGRATE``) fails to send,
+the migration itself has already completed successfully and the new SA
+is installed. The operation returns success, 0, with an extack warning,
+but listeners will not receive the migration event.

--
2.47.3