@@ -9,5 +9,6 @@ XFRM Framework
 
    xfrm_device
    xfrm_proc
+   xfrm_migrate_state
    xfrm_sync
    xfrm_sysctl

@@ -0,0 +1,129 @@
+.. 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, and other SA-specific parameters.
+
+Because IKE daemons such as strongSwan manage policies independently of
+the kernel, this interface allows precise per-SA migration without
+requiring policy involvement. Optional XFRM attributes follow an
+omit-to-inherit model.
+
+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``::
+
+    struct xfrm_user_migrate_state {
+        struct xfrm_usersa_id id;       /* spi, daddr, proto, family */
+        xfrm_address_t        new_daddr;
+        xfrm_address_t        new_saddr;
+        __u16                 new_family;
+        __u16                 reserved;
+        __u32                 new_reqid;
+        struct xfrm_mark      old_mark; /* SA lookup */
+    };
+
+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)
+
+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. Set ``ifindex=0`` to remove offload.
+   * - ``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``).
+
+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.
+
+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 deletes the old SA and installs
+the new one with the sequence counter and replay window copied. 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
+
+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.