1.. SPDX-License-Identifier: GPL-2.0
2
3===============================================
4XFRM device - offloading the IPsec computations
5===============================================
6
7Shannon Nelson <shannon.nelson@oracle.com>
8Leon Romanovsky <leonro@nvidia.com>
9
10
11Overview
12========
13
14IPsec is a useful feature for securing network traffic, but the
15computational cost is high: a 10Gbps link can easily be brought down
16to under 1Gbps, depending on the traffic and link configuration.
17Luckily, there are NICs that offer a hardware based IPsec offload which
18can radically increase throughput and decrease CPU utilization.  The XFRM
19Device interface allows NIC drivers to offer to the stack access to the
20hardware offload.
21
22Right now, there are two types of hardware offload that kernel supports.
23 * IPsec crypto offload:
24   * NIC performs encrypt/decrypt
25   * Kernel does everything else
26 * IPsec packet offload:
27   * NIC performs encrypt/decrypt
28   * NIC does encapsulation
29   * Kernel and NIC have SA and policy in-sync
30   * NIC handles the SA and policies states
31   * The Kernel talks to the keymanager
32
33Userland access to the offload is typically through a system such as
34libreswan or KAME/raccoon, but the iproute2 'ip xfrm' command set can
35be handy when experimenting.  An example command might look something
36like this for crypto offload:
37
38  ip x s add proto esp dst 14.0.0.70 src 14.0.0.52 spi 0x07 mode transport \
39     reqid 0x07 replay-window 32 \
40     aead 'rfc4106(gcm(aes))' 0x44434241343332312423222114131211f4f3f2f1 128 \
41     sel src 14.0.0.52/24 dst 14.0.0.70/24 proto tcp \
42     offload dev eth4 dir in
43
44and for packet offload
45
46  ip x s add proto esp dst 14.0.0.70 src 14.0.0.52 spi 0x07 mode transport \
47     reqid 0x07 replay-window 32 \
48     aead 'rfc4106(gcm(aes))' 0x44434241343332312423222114131211f4f3f2f1 128 \
49     sel src 14.0.0.52/24 dst 14.0.0.70/24 proto tcp \
50     offload packet dev eth4 dir in
51
52  ip x p add src 14.0.0.70 dst 14.0.0.52 offload packet dev eth4 dir in
53  tmpl src 14.0.0.70 dst 14.0.0.52 proto esp reqid 10000 mode transport
54
55Yes, that's ugly, but that's what shell scripts and/or libreswan are for.
56
57
58
59Callbacks to implement
60======================
61
62::
63
64  /* from include/linux/netdevice.h */
65  struct xfrmdev_ops {
66        /* Crypto and Packet offload callbacks */
67	int	(*xdo_dev_state_add) (struct xfrm_state *x, struct netlink_ext_ack *extack);
68	void	(*xdo_dev_state_delete) (struct xfrm_state *x);
69	void	(*xdo_dev_state_free) (struct xfrm_state *x);
70	bool	(*xdo_dev_offload_ok) (struct sk_buff *skb,
71				       struct xfrm_state *x);
72	void    (*xdo_dev_state_advance_esn) (struct xfrm_state *x);
73
74        /* Solely packet offload callbacks */
75	void    (*xdo_dev_state_update_curlft) (struct xfrm_state *x);
76	int	(*xdo_dev_policy_add) (struct xfrm_policy *x, struct netlink_ext_ack *extack);
77	void	(*xdo_dev_policy_delete) (struct xfrm_policy *x);
78	void	(*xdo_dev_policy_free) (struct xfrm_policy *x);
79  };
80
81The NIC driver offering ipsec offload will need to implement callbacks
82relevant to supported offload to make the offload available to the network
83stack's XFRM subsystem. Additionally, the feature bits NETIF_F_HW_ESP and
84NETIF_F_HW_ESP_TX_CSUM will signal the availability of the offload.
85
86
87
88Flow
89====
90
91At probe time and before the call to register_netdev(), the driver should
92set up local data structures and XFRM callbacks, and set the feature bits.
93The XFRM code's listener will finish the setup on NETDEV_REGISTER.
94
95::
96
97		adapter->netdev->xfrmdev_ops = &ixgbe_xfrmdev_ops;
98		adapter->netdev->features |= NETIF_F_HW_ESP;
99		adapter->netdev->hw_enc_features |= NETIF_F_HW_ESP;
100
101When new SAs are set up with a request for "offload" feature, the
102driver's xdo_dev_state_add() will be given the new SA to be offloaded
103and an indication of whether it is for Rx or Tx.  The driver should
104
105	- verify the algorithm is supported for offloads
106	- store the SA information (key, salt, target-ip, protocol, etc)
107	- enable the HW offload of the SA
108	- return status value:
109
110		===========   ===================================
111		0             success
112		-EOPNETSUPP   offload not supported, try SW IPsec,
113                              not applicable for packet offload mode
114		other         fail the request
115		===========   ===================================
116
117The driver can also set an offload_handle in the SA, an opaque void pointer
118that can be used to convey context into the fast-path offload requests::
119
120		xs->xso.offload_handle = context;
121
122
123When the network stack is preparing an IPsec packet for an SA that has
124been setup for offload, it first calls into xdo_dev_offload_ok() with
125the skb and the intended offload state to ask the driver if the offload
126will serviceable.  This can check the packet information to be sure the
127offload can be supported (e.g. IPv4 or IPv6, no IPv4 options, etc) and
128return true of false to signify its support.
129
130Crypto offload mode:
131When ready to send, the driver needs to inspect the Tx packet for the
132offload information, including the opaque context, and set up the packet
133send accordingly::
134
135		xs = xfrm_input_state(skb);
136		context = xs->xso.offload_handle;
137		set up HW for send
138
139The stack has already inserted the appropriate IPsec headers in the
140packet data, the offload just needs to do the encryption and fix up the
141header values.
142
143
144When a packet is received and the HW has indicated that it offloaded a
145decryption, the driver needs to add a reference to the decoded SA into
146the packet's skb.  At this point the data should be decrypted but the
147IPsec headers are still in the packet data; they are removed later up
148the stack in xfrm_input().
149
150	find and hold the SA that was used to the Rx skb::
151
152		get spi, protocol, and destination IP from packet headers
153		xs = find xs from (spi, protocol, dest_IP)
154		xfrm_state_hold(xs);
155
156	store the state information into the skb::
157
158		sp = secpath_set(skb);
159		if (!sp) return;
160		sp->xvec[sp->len++] = xs;
161		sp->olen++;
162
163	indicate the success and/or error status of the offload::
164
165		xo = xfrm_offload(skb);
166		xo->flags = CRYPTO_DONE;
167		xo->status = crypto_status;
168
169	hand the packet to napi_gro_receive() as usual
170
171In ESN mode, xdo_dev_state_advance_esn() is called from xfrm_replay_advance_esn().
172Driver will check packet seq number and update HW ESN state machine if needed.
173
174Packet offload mode:
175HW adds and deletes XFRM headers. So in RX path, XFRM stack is bypassed if HW
176reported success. In TX path, the packet lefts kernel without extra header
177and not encrypted, the HW is responsible to perform it.
178
179When the SA is removed by the user, the driver's xdo_dev_state_delete()
180and xdo_dev_policy_delete() are asked to disable the offload.  Later,
181xdo_dev_state_free() and xdo_dev_policy_free() are called from a garbage
182collection routine after all reference counts to the state and policy
183have been removed and any remaining resources can be cleared for the
184offload state.  How these are used by the driver will depend on specific
185hardware needs.
186
187As a netdev is set to DOWN the XFRM stack's netdev listener will call
188xdo_dev_state_delete(), xdo_dev_policy_delete(), xdo_dev_state_free() and
189xdo_dev_policy_free() on any remaining offloaded states.
190
191Outcome of HW handling packets, the XFRM core can't count hard, soft limits.
192The HW/driver are responsible to perform it and provide accurate data when
193xdo_dev_state_update_curlft() is called. In case of one of these limits
194occuried, the driver needs to call to xfrm_state_check_expire() to make sure
195that XFRM performs rekeying sequence.
196