136 lines
5 KiB
Plaintext
136 lines
5 KiB
Plaintext
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===============================================
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XFRM device - offloading the IPsec computations
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===============================================
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Shannon Nelson <shannon.nelson@oracle.com>
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Overview
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========
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IPsec is a useful feature for securing network traffic, but the
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computational cost is high: a 10Gbps link can easily be brought down
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to under 1Gbps, depending on the traffic and link configuration.
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Luckily, there are NICs that offer a hardware based IPsec offload which
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can radically increase throughput and decrease CPU utilization. The XFRM
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Device interface allows NIC drivers to offer to the stack access to the
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hardware offload.
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Userland access to the offload is typically through a system such as
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libreswan or KAME/raccoon, but the iproute2 'ip xfrm' command set can
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be handy when experimenting. An example command might look something
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like this:
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ip x s add proto esp dst 14.0.0.70 src 14.0.0.52 spi 0x07 mode transport \
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reqid 0x07 replay-window 32 \
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aead 'rfc4106(gcm(aes))' 0x44434241343332312423222114131211f4f3f2f1 128 \
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sel src 14.0.0.52/24 dst 14.0.0.70/24 proto tcp \
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offload dev eth4 dir in
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Yes, that's ugly, but that's what shell scripts and/or libreswan are for.
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Callbacks to implement
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======================
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/* from include/linux/netdevice.h */
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struct xfrmdev_ops {
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int (*xdo_dev_state_add) (struct xfrm_state *x);
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void (*xdo_dev_state_delete) (struct xfrm_state *x);
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void (*xdo_dev_state_free) (struct xfrm_state *x);
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bool (*xdo_dev_offload_ok) (struct sk_buff *skb,
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struct xfrm_state *x);
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void (*xdo_dev_state_advance_esn) (struct xfrm_state *x);
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};
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The NIC driver offering ipsec offload will need to implement these
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callbacks to make the offload available to the network stack's
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XFRM subsytem. Additionally, the feature bits NETIF_F_HW_ESP and
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NETIF_F_HW_ESP_TX_CSUM will signal the availability of the offload.
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Flow
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====
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At probe time and before the call to register_netdev(), the driver should
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set up local data structures and XFRM callbacks, and set the feature bits.
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The XFRM code's listener will finish the setup on NETDEV_REGISTER.
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adapter->netdev->xfrmdev_ops = &ixgbe_xfrmdev_ops;
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adapter->netdev->features |= NETIF_F_HW_ESP;
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adapter->netdev->hw_enc_features |= NETIF_F_HW_ESP;
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When new SAs are set up with a request for "offload" feature, the
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driver's xdo_dev_state_add() will be given the new SA to be offloaded
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and an indication of whether it is for Rx or Tx. The driver should
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- verify the algorithm is supported for offloads
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- store the SA information (key, salt, target-ip, protocol, etc)
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- enable the HW offload of the SA
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The driver can also set an offload_handle in the SA, an opaque void pointer
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that can be used to convey context into the fast-path offload requests.
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xs->xso.offload_handle = context;
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When the network stack is preparing an IPsec packet for an SA that has
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been setup for offload, it first calls into xdo_dev_offload_ok() with
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the skb and the intended offload state to ask the driver if the offload
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will serviceable. This can check the packet information to be sure the
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offload can be supported (e.g. IPv4 or IPv6, no IPv4 options, etc) and
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return true of false to signify its support.
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When ready to send, the driver needs to inspect the Tx packet for the
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offload information, including the opaque context, and set up the packet
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send accordingly.
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xs = xfrm_input_state(skb);
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context = xs->xso.offload_handle;
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set up HW for send
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The stack has already inserted the appropriate IPsec headers in the
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packet data, the offload just needs to do the encryption and fix up the
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header values.
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When a packet is received and the HW has indicated that it offloaded a
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decryption, the driver needs to add a reference to the decoded SA into
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the packet's skb. At this point the data should be decrypted but the
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IPsec headers are still in the packet data; they are removed later up
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the stack in xfrm_input().
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find and hold the SA that was used to the Rx skb
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get spi, protocol, and destination IP from packet headers
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xs = find xs from (spi, protocol, dest_IP)
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xfrm_state_hold(xs);
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store the state information into the skb
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skb->sp = secpath_dup(skb->sp);
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skb->sp->xvec[skb->sp->len++] = xs;
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skb->sp->olen++;
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indicate the success and/or error status of the offload
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xo = xfrm_offload(skb);
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xo->flags = CRYPTO_DONE;
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xo->status = crypto_status;
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hand the packet to napi_gro_receive() as usual
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In ESN mode, xdo_dev_state_advance_esn() is called from xfrm_replay_advance_esn().
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Driver will check packet seq number and update HW ESN state machine if needed.
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When the SA is removed by the user, the driver's xdo_dev_state_delete()
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is asked to disable the offload. Later, xdo_dev_state_free() is called
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from a garbage collection routine after all reference counts to the state
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have been removed and any remaining resources can be cleared for the
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offload state. How these are used by the driver will depend on specific
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hardware needs.
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As a netdev is set to DOWN the XFRM stack's netdev listener will call
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xdo_dev_state_delete() and xdo_dev_state_free() on any remaining offloaded
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states.
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