xfrm: iptfs: fix skb_put() panic on non-linear skb during reassembly

In the Linux kernel, the following vulnerability has been resolved:

xfrm: iptfs: fix skb_put() panic on non-linear skb during reassembly

In iptfs_reassem_cont(), IP-TFS attempts to append data to the new inner
packet 'newskb' that is being reassembled. First a zero-copy approach is
tried if it succeeds then newskb becomes non-linear.

When a subsequent fragment in the same datagram does not meet the
fast-path conditions, a memory copy is performed. It calls skb_put() to
append the data and as newskb is non-linear it triggers
SKB_LINEAR_ASSERT check.

Oops: invalid opcode: 0000 [#1] SMP NOPTI
[...]
RIP: 0010:skb_put+0x3c/0x40
[...]
Call Trace:
<IRQ>
iptfs_reassem_cont+0x1ab/0x5e0 [xfrm_iptfs]
iptfs_input_ordered+0x2af/0x380 [xfrm_iptfs]
iptfs_input+0x122/0x3e0 [xfrm_iptfs]
xfrm_input+0x91e/0x1a50
xfrm4_esp_rcv+0x3a/0x110
ip_protocol_deliver_rcu+0x1d7/0x1f0
ip_local_deliver_finish+0xbe/0x1e0
__netif_receive_skb_core.constprop.0+0xb56/0x1120
__netif_receive_skb_list_core+0x133/0x2b0
netif_receive_skb_list_internal+0x1ff/0x3f0
napi_complete_done+0x81/0x220
virtnet_poll+0x9d6/0x116e [virtio_net]
__napi_poll.constprop.0+0x2b/0x270
net_rx_action+0x162/0x360
handle_softirqs+0xdc/0x510
__irq_exit_rcu+0xe7/0x110
irq_exit_rcu+0xe/0x20
common_interrupt+0x85/0xa0
</IRQ>
<TASK>

Fix this by checking if the skb is non-linear. If it is, linearize it by
calling skb_linearize(). As the initial allocation of newskb originally
reserved enough tailroom for the entire reassembled packet we do not
need to check if we have enough tailroom or extend it.