nitro_enclaves: Add overview documentation

   uml/user_mode_linux

   paravirt_ops

   guest-halt-polling

   ne_overview

.. only:: html and subproject

.. SPDX-License-Identifier: GPL-2.0

==============

Nitro Enclaves

==============

Overview

========

Nitro Enclaves (NE) is a new Amazon Elastic Compute Cloud (EC2) capability

that allows customers to carve out isolated compute environments within EC2

instances [1].

For example, an application that processes sensitive data and runs in a VM,

can be separated from other applications running in the same VM. This

application then runs in a separate VM than the primary VM, namely an enclave.

An enclave runs alongside the VM that spawned it. This setup matches low latency

applications needs. The resources that are allocated for the enclave, such as

memory and CPUs, are carved out of the primary VM. Each enclave is mapped to a

process running in the primary VM, that communicates with the NE driver via an

ioctl interface.

In this sense, there are two components:

1. An enclave abstraction process - a user space process running in the primary

VM guest that uses the provided ioctl interface of the NE driver to spawn an

enclave VM (that's 2 below).

There is a NE emulated PCI device exposed to the primary VM. The driver for this

new PCI device is included in the NE driver.

The ioctl logic is mapped to PCI device commands e.g. the NE_START_ENCLAVE ioctl

maps to an enclave start PCI command. The PCI device commands are then

translated into  actions taken on the hypervisor side; that's the Nitro

hypervisor running on the host where the primary VM is running. The Nitro

hypervisor is based on core KVM technology.

2. The enclave itself - a VM running on the same host as the primary VM that

spawned it. Memory and CPUs are carved out of the primary VM and are dedicated

for the enclave VM. An enclave does not have persistent storage attached.

The memory regions carved out of the primary VM and given to an enclave need to

be aligned 2 MiB / 1 GiB physically contiguous memory regions (or multiple of

this size e.g. 8 MiB). The memory can be allocated e.g. by using hugetlbfs from

user space [2][3]. The memory size for an enclave needs to be at least 64 MiB.

The enclave memory and CPUs need to be from the same NUMA node.

An enclave runs on dedicated cores. CPU 0 and its CPU siblings need to remain

available for the primary VM. A CPU pool has to be set for NE purposes by an

user with admin capability. See the cpu list section from the kernel

documentation [4] for how a CPU pool format looks.

An enclave communicates with the primary VM via a local communication channel,

using virtio-vsock [5]. The primary VM has virtio-pci vsock emulated device,

while the enclave VM has a virtio-mmio vsock emulated device. The vsock device

uses eventfd for signaling. The enclave VM sees the usual interfaces - local

APIC and IOAPIC - to get interrupts from virtio-vsock device. The virtio-mmio

device is placed in memory below the typical 4 GiB.

The application that runs in the enclave needs to be packaged in an enclave

image together with the OS ( e.g. kernel, ramdisk, init ) that will run in the

enclave VM. The enclave VM has its own kernel and follows the standard Linux

boot protocol [6].

The kernel bzImage, the kernel command line, the ramdisk(s) are part of the

Enclave Image Format (EIF); plus an EIF header including metadata such as magic

number, eif version, image size and CRC.

Hash values are computed for the entire enclave image (EIF), the kernel and

ramdisk(s). That's used, for example, to check that the enclave image that is

loaded in the enclave VM is the one that was intended to be run.

These crypto measurements are included in a signed attestation document

generated by the Nitro Hypervisor and further used to prove the identity of the

enclave; KMS is an example of service that NE is integrated with and that checks

the attestation doc.

The enclave image (EIF) is loaded in the enclave memory at offset 8 MiB. The

init process in the enclave connects to the vsock CID of the primary VM and a

predefined port - 9000 - to send a heartbeat value - 0xb7. This mechanism is

used to check in the primary VM that the enclave has booted. The CID of the

primary VM is 3.

If the enclave VM crashes or gracefully exits, an interrupt event is received by

the NE driver. This event is sent further to the user space enclave process

running in the primary VM via a poll notification mechanism. Then the user space

enclave process can exit.

[1] https://aws.amazon.com/ec2/nitro/nitro-enclaves/

[2] https://www.kernel.org/doc/html/latest/admin-guide/mm/hugetlbpage.html

[3] https://lwn.net/Articles/807108/

[4] https://www.kernel.org/doc/html/latest/admin-guide/kernel-parameters.html

[5] https://man7.org/linux/man-pages/man7/vsock.7.html

[6] https://www.kernel.org/doc/html/latest/x86/boot.html