[PATCH v4 17/18] nitro_enclaves: Add overview documentation

[Andra Paraschiv <andraprs@amazon.com>]

Signed-off-by: Andra Paraschiv <andraprs@amazon.com>
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Changelog

v3 -> v4

* Update doc type from .txt to .rst.
* Update documentation based on the changes from v4.

v2 -> v3

* No changes.

v1 -> v2

* New in v2.
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 Documentation/nitro_enclaves/ne_overview.rst | 87 ++++++++++++++++++++
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 create mode 100644 Documentation/nitro_enclaves/ne_overview.rst

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+Nitro Enclaves
+==============
+
+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 CPU, 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.
+
+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.
+
+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/Documentation/vm/hugetlbpage.txt
+[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
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2.20.1 (Apple Git-117)




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