coco.rst

  1.. SPDX-License-Identifier: GPL-2.0
  2
  3==============================
  4Confidential Computing secrets
  5==============================
  6
  7This document describes how Confidential Computing secret injection is handled
  8from the firmware to the operating system, in the EFI driver and the efi_secret
  9kernel module.
 10
 11
 12Introduction
 13============
 14
 15Confidential Computing (coco) hardware such as AMD SEV (Secure Encrypted
 16Virtualization) allows guest owners to inject secrets into the VMs
 17memory without the host/hypervisor being able to read them.  In SEV,
 18secret injection is performed early in the VM launch process, before the
 19guest starts running.
 20
 21The efi_secret kernel module allows userspace applications to access these
 22secrets via securityfs.
 23
 24
 25Secret data flow
 26================
 27
 28The guest firmware may reserve a designated memory area for secret injection,
 29and publish its location (base GPA and length) in the EFI configuration table
 30under a ``LINUX_EFI_COCO_SECRET_AREA_GUID`` entry
 31(``adf956ad-e98c-484c-ae11-b51c7d336447``).  This memory area should be marked
 32by the firmware as ``EFI_RESERVED_TYPE``, and therefore the kernel should not
 33be use it for its own purposes.
 34
 35During the VM's launch, the virtual machine manager may inject a secret to that
 36area.  In AMD SEV and SEV-ES this is performed using the
 37``KVM_SEV_LAUNCH_SECRET`` command (see [sev]_).  The structure of the injected
 38Guest Owner secret data should be a GUIDed table of secret values; the binary
 39format is described in ``drivers/virt/coco/efi_secret/efi_secret.c`` under
 40"Structure of the EFI secret area".
 41
 42On kernel start, the kernel's EFI driver saves the location of the secret area
 43(taken from the EFI configuration table) in the ``efi.coco_secret`` field.
 44Later it checks if the secret area is populated: it maps the area and checks
 45whether its content begins with ``EFI_SECRET_TABLE_HEADER_GUID``
 46(``1e74f542-71dd-4d66-963e-ef4287ff173b``).  If the secret area is populated,
 47the EFI driver will autoload the efi_secret kernel module, which exposes the
 48secrets to userspace applications via securityfs.  The details of the
 49efi_secret filesystem interface are in [secrets-coco-abi]_.
 50
 51
 52Application usage example
 53=========================
 54
 55Consider a guest performing computations on encrypted files.  The Guest Owner
 56provides the decryption key (= secret) using the secret injection mechanism.
 57The guest application reads the secret from the efi_secret filesystem and
 58proceeds to decrypt the files into memory and then performs the needed
 59computations on the content.
 60
 61In this example, the host can't read the files from the disk image
 62because they are encrypted.  Host can't read the decryption key because
 63it is passed using the secret injection mechanism (= secure channel).
 64Host can't read the decrypted content from memory because it's a
 65confidential (memory-encrypted) guest.
 66
 67Here is a simple example for usage of the efi_secret module in a guest
 68to which an EFI secret area with 4 secrets was injected during launch::
 69
 70	# ls -la /sys/kernel/security/secrets/coco
 71	total 0
 72	drwxr-xr-x 2 root root 0 Jun 28 11:54 .
 73	drwxr-xr-x 3 root root 0 Jun 28 11:54 ..
 74	-r--r----- 1 root root 0 Jun 28 11:54 736870e5-84f0-4973-92ec-06879ce3da0b
 75	-r--r----- 1 root root 0 Jun 28 11:54 83c83f7f-1356-4975-8b7e-d3a0b54312c6
 76	-r--r----- 1 root root 0 Jun 28 11:54 9553f55d-3da2-43ee-ab5d-ff17f78864d2
 77	-r--r----- 1 root root 0 Jun 28 11:54 e6f5a162-d67f-4750-a67c-5d065f2a9910
 78
 79	# hd /sys/kernel/security/secrets/coco/e6f5a162-d67f-4750-a67c-5d065f2a9910
 80	00000000  74 68 65 73 65 2d 61 72  65 2d 74 68 65 2d 6b 61  |these-are-the-ka|
 81	00000010  74 61 2d 73 65 63 72 65  74 73 00 01 02 03 04 05  |ta-secrets......|
 82	00000020  06 07                                             |..|
 83	00000022
 84
 85	# rm /sys/kernel/security/secrets/coco/e6f5a162-d67f-4750-a67c-5d065f2a9910
 86
 87	# ls -la /sys/kernel/security/secrets/coco
 88	total 0
 89	drwxr-xr-x 2 root root 0 Jun 28 11:55 .
 90	drwxr-xr-x 3 root root 0 Jun 28 11:54 ..
 91	-r--r----- 1 root root 0 Jun 28 11:54 736870e5-84f0-4973-92ec-06879ce3da0b
 92	-r--r----- 1 root root 0 Jun 28 11:54 83c83f7f-1356-4975-8b7e-d3a0b54312c6
 93	-r--r----- 1 root root 0 Jun 28 11:54 9553f55d-3da2-43ee-ab5d-ff17f78864d2
 94
 95
 96References
 97==========
 98
 99See [sev-api-spec]_ for more info regarding SEV ``LAUNCH_SECRET`` operation.
100
101.. [sev] Documentation/virt/kvm/x86/amd-memory-encryption.rst
102.. [secrets-coco-abi] Documentation/ABI/testing/securityfs-secrets-coco
103.. [sev-api-spec] https://www.amd.com/system/files/TechDocs/55766_SEV-KM_API_Specification.pdf