7ac3945d8e
Changesetdaec8d4083("Documentation: KVM: add separate directories for architecture-specific documentation") renamed: Documentation/virt/kvm/amd-memory-encryption.rst to: Documentation/virt/kvm/x86/amd-memory-encryption.rst. Update the cross-references accordingly. Fixes:daec8d4083("Documentation: KVM: add separate directories for architecture-specific documentation") Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org> Link: https://lore.kernel.org/r/fd80db889e34aae87a4ca88cad94f650723668f4.1656234456.git.mchehab@kernel.org Signed-off-by: Jonathan Corbet <corbet@lwn.net>
104 lines
4.4 KiB
ReStructuredText
104 lines
4.4 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
|
|
|
|
==============================
|
|
Confidential Computing secrets
|
|
==============================
|
|
|
|
This document describes how Confidential Computing secret injection is handled
|
|
from the firmware to the operating system, in the EFI driver and the efi_secret
|
|
kernel module.
|
|
|
|
|
|
Introduction
|
|
============
|
|
|
|
Confidential Computing (coco) hardware such as AMD SEV (Secure Encrypted
|
|
Virtualization) allows guest owners to inject secrets into the VMs
|
|
memory without the host/hypervisor being able to read them. In SEV,
|
|
secret injection is performed early in the VM launch process, before the
|
|
guest starts running.
|
|
|
|
The efi_secret kernel module allows userspace applications to access these
|
|
secrets via securityfs.
|
|
|
|
|
|
Secret data flow
|
|
================
|
|
|
|
The guest firmware may reserve a designated memory area for secret injection,
|
|
and publish its location (base GPA and length) in the EFI configuration table
|
|
under a ``LINUX_EFI_COCO_SECRET_AREA_GUID`` entry
|
|
(``adf956ad-e98c-484c-ae11-b51c7d336447``). This memory area should be marked
|
|
by the firmware as ``EFI_RESERVED_TYPE``, and therefore the kernel should not
|
|
be use it for its own purposes.
|
|
|
|
During the VM's launch, the virtual machine manager may inject a secret to that
|
|
area. In AMD SEV and SEV-ES this is performed using the
|
|
``KVM_SEV_LAUNCH_SECRET`` command (see [sev]_). The strucutre of the injected
|
|
Guest Owner secret data should be a GUIDed table of secret values; the binary
|
|
format is described in ``drivers/virt/coco/efi_secret/efi_secret.c`` under
|
|
"Structure of the EFI secret area".
|
|
|
|
On kernel start, the kernel's EFI driver saves the location of the secret area
|
|
(taken from the EFI configuration table) in the ``efi.coco_secret`` field.
|
|
Later it checks if the secret area is populated: it maps the area and checks
|
|
whether its content begins with ``EFI_SECRET_TABLE_HEADER_GUID``
|
|
(``1e74f542-71dd-4d66-963e-ef4287ff173b``). If the secret area is populated,
|
|
the EFI driver will autoload the efi_secret kernel module, which exposes the
|
|
secrets to userspace applications via securityfs. The details of the
|
|
efi_secret filesystem interface are in [secrets-coco-abi]_.
|
|
|
|
|
|
Application usage example
|
|
=========================
|
|
|
|
Consider a guest performing computations on encrypted files. The Guest Owner
|
|
provides the decryption key (= secret) using the secret injection mechanism.
|
|
The guest application reads the secret from the efi_secret filesystem and
|
|
proceeds to decrypt the files into memory and then performs the needed
|
|
computations on the content.
|
|
|
|
In this example, the host can't read the files from the disk image
|
|
because they are encrypted. Host can't read the decryption key because
|
|
it is passed using the secret injection mechanism (= secure channel).
|
|
Host can't read the decrypted content from memory because it's a
|
|
confidential (memory-encrypted) guest.
|
|
|
|
Here is a simple example for usage of the efi_secret module in a guest
|
|
to which an EFI secret area with 4 secrets was injected during launch::
|
|
|
|
# ls -la /sys/kernel/security/secrets/coco
|
|
total 0
|
|
drwxr-xr-x 2 root root 0 Jun 28 11:54 .
|
|
drwxr-xr-x 3 root root 0 Jun 28 11:54 ..
|
|
-r--r----- 1 root root 0 Jun 28 11:54 736870e5-84f0-4973-92ec-06879ce3da0b
|
|
-r--r----- 1 root root 0 Jun 28 11:54 83c83f7f-1356-4975-8b7e-d3a0b54312c6
|
|
-r--r----- 1 root root 0 Jun 28 11:54 9553f55d-3da2-43ee-ab5d-ff17f78864d2
|
|
-r--r----- 1 root root 0 Jun 28 11:54 e6f5a162-d67f-4750-a67c-5d065f2a9910
|
|
|
|
# hd /sys/kernel/security/secrets/coco/e6f5a162-d67f-4750-a67c-5d065f2a9910
|
|
00000000 74 68 65 73 65 2d 61 72 65 2d 74 68 65 2d 6b 61 |these-are-the-ka|
|
|
00000010 74 61 2d 73 65 63 72 65 74 73 00 01 02 03 04 05 |ta-secrets......|
|
|
00000020 06 07 |..|
|
|
00000022
|
|
|
|
# rm /sys/kernel/security/secrets/coco/e6f5a162-d67f-4750-a67c-5d065f2a9910
|
|
|
|
# ls -la /sys/kernel/security/secrets/coco
|
|
total 0
|
|
drwxr-xr-x 2 root root 0 Jun 28 11:55 .
|
|
drwxr-xr-x 3 root root 0 Jun 28 11:54 ..
|
|
-r--r----- 1 root root 0 Jun 28 11:54 736870e5-84f0-4973-92ec-06879ce3da0b
|
|
-r--r----- 1 root root 0 Jun 28 11:54 83c83f7f-1356-4975-8b7e-d3a0b54312c6
|
|
-r--r----- 1 root root 0 Jun 28 11:54 9553f55d-3da2-43ee-ab5d-ff17f78864d2
|
|
|
|
|
|
References
|
|
==========
|
|
|
|
See [sev-api-spec]_ for more info regarding SEV ``LAUNCH_SECRET`` operation.
|
|
|
|
.. [sev] Documentation/virt/kvm/x86/amd-memory-encryption.rst
|
|
.. [secrets-coco-abi] Documentation/ABI/testing/securityfs-secrets-coco
|
|
.. [sev-api-spec] https://www.amd.com/system/files/TechDocs/55766_SEV-KM_API_Specification.pdf
|