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1.. SPDX-License-Identifier: GPL-2.02 3==============================4Confidential Computing secrets5==============================6 7This document describes how Confidential Computing secret injection is handled8from the firmware to the operating system, in the EFI driver and the efi_secret9kernel module.10 11 12Introduction13============14 15Confidential Computing (coco) hardware such as AMD SEV (Secure Encrypted16Virtualization) allows guest owners to inject secrets into the VMs17memory without the host/hypervisor being able to read them. In SEV,18secret injection is performed early in the VM launch process, before the19guest starts running.20 21The efi_secret kernel module allows userspace applications to access these22secrets via securityfs.23 24 25Secret data flow26================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 table30under a ``LINUX_EFI_COCO_SECRET_AREA_GUID`` entry31(``adf956ad-e98c-484c-ae11-b51c7d336447``). This memory area should be marked32by the firmware as ``EFI_RESERVED_TYPE``, and therefore the kernel should not33be use it for its own purposes.34 35During the VM's launch, the virtual machine manager may inject a secret to that36area. In AMD SEV and SEV-ES this is performed using the37``KVM_SEV_LAUNCH_SECRET`` command (see [sev]_). The structure of the injected38Guest Owner secret data should be a GUIDed table of secret values; the binary39format is described in ``drivers/virt/coco/efi_secret/efi_secret.c`` under40"Structure of the EFI secret area".41 42On kernel start, the kernel's EFI driver saves the location of the secret area43(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 checks45whether 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 the48secrets to userspace applications via securityfs. The details of the49efi_secret filesystem interface are in [secrets-coco-abi]_.50 51 52Application usage example53=========================54 55Consider a guest performing computations on encrypted files. The Guest Owner56provides the decryption key (= secret) using the secret injection mechanism.57The guest application reads the secret from the efi_secret filesystem and58proceeds to decrypt the files into memory and then performs the needed59computations on the content.60 61In this example, the host can't read the files from the disk image62because they are encrypted. Host can't read the decryption key because63it is passed using the secret injection mechanism (= secure channel).64Host can't read the decrypted content from memory because it's a65confidential (memory-encrypted) guest.66 67Here is a simple example for usage of the efi_secret module in a guest68to which an EFI secret area with 4 secrets was injected during launch::69 70 # ls -la /sys/kernel/security/secrets/coco71 total 072 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-06879ce3da0b75 -r--r----- 1 root root 0 Jun 28 11:54 83c83f7f-1356-4975-8b7e-d3a0b54312c676 -r--r----- 1 root root 0 Jun 28 11:54 9553f55d-3da2-43ee-ab5d-ff17f78864d277 -r--r----- 1 root root 0 Jun 28 11:54 e6f5a162-d67f-4750-a67c-5d065f2a991078 79 # hd /sys/kernel/security/secrets/coco/e6f5a162-d67f-4750-a67c-5d065f2a991080 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 0000002284 85 # rm /sys/kernel/security/secrets/coco/e6f5a162-d67f-4750-a67c-5d065f2a991086 87 # ls -la /sys/kernel/security/secrets/coco88 total 089 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-06879ce3da0b92 -r--r----- 1 root root 0 Jun 28 11:54 83c83f7f-1356-4975-8b7e-d3a0b54312c693 -r--r----- 1 root root 0 Jun 28 11:54 9553f55d-3da2-43ee-ab5d-ff17f78864d294 95 96References97==========98 99See [sev-api-spec]_ for more info regarding SEV ``LAUNCH_SECRET`` operation.100 101.. [sev] Documentation/virt/kvm/x86/amd-memory-encryption.rst102.. [secrets-coco-abi] Documentation/ABI/testing/securityfs-secrets-coco103.. [sev-api-spec] https://www.amd.com/system/files/TechDocs/55766_SEV-KM_API_Specification.pdf104