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 strucutre 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 104