1 .. SPDX-License-Identifier: GPL-2.0 2 3 ============================== 4 Confidential Computing secrets 5 ============================== 6 7 This document describes how Confidential Computing secret injection is handled 8 from the firmware to the operating system, in the EFI driver and the efi_secret 9 kernel module. 10 11 12 Introduction 13 ============ 14 15 Confidential Computing (coco) hardware such as AMD SEV (Secure Encrypted 16 Virtualization) allows guest owners to inject secrets into the VMs 17 memory without the host/hypervisor being able to read them. In SEV, 18 secret injection is performed early in the VM launch process, before the 19 guest starts running. 20 21 The efi_secret kernel module allows userspace applications to access these 22 secrets via securityfs. 23 24 25 Secret data flow 26 ================ 27 28 The guest firmware may reserve a designated memory area for secret injection, 29 and publish its location (base GPA and length) in the EFI configuration table 30 under a ``LINUX_EFI_COCO_SECRET_AREA_GUID`` entry 31 (``adf956ad-e98c-484c-ae11-b51c7d336447``). This memory area should be marked 32 by the firmware as ``EFI_RESERVED_TYPE``, and therefore the kernel should not 33 be use it for its own purposes. 34 35 During the VM's launch, the virtual machine manager may inject a secret to that 36 area. In AMD SEV and SEV-ES this is performed using the 37 ``KVM_SEV_LAUNCH_SECRET`` command (see [sev]_). The structure of the injected 38 Guest Owner secret data should be a GUIDed table of secret values; the binary 39 format is described in ``drivers/virt/coco/efi_secret/efi_secret.c`` under 40 "Structure of the EFI secret area". 41 42 On 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. 44 Later it checks if the secret area is populated: it maps the area and checks 45 whether its content begins with ``EFI_SECRET_TABLE_HEADER_GUID`` 46 (``1e74f542-71dd-4d66-963e-ef4287ff173b``). If the secret area is populated, 47 the EFI driver will autoload the efi_secret kernel module, which exposes the 48 secrets to userspace applications via securityfs. The details of the 49 efi_secret filesystem interface are in [secrets-coco-abi]_. 50 51 52 Application usage example 53 ========================= 54 55 Consider a guest performing computations on encrypted files. The Guest Owner 56 provides the decryption key (= secret) using the secret injection mechanism. 57 The guest application reads the secret from the efi_secret filesystem and 58 proceeds to decrypt the files into memory and then performs the needed 59 computations on the content. 60 61 In this example, the host can't read the files from the disk image 62 because they are encrypted. Host can't read the decryption key because 63 it is passed using the secret injection mechanism (= secure channel). 64 Host can't read the decrypted content from memory because it's a 65 confidential (memory-encrypted) guest. 66 67 Here is a simple example for usage of the efi_secret module in a guest 68 to 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 96 References 97 ========== 98 99 See [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
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