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