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]{.citation}). The structure 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]{.citation}.

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]{.citation} for more info regarding SEV LAUNCH_SECRET operation.

::: {#citations}

[secrets-coco-abi]{#secrets-coco-abi .citation-label}

: Documentation/ABI/testing/securityfs-secrets-coco

[sev]{#sev .citation-label}

: Documentation/virt/kvm/x86/amd-memory-encryption.rst

[sev-api-spec]{#sev-api-spec .citation-label}

: https://www.amd.com/system/files/TechDocs/55766_SEV-KM_API_Specification.pdf :::