From 106b4a2a78b2a2f14885604891957b84bba502db Mon Sep 17 00:00:00 2001 From: Adrian Betschart Date: Fri, 11 Sep 2026 17:13:14 +0200 Subject: drm/amd/display: Try RGB before YCbCr 4:4:4 in stream validation amdgpu_dm_create_validate_stream_for_sink() walks encoding_order[] and uses the first encoding that validates. YCbCr 4:4:4 is listed before RGB, so an HDMI sink that advertises 4:4:4 gets YCbCr 4:4:4 whenever the "color format" property is left at AUTO, even though RGB fits the same link. That contradicts the documented AUTO behaviour for HDMI in enum drm_connector_color_format (RGB, falling back to YCbCr 4:2:0 only when the bandwidth is not available or the mode is 4:2:0-only), which the amdgpu implementation of the property also describes. It also leaves the "Broadcast RGB" property without effect on such sinks, since the quantization range it selects only applies to RGB output. Try RGB first. The mask still holds every encoding the sink supports, so a mode that cannot carry RGB falls back exactly as before. For reference, v7.2 picked RGB here unless YCbCr 4:4:4 was forced through debugfs, while earlier kernels picked YCbCr 4:4:4 for any HDMI sink that advertised it. Fixes: 0b0ff65d3ca1 ("drm/amd/display: Refactor stream validation") Suggested-by: Adolfo Rodrigues Assisted-by: Claude Code:claude-fable-5-1 Signed-off-by: Adrian Betschart Reviewed-by: Fangzhi Zuo Tested-by: Adolfo Rodrigues Tested-by: Daniel Wheeler Signed-off-by: Alex Deucher (cherry picked from commit 7de432bc753133764dc40d37f9388da56de251e7) --- Documentation/security/secrets/coco.rst | 103 +++++++++++++++++++++++++++++++ Documentation/security/secrets/index.rst | 9 +++ 2 files changed, 112 insertions(+) create mode 100644 Documentation/security/secrets/coco.rst create mode 100644 Documentation/security/secrets/index.rst (limited to 'Documentation/security/secrets') diff --git a/Documentation/security/secrets/coco.rst b/Documentation/security/secrets/coco.rst new file mode 100644 index 000000000..a1210db8e --- /dev/null +++ b/Documentation/security/secrets/coco.rst @@ -0,0 +1,103 @@ +.. SPDX-License-Identifier: GPL-2.0 + +============================== +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]_). 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]_. + + +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]_ for more info regarding SEV ``LAUNCH_SECRET`` operation. + +.. [sev] Documentation/virt/kvm/x86/amd-memory-encryption.rst +.. [secrets-coco-abi] Documentation/ABI/testing/securityfs-secrets-coco +.. [sev-api-spec] https://www.amd.com/system/files/TechDocs/55766_SEV-KM_API_Specification.pdf diff --git a/Documentation/security/secrets/index.rst b/Documentation/security/secrets/index.rst new file mode 100644 index 000000000..ced34e9c4 --- /dev/null +++ b/Documentation/security/secrets/index.rst @@ -0,0 +1,9 @@ +.. SPDX-License-Identifier: GPL-2.0 + +===================== +Secrets documentation +===================== + +.. toctree:: + + coco -- cgit v1.3.1