[PATCH v5 0/4] Allow guest access to EFI confidential computing secret area

From: Dov Murik
Date: Thu Nov 18 2021 - 06:35:09 EST

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.

OVMF already reserves designated area for secret injection (in its
AmdSev package; see edk2 commit 01726b6d23d4 "OvmfPkg/AmdSev: Expose the
Sev Secret area using a configuration table" [1]), but the secrets were
not available in the guest kernel.

The patch series keeps the address of the EFI-provided memory for
injected secrets, and optionally exposes the secrets to userspace via
securityfs using a new efi_secret kernel module.

The first patch in EFI keeps the address of the secret area as passed in
the EFI configuration table. The second patch is a quirk fix for older
firmwares didn't mark the secrets page as EFI_RESERVED_TYPE. The third
patch introduces the new efi_secret module that exposes the content of
the secret entries as securityfs files, and allows clearing out secrets
with a file unlink interface. The last patch documents the data flow of
confidential computing secret injection.

As a 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.

This has been tested with AMD SEV and SEV-ES guests, but the kernel side
of handling the secret area has no SEV-specific dependencies, and
therefore might be usable (perhaps with minor changes) for any
confidential computing hardware that can publish the secret area via the
standard EFI config table entry.

To enable this functionality, set CONFIG_EFI_SECRET=m when building the
guest kernel.

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:

# modprobe efi_secret
# ls -la /sys/kernel/security/coco/efi_secret
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

# xxd /sys/kernel/security/coco/efi_secret/e6f5a162-d67f-4750-a67c-5d065f2a9910
00000000: 7468 6573 652d 6172 652d 7468 652d 6b61 these-are-the-ka
00000010: 7461 2d73 6563 7265 7473 0001 0203 0405 ta-secrets......
00000020: 0607 ..

# rm /sys/kernel/security/coco/efi_secret/e6f5a162-d67f-4750-a67c-5d065f2a9910

# ls -la /sys/kernel/security/coco/efi_secret
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

[1] https://github.com/tianocore/edk2/commit/01726b6d23d4


v5 changes:
- Simplify EFI code: instead of copying the secret area, the firmware
marks the secret area as EFI_RESERVED_TYPE, and then the uefi_init()
code just keeps the pointer as it appears in the EFI configuration
table. The use of reserved pages is similar to the AMD SEV-SNP
patches for handling SNP-Secrets and SNP-CPUID pages.
- In order to handle OVMF releases out there which mark the
confidential computing secrets page as EFI_BOOT_SERVICES_DATA, add
efi/libstub code that detects this and fixes the E820 map to reserve
this page.
- In the efi_secret module code, map the secrets page using
ioremap_encrypted (again, similar to the AMD SEV-SNP guest patches
for accessing SNP-Secrets and SNP-CPUID pages).
- Add documentation in Documentation/security/coco/efi_secret.

v4: https://lore.kernel.org/linux-coco/20211020061408.3447533-1-dovmurik@xxxxxxxxxxxxx/
v4 changes:
- Guard all the new EFI and efi-stub code (patches 1+2) with #ifdef
CONFIG_EFI_COCO_SECRET (thanks Greg KH). Selecting
CONFIG_EFI_SECRET=m (patch 3) will enable the EFI parts as well.
- Guard call to clflush_cache_range() with #ifdef CONFIG_X86
(Reported-by: kernel test robot <lkp@xxxxxxxxx>)

v3: https://lore.kernel.org/linux-coco/20211014130848.592611-1-dovmurik@xxxxxxxxxxxxx/
v3 changes:
- Rename the module to efi_secret
- Remove the exporting of clean_cache_range
- Use clflush_cache_range in wipe_memory
- Document function wipe_memory
- Initialize efi.coco_secret to EFI_INVALID_TABLE_ADDR to correctly detect
when there's no secret area published in the EFI configuration tables

v2: https://lore.kernel.org/linux-coco/20211007061838.1381129-1-dovmurik@xxxxxxxxxxxxx
v2 changes:
- Export clean_cache_range()
- When deleteing a secret, call clean_cache_range() after explicit_memzero
- Add Documentation/ABI/testing/securityfs-coco-sev_secret

v1: https://lore.kernel.org/linux-coco/20210809190157.279332-1-dovmurik@xxxxxxxxxxxxx/

RFC: https://lore.kernel.org/linux-coco/20210628183431.953934-1-dovmurik@xxxxxxxxxxxxx/

Dov Murik (4):
efi: Save location of EFI confidential computing area
efi/libstub: Reserve confidential computing secret area
virt: Add efi_secret module to expose confidential computing secrets
docs: security: Add coco/efi_secret documentation

.../ABI/testing/securityfs-coco-efi_secret | 50 +++
Documentation/security/coco/efi_secret.rst | 103 ++++++
Documentation/security/coco/index.rst | 9 +
Documentation/security/index.rst | 1 +
arch/x86/platform/efi/efi.c | 3 +
drivers/firmware/efi/Kconfig | 16 +
drivers/firmware/efi/efi.c | 6 +
drivers/firmware/efi/libstub/x86-stub.c | 28 ++
drivers/virt/Kconfig | 3 +
drivers/virt/Makefile | 1 +
drivers/virt/coco/efi_secret/Kconfig | 11 +
drivers/virt/coco/efi_secret/Makefile | 2 +
drivers/virt/coco/efi_secret/efi_secret.c | 341 ++++++++++++++++++
include/linux/efi.h | 7 +
14 files changed, 581 insertions(+)
create mode 100644 Documentation/ABI/testing/securityfs-coco-efi_secret
create mode 100644 Documentation/security/coco/efi_secret.rst
create mode 100644 Documentation/security/coco/index.rst
create mode 100644 drivers/virt/coco/efi_secret/Kconfig
create mode 100644 drivers/virt/coco/efi_secret/Makefile
create mode 100644 drivers/virt/coco/efi_secret/efi_secret.c

base-commit: 42eb8fdac2fc5d62392dcfcf0253753e821a97b0