[PATCH v10 0/4] Allow guest access to EFI confidential computing secret area

From: Dov Murik
Date: Tue Apr 12 2022 - 19:33:24 EST

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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 exposes the secrets to userspace via securityfs
using a new efi_secret kernel module. The module is autoloaded (by the
EFI driver) if the secret area is populated.

The first patch in EFI keeps the address of the secret area as passed in
the EFI configuration table. The second 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 third patch auto-loads the efi_secret module during
startup if the injected secrets area is populated. 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:

...
[ 0.000000] efi: EFI v2.70 by EDK II
[ 0.000000] efi: CocoSecret=0x7f222680 SMBIOS=0x7f541000 ACPI=0x7f77e000 ACPI 2.0=0x7f77e014 MEMATTR=0x7ea16418
...
[ 1.127627] Run /init as init process
Loading, please wait...
Starting version 245.4-4ubuntu3.15
...
[ 0.763204] efi_secret efi_secret.0: Created 4 entries in securityfs secrets/coco
...

# 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

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

---

v10 changes:
- Add #ifdef CONFIG_EFI_COCO_SECRET to platform device registration in
patch 3.

v9: https://lore.kernel.org/linux-coco/20220331215607.3182232-1-dovmurik@xxxxxxxxxxxxx/
v9 changes:
- Change the module into a platform driver (thanks Ard)
- Remove special auto-loading code in efi; instead register a platform
device (udev will load the efi_secret module) (thanks Ard)
- Change logging in the efi_secret module to dev_err() etc.
- efi_secret: first check that the secret area header is valid; only then start
creating securityfs dirs.

v8: https://lore.kernel.org/linux-coco/20220228114254.1099945-1-dovmurik@xxxxxxxxxxxxx/
v8 changes:
- Change path of filesystem to <securityfs>/secrets/coco and fix the
documentation accordingly (Thanks Gerd, Matthew)
- Remove patch 2/5 (of v7) because the latest OVMF release (edk2-stable202202)
already contains the fix to mark the launch secret page as EFI_RESERVED_TYPE.

v7: https://lore.kernel.org/linux-coco/20220201124413.1093099-1-dovmurik@xxxxxxxxxxxxx/
v7 changes:
- Improve description of efi_secret module in Kconfig.
- Fix sparse warnings on pointer address space mismatch
(Reported-by: kernel test robot <lkp@xxxxxxxxx>)

v6: https://lore.kernel.org/linux-coco/20211129114251.3741721-1-dovmurik@xxxxxxxxxxxxx/
v6 changes:
- Autoload the efi_secret module if the secret area is populated
(thanks Greg KH).
- efi_secret: Depend on X86_64 because we use ioremap_encrypted() which
is only defined for this arch.
- efi_secret.c: Remove unneeded tableheader_guid local variable.
- Documentation fixes.

v5: https://lore.kernel.org/linux-coco/20211118113359.642571-1-dovmurik@xxxxxxxxxxxxx/
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
virt: Add efi_secret module to expose confidential computing secrets
efi: Register efi_secret platform device if EFI secret area is
declared
docs: security: Add secrets/coco documentation

Documentation/ABI/testing/securityfs-secrets-coco | 51 +++
Documentation/security/index.rst | 1 +
Documentation/security/secrets/coco.rst | 103 ++++++
Documentation/security/secrets/index.rst | 9 +
arch/x86/platform/efi/efi.c | 3 +
drivers/firmware/efi/Kconfig | 16 +
drivers/firmware/efi/efi.c | 11 +
drivers/virt/Kconfig | 3 +
drivers/virt/Makefile | 1 +
drivers/virt/coco/efi_secret/Kconfig | 16 +
drivers/virt/coco/efi_secret/Makefile | 2 +
drivers/virt/coco/efi_secret/efi_secret.c | 349 ++++++++++++++++++++
include/linux/efi.h | 10 +
13 files changed, 575 insertions(+)
create mode 100644 Documentation/ABI/testing/securityfs-secrets-coco
create mode 100644 Documentation/security/secrets/coco.rst
create mode 100644 Documentation/security/secrets/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: 7e57714cd0ad2d5bb90e50b5096a0e671dec1ef3
--
2.25.1

Next message: Dov Murik: "[PATCH v10 4/4] docs: security: Add secrets/coco documentation"
Previous message: Rob Herring: "Re: [PATCH v2 1/2] dt-bindings: i2c: add property to avoid device detection"
Next in thread: Dov Murik: "[PATCH v10 3/4] efi: Register efi_secret platform device if EFI secret area is declared"
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