[v4,0/3] Allow guest access to EFI confidential computing secret area

Message

Dov Murik Oct. 20, 2021, 6:14 a.m. UTC

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 copies the secrets from the EFI-provided memory to
kernel reserved memory, and optionally exposes them to userspace via
securityfs using a new efi_secret kernel module.

The first patch in efi/libstub copies the secret area from the EFI
memory to specially allocated memory; the second patch reserves that
memory block; and 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.

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

---

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 <[e-mail]>)

v3: https://lore.kernel.org/linux-coco/[e-mail]/
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/[e-mail]
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/[e-mail]/

RFC: https://lore.kernel.org/linux-coco/[e-mail]/



Dov Murik (3):
  efi/libstub: Copy confidential computing secret area
  efi: Reserve confidential computing secret area
  virt: Add efi_secret module to expose confidential computing secrets

 .../ABI/testing/securityfs-coco-efi_secret    |  50 +++
 arch/x86/platform/efi/efi.c                   |   3 +
 drivers/firmware/efi/Kconfig                  |  12 +
 drivers/firmware/efi/Makefile                 |   1 +
 drivers/firmware/efi/coco.c                   |  41 +++
 drivers/firmware/efi/efi.c                    |   8 +
 drivers/firmware/efi/libstub/Makefile         |   1 +
 drivers/firmware/efi/libstub/coco.c           |  68 ++++
 drivers/firmware/efi/libstub/efi-stub.c       |   2 +
 drivers/firmware/efi/libstub/efistub.h        |   6 +
 drivers/firmware/efi/libstub/x86-stub.c       |   2 +
 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     | 326 ++++++++++++++++++
 include/linux/efi.h                           |  16 +
 17 files changed, 553 insertions(+)
 create mode 100644 Documentation/ABI/testing/securityfs-coco-efi_secret
 create mode 100644 drivers/firmware/efi/coco.c
 create mode 100644 drivers/firmware/efi/libstub/coco.c
 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: 348949d9a4440abdab3b1dc99a9bb660e8c7da7c