[PATCH v1 00/21] Appended Signature Secure Boot Support for PowerPC

[Sudhakar Kuppusamy]

Linux on Power LPAR secure boot ensures the integrity of the Linux boot
stack. The hypervisor and partition firmware are part of the core root of
trust. The partition firmware verifies the signature on the GRUB image
before handing control to GRUB. Similarly, GRUB verifies the signature on
the kernel image before booting the OS. This ensures that every image
running at the boot time is verified and trusted. UEFI platforms relies
on PECOFF based signature scheme. Since Power is not a UEFI platform, an
alternative mechanism is needed. Power already uses appended signatures
on the Linux Kernel, and is now extended to sign the grub as well.

Linux on Power also allows multiple signers, and if any one of the
signature passes, then the image passes the validation. Appended signature
scheme uses CMS structure to contain signatures. On Power, the multiple
signature support relies on the multiple signers features already supported
by CMS standards. It does require that all the signers should sign at the
same time and are not allowed to add or remove the signatures randomly.

By default, Linux LPAR secure boot uses static key management[1]. This means
that each image embeds the keys it needs to verify the image it loads.
For example, the keys used to verify the GRUB image are built into the
firmware image. Similarly, the keys used for verifying the kernel image
are built into the GRUB image. These are pre-defined keys and they cannot
be modified at runtime. The drawback of this approach is that key rotations
results in both firmware and OS updates. This is where dynamic key
management is useful.

An admin can switch from static keys to dynamic keys by coordinating with
Hardware Management Console(HMC) admin and enabling the required flags
for the given LPAR.

The dynamic key management relies on the Platform KeyStore(PKS)[2] storage
allocation for each LPAR with individually managed access controls to
store sensitive information securely. Once switched to dynamic keys, HMC
advertises this flag to the PowerVM, which then initializes the PKS
with the default secvars. It also creates a variable SB_VERSION that
represents the secure boot key management mode. The default secvars are
used by Partition firmware, grub and the linux kernel to reads keys for
verification. These secvars can be managed by user interface exposed via
linux kernel. The linux kernel already supports this interface and
it is available in the upstream kernel.

This patchset adds the appended signature support both for signing and
verification and the key management to the grub component. The whole
patchset can be split into following four main parts:

The series has following four main parts:

1.) Sign grub.elf with an appended signature. (Patches 1 - 3)

These patches provide some infrastructure and documentation for
signing grub's core.elf with a Linux-kernel-module style appended
signature.

An appended signature is a 'dumb' signature over the contents of a
file. (It is distinct from schemes like Authenticode that are aware of
the structure of the file and only sign certain parts.) The signature
is wrapped in a PKCS#7 message, and is appended to the signed file
along with some metadata and a magic string. The signatures are
validated against a public key which is usually provided as an x509
certificate.

Because some platforms, such as powerpc-ieee1275, may load grub from a
raw disk partition rather than a filesystem, we extend grub-install to
add an ELF note that allows us to specify the size and location of the
signature.

2.) Enable appended signature verification using builtin keys (Patches 4 - 10).

Part of a secure boot chain is allowing grub to verify the boot
kernel. For UEFI platforms, this is usually delegated to the
shim. However, for platforms that do not implement UEFI, an
alternative scheme is required.

This part teaches grub how to verify Linux kernel-style appended
signatures. Kernels on powerpc are already signed with this scheme and
can be verified by IMA for kexec.

As PKCS#7 messages and x509 certificates are both based on ASN.1, we
import libtasn1 to parse them. Because ASN.1 isn't self-documenting,
we import from GNUTLS the information we need to navigate their
structure.

This section is composed of the following patches:

 - patch 4 is a small fix to allow persistent modules to work on the
   emu target.

 - patches 5 and 6 are small refactorings.

 - patch 7 allows x509 certificates to be built in to the grub core
   in much the same way as PGP keys.

 - patch 8 brings in the code from GNUTLS that allows us to parse
   PKCS#7 and x509 with libtasn1.

 - patch 9 is our PKCS#7 and x509 parser. They're minimal and fairly
   strict parsers that extract only the bits we need to verify the
   signatures.

 - patch 10 is the guts of the appended signature verifier. It uses
   the verifier infrastructure like pgp, and adds a number of
   user-friendly commands that mirror the pgp module.

 - patch 11 adds tests, and patch 12 adds documentation.

3.) Enable lockdown if secure boot is enabled (Patch 13)

If the 'ibm,secure-boot' property of the root node is 2 or greater,
enter lockdown.The main appended signature module now tests for lockdown to
enter 'forced' mode.

4.) Enable accessing keys dynamically from Platform KeyStore (Patch 14 - 21)

This part teaches grub how to read db and dbx variables from platform keystore
using client interface call then load keys from those two variable, and use it
to verify Linux kernel.

This section is composed of the following patches:

 - patch 14 is an exposes an interface in ieee1275 for reading secure boot 
variable
   db and dbx from Platform Keystore.

 - patch 15 is a read secure boot variables such as db and dbx from PKS and 
extract certificates from ESL.

 - patch 16 is creates the trusted and distrusted lists.

 - patch 17 is verify the kernel using trusted and distrusted lists

 - patch 18 sets the use_static_keys flag if DB not available in PKS,
   and patch 19  is reads the DB default keys from ELF Note and
   store it in trusted lists if use_static_keys flag is set.

 - patch 20 adds trusted and distrusted commands, and patch 21 adds 
documentation.


[1]https://www.ibm.com/docs/en/linux-on-systems?topic=servers-guest-secure-boot-static-keys
[2]https://community.ibm.com/community/user/power/blogs/chris-engel1/2020/11/20/powervm-introduces-the-platform-keystore

Alastair D'Silva (1):
  grub-install: support embedding x509 certificates

Daniel Axtens (11):
  docs/grub: Document signing grub under UEFI
  docs/grub: Document signing grub with an appended signature
  dl: provide a fake grub_dl_set_persistent for the emu target
  pgp: factor out rsa_pad
  crypto: move storage for grub_crypto_pk_* to crypto.c
  appended signatures: import GNUTLS's ASN.1 description files
  appended signatures: parse PKCS#7 signedData and X.509 certificates
  appended signatures: support verifying appended signatures
  appended signatures: verification tests
  appended signatures: documentation
  ieee1275: enter lockdown based on /ibm,secure-boot

Rashmica Gupta (1):
  powerpc-ieee1275: Add support for signing grub with an appended
    signature

Sudhakar Kuppusamy (8):
  ieee1275: Platform Keystore (PKS) Support
  ieee1275: Read the DB and DBX secure boot variables
  appendedsig: The creation of trusted and distrusted lists
  appendedsig: While verifying the kernel, use trusted and distrusted
    lists
  ieee1275: set use_static_keys flag
  appendedsig: Reads the default DB keys from ELF Note
  appendedsig: The grub command's trusted and distrusted support
  appendedsig: documentation

 docs/grub.texi                                |  295 +++-
 grub-core/Makefile.am                         |    1 +
 grub-core/Makefile.core.def                   |   30 +
 grub-core/commands/appendedsig/appendedsig.c  | 1455 +++++++++++++++++
 grub-core/commands/appendedsig/appendedsig.h  |  110 ++
 grub-core/commands/appendedsig/asn1util.c     |   99 ++
 .../commands/appendedsig/gnutls_asn1_tab.c    |  121 ++
 grub-core/commands/appendedsig/pkcs7.c        |  473 ++++++
 .../commands/appendedsig/pkix_asn1_tab.c      |  484 ++++++
 grub-core/commands/appendedsig/x509.c         |  981 +++++++++++
 grub-core/commands/pgp.c                      |   34 +-
 grub-core/kern/ieee1275/ieee1275.c            |  117 ++
 grub-core/kern/ieee1275/init.c                |   38 +
 grub-core/kern/ieee1275/platform_keystore.c   |  350 ++++
 grub-core/lib/crypto.c                        |    4 +
 grub-core/lib/pkcs1_v15.c                     |   59 +
 grub-core/tests/appended_signature_test.c     |  258 +++
 grub-core/tests/appended_signatures.h         |  975 +++++++++++
 grub-core/tests/lib/functional_test.c         |    1 +
 include/grub/dl.h                             |   11 +
 include/grub/file.h                           |    2 +
 include/grub/ieee1275/ieee1275.h              |   14 +
 include/grub/kernel.h                         |    2 +
 include/grub/lockdown.h                       |    3 +-
 include/grub/pkcs1_v15.h                      |   27 +
 include/grub/platform_keystore.h              |  235 +++
 include/grub/util/install.h                   |   10 +-
 include/grub/util/mkimage.h                   |    4 +-
 util/grub-install-common.c                    |   34 +-
 util/grub-mkimage.c                           |   25 +-
 util/grub-mkimagexx.c                         |   38 +-
 util/mkimage.c                                |   39 +-
 32 files changed, 6260 insertions(+), 69 deletions(-)
 create mode 100644 grub-core/commands/appendedsig/appendedsig.c
 create mode 100644 grub-core/commands/appendedsig/appendedsig.h
 create mode 100644 grub-core/commands/appendedsig/asn1util.c
 create mode 100644 grub-core/commands/appendedsig/gnutls_asn1_tab.c
 create mode 100644 grub-core/commands/appendedsig/pkcs7.c
 create mode 100644 grub-core/commands/appendedsig/pkix_asn1_tab.c
 create mode 100644 grub-core/commands/appendedsig/x509.c
 create mode 100644 grub-core/kern/ieee1275/platform_keystore.c
 create mode 100644 grub-core/lib/pkcs1_v15.c
 create mode 100644 grub-core/tests/appended_signature_test.c
 create mode 100644 grub-core/tests/appended_signatures.h
 create mode 100644 include/grub/pkcs1_v15.h
 create mode 100644 include/grub/platform_keystore.h

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2.43.5