Re: [PATCH v1 0/8] Measured SEV boot with kernel/initrd/cmdline

["Brijesh Singh" <brijesh.singh@amd.com>]

On 5/25/21 12:31 AM, Dov Murik wrote:
> Booting with SEV prevented the loading of kernel, initrd, and kernel
> command-line via QEMU fw_cfg interface because they arrive from the VMM
> which is untrusted in SEV.
>
> However, in some cases the kernel, initrd, and cmdline are not secret
> but should not be modified by the host.  In such a case, we want to
> verify inside the trusted VM that the kernel, initrd, and cmdline are
> indeed the ones expected by the Guest Owner, and only if that is the
> case go on and boot them up (removing the need for grub inside OVMF in
> that mode).
>
> This patch series declares a new page in MEMFD which will contain the
> hashes of these three blobs (kernel, initrd, cmdline), each under its
> own GUID entry.  This tables of hashes is populated by QEMU before
> launch, and encrypted as part of the initial VM memory; this makes sure
> theses hashes are part of the SEV measurement (which has to be approved
> by the Guest Owner for secret injection, for example).  Note that this
> requires a new QEMU patch which will be submitted soon.

I have not looked at the patches, but trying to brainstorm if we can
avoid reserving a new page in the MEMFD and use the existing EDK2
infrastructure to verify the blobs (kernel, initrd) loaded through the
FW_CFG interface in the guest memory.

If I understand correctly, then in your proposed approach, guest owner
wants to ensure that the hypevisor passing its preferred kernel, initrd
and cmdline. The guest owner basically knows the hashes of these
components in advance. So, can we do something like this:

- The secret blob provided by the guest owner should contains the hashes
(sha384) of these components.

- Use openssl API available in the edk2 to calculate the hash while
loading the kernel, initrd and cmdline.

- Before booting the kernel, compare the calculated hash with the one
listed in the secret page. If they don't match then fail otherwise continue.

Did I miss something ?

-Brijesh

> OVMF parses the table of hashes populated by QEMU (patch 5), and as it
> reads the fw_cfg blobs from QEMU, it will verify each one against the
> expected hash (kernel and initrd verifiers are introduced in patch 6,
> and command-line verifier is introduced in patches 7+8).  This is all
> done inside the trusted VM context.  If all the hashes are correct, boot
> of the kernel is allowed to continue.
>
> Any attempt by QEMU to modify the kernel, initrd, cmdline (including
> dropping one of them), or to modify the OVMF code that verifies those
> hashes, will cause the initial SEV measurement to change and therefore
> will be detectable by the Guest Owner during launch before secret
> injection.
>
> Cc: Laszlo Ersek <lersek@redhat.com>
> Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
> Cc: Jordan Justen <jordan.l.justen@intel.com>
> Cc: Ashish Kalra <ashish.kalra@amd.com>
> Cc: Brijesh Singh <brijesh.singh@amd.com>
> Cc: Erdem Aktas <erdemaktas@google.com>
> Cc: James Bottomley <jejb@linux.ibm.com>
> Cc: Jiewen Yao <jiewen.yao@intel.com>
> Cc: Min Xu <min.m.xu@intel.com>
> Cc: Tom Lendacky <thomas.lendacky@amd.com>
>
> James Bottomley (8):
>   OvmfPkg/AmdSev/SecretDxe: fix header comment to generic naming
>   OvmfPkg: PlatformBootManagerLibGrub: Allow executing kernel via fw_cfg
>   OvmfPkg/AmdSev: add a page to the MEMFD for firmware config hashes
>   OvmfPkg/QemuKernelLoaderFsDxe: Add ability to verify loaded items
>   OvmfPkg/AmdSev: Add library to find encrypted hashes for the FwCfg
>     device
>   OvmfPkg/AmdSev: Add firmware file plugin to verifier
>   OvmfPkg: GenericQemuLoadImageLib: Allow verifying fw_cfg command line
>   OvmfPkg/AmdSev: add SevQemuLoadImageLib
>
>  OvmfPkg/OvmfPkg.dec                                                       |  10 ++
>  OvmfPkg/AmdSev/AmdSevX64.dsc                                              |   9 +-
>  OvmfPkg/AmdSev/AmdSevX64.fdf                                              |   3 +
>  OvmfPkg/AmdSev/Library/SevFwCfgVerifier/SevFwCfgVerifier.inf              |  30 +++++
>  OvmfPkg/AmdSev/Library/SevHashFinderLib/SevHashFinderLib.inf              |  34 ++++++
>  OvmfPkg/AmdSev/Library/SevQemuLoadImageLib/SevQemuLoadImageLib.inf        |  30 +++++
>  OvmfPkg/Library/PlatformBootManagerLibGrub/PlatformBootManagerLibGrub.inf |   2 +
>  OvmfPkg/ResetVector/ResetVector.inf                                       |   2 +
>  OvmfPkg/AmdSev/Include/Library/SevHashFinderLib.h                         |  47 ++++++++
>  OvmfPkg/Include/Library/QemuFwCfgLib.h                                    |  35 ++++++
>  OvmfPkg/Library/PlatformBootManagerLibGrub/BdsPlatform.h                  |  11 ++
>  OvmfPkg/AmdSev/Library/SevFwCfgVerifier/SevFwCfgVerifier.c                |  60 ++++++++++
>  OvmfPkg/AmdSev/Library/SevHashFinderLib/SevHashFinderLib.c                | 126 ++++++++++++++++++++
>  OvmfPkg/AmdSev/Library/SevQemuLoadImageLib/SevQemuLoadImageLib.c          |  52 ++++++++
>  OvmfPkg/AmdSev/SecretDxe/SecretDxe.c                                      |   2 +-
>  OvmfPkg/Library/GenericQemuLoadImageLib/GenericQemuLoadImageLib.c         |  29 +++++
>  OvmfPkg/Library/PlatformBootManagerLibGrub/BdsPlatform.c                  |   5 +
>  OvmfPkg/Library/PlatformBootManagerLibGrub/QemuKernel.c                   |  50 ++++++++
>  OvmfPkg/QemuKernelLoaderFsDxe/QemuKernelLoaderFsDxe.c                     |  31 +++++
>  OvmfPkg/ResetVector/Ia16/ResetVectorVtf0.asm                              |  20 ++++
>  OvmfPkg/ResetVector/ResetVector.nasmb                                     |   2 +
>  21 files changed, 587 insertions(+), 3 deletions(-)
>  create mode 100644 OvmfPkg/AmdSev/Library/SevFwCfgVerifier/SevFwCfgVerifier.inf
>  create mode 100644 OvmfPkg/AmdSev/Library/SevHashFinderLib/SevHashFinderLib.inf
>  create mode 100644 OvmfPkg/AmdSev/Library/SevQemuLoadImageLib/SevQemuLoadImageLib.inf
>  create mode 100644 OvmfPkg/AmdSev/Include/Library/SevHashFinderLib.h
>  create mode 100644 OvmfPkg/AmdSev/Library/SevFwCfgVerifier/SevFwCfgVerifier.c
>  create mode 100644 OvmfPkg/AmdSev/Library/SevHashFinderLib/SevHashFinderLib.c
>  create mode 100644 OvmfPkg/AmdSev/Library/SevQemuLoadImageLib/SevQemuLoadImageLib.c
>  create mode 100644 OvmfPkg/Library/PlatformBootManagerLibGrub/QemuKernel.c
>