Re: [edk2-devel] [PATCH v1 0/8] Measured SEV boot with kernel/initrd/cmdline

["Laszlo Ersek" <lersek@redhat.com>]

On 06/01/21 15:20, Ard Biesheuvel wrote:
> On Tue, 1 Jun 2021 at 14:12, Laszlo Ersek <lersek@redhat.com> wrote:
>>
> ...
>> - A major complication for hashing all three of: kernel, initrd,
>> cmdline, is that the *fetching* of this triplet is split between two
>> places. (Well, it is split between *three* places in fact, but I'm
>> going to ignore LinuxInitrdDynamicShellCommand for now, because the
>> AmdSevX64 platform sets BUILD_SHELL to FALSE for production.)
>>
>> The kernel and the initrd are fetched in QemuKernelLoaderFsDxe, but
>> the command line is fetched in (both) QemuLoadImageLib instances.
>> This requires that all these modules be littered with hashing as
>> well, which I find *really bad*. Even if we factor out the actual
>> logic, I strongly dislike having *just hooks* for hashing in multiple
>> modules.
>>
>> Now, please refer to efc52d67e157 ("OvmfPkg/QemuKernelLoaderFsDxe:
>> don't expose kernel command line", 2020-03-05). If we first
>>
>> (a) reverted that commit, and
>>
>> (b) modified *both* QemuLoadImageLib instances, to load the kernel
>> command line from the *synthetic filesystem* (rather than directly
>> from fw_cfg),
>>
>> then we could centralize the hashing to just QemuKernelLoaderFsDxe.
>>
>> Ard -- what's your thought on this?
>>
>
> I don't have any problems with that - I take it this means we can drop
> the QemuFwCfgLib dependency from GenericQemuLoadImageLib altogether,
> right?

A bit more work is needed for that (but I agree it should be done),
because we have this additionally:

  QemuFwCfgSelectItem (QemuFwCfgItemInitrdSize);
  InitrdSize = (UINTN)QemuFwCfgRead32 ();

  if (InitrdSize > 0) {
    //
    // Append ' initrd=initrd' in UTF-16.
    //
    KernelLoadedImage->LoadOptionsSize += sizeof (L" initrd=initrd") - 2;
  }

This should also be rebased on top of the synthetic filesystem [*], and
then no more QemuFwCfgLib calls should remain.

[*] From StubFileOpen() in "QemuKernelLoaderFsDxe.c", it seems that we
    successfully open zero-sized fw_cfg files. (We also list them, when
    reading the root directory, in StubFileRead()). That's not a problem
    at all, but it means that, after opening the initrd file temporarily
    in QemuLoadImageLib, EFI_FILE_PROTOCOL.GetInfo() should be used for
    fetching an EFI_FILE_INFO, and then EFI_FILE_INFO.FileSize needs to
    be compared to 0.


>
>>
>> And then, we could eliminate the dynamic callback registration, plus
>> the separate SevFwCfgVerifier, SevHashFinderLib, and
>> SevQemuLoadImageLib stuff.
>>
>> We'd only need one new lib class, with *statically linked* hooks for
>> QemuKernelLoaderFsDxe, and two instances of this new class, a Null
>> one, and an actual (SEV hash verifier) one. The latter instance would
>> locate the hash values, calculate the fresh hashes, and perform the
>> comparisons. Only the AmdSevX64 platform would use the non-Null
>> instance of this library class.
>>
>> (NB QemuKernelLoaderFsDxe is used by some ArmVirtPkg platforms, so
>> resolutions to the Null instance would be required there too.)
>>
>
> This sounds like a good approach to me.

Thank you!
Laszlo

>
>
>>
>>>
>>> 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
>>>
>>
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