Re: [PATCH v2 1/2] OvmfPkg/AmdSevDxe: Clear the C-bit from SMM Saved State

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

Hi Laszlo,


On 2/28/18 1:06 PM, Laszlo Ersek wrote:
> Hi Brijesh,
>
> On 02/28/18 17:14, Brijesh Singh wrote:
>> When OVMF is built with SMM, SMMSaved State area (SMM_DEFAULT_SMBASE +
>> SMRAM_SAVE_STATE_MAP_OFFSET) contains data which need to be accessed by
>> both guest and hypervisor. Since the data need to be accessed by both
>> hence we must map the SMMSaved State area as unencrypted (i.e C-bit
>> cleared).
>>
>> This patch clears the SavedStateArea address before SMBASE relocation.
>> Currently, we do not clear the SavedStateArea address after SMBASE is
>> relocated due to the following reasons:
>>
>> 1) Guest BIOS never access the relocated SavedStateArea.
>>
>> 2) The C-bit works on page-aligned address, but the SavedStateArea
>> address is not a page-aligned. Theoretically, we could roundup the address
>> and clear the C-bit of aligned address but looking carefully we found
>> that some portion of the page contains code -- which will causes a bigger
>> issue for the SEV guest. When SEV is enabled, all the code must be
>> encrypted otherwise hardware will cause trap.
>>
>> Cc: Jordan Justen <jordan.l.justen@intel.com>
>> Cc: Laszlo Ersek <lersek@redhat.com>
>> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> Contributed-under: TianoCore Contribution Agreement 1.1
>> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
>> ---
>>  OvmfPkg/AmdSevDxe/AmdSevDxe.inf                         |  4 +++
>>  OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.inf |  1 +
>>  OvmfPkg/AmdSevDxe/AmdSevDxe.c                           | 35 ++++++++++++++++++++
>>  OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.c   | 21 ++++++++++++
>>  4 files changed, 61 insertions(+)
> I've been staring at this patch for ~2 hours now (I've also read your
> other email). I like this approach (and the comments / commit message),
> but IMO an important detail is missing.
>
> I started writing up my notes, but the list got very long. Is it OK with
> you if I send my ideas as a patch set (replacing just this patch)? I
> think I'd like to turn this patch into 3-4 patches.

Yes,  patches are appreciated.  thanks.

>
> Thanks!
> Laszlo
>
>
>> diff --git a/OvmfPkg/AmdSevDxe/AmdSevDxe.inf b/OvmfPkg/AmdSevDxe/AmdSevDxe.inf
>> index 41635a57a454..162ed98a2fbe 100644
>> --- a/OvmfPkg/AmdSevDxe/AmdSevDxe.inf
>> +++ b/OvmfPkg/AmdSevDxe/AmdSevDxe.inf
>> @@ -29,6 +29,7 @@ [Packages]
>>    MdePkg/MdePkg.dec
>>    MdeModulePkg/MdeModulePkg.dec
>>    OvmfPkg/OvmfPkg.dec
>> +  UefiCpuPkg/UefiCpuPkg.dec
>>  
>>  [LibraryClasses]
>>    BaseLib
>> @@ -41,3 +42,6 @@ [LibraryClasses]
>>  
>>  [Depex]
>>    TRUE
>> +
>> +[FeaturePcd]
>> +  gUefiOvmfPkgTokenSpaceGuid.PcdSmmSmramRequire
>> diff --git a/OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.inf b/OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.inf
>> index 31edf3a9c1fd..ba564abb787b 100644
>> --- a/OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.inf
>> +++ b/OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.inf
>> @@ -36,3 +36,4 @@ [LibraryClasses]
>>    PcdLib
>>    DebugLib
>>    SmmServicesTableLib
>> +  MemEncryptSevLib
>> diff --git a/OvmfPkg/AmdSevDxe/AmdSevDxe.c b/OvmfPkg/AmdSevDxe/AmdSevDxe.c
>> index e472096320ea..5803e8655049 100644
>> --- a/OvmfPkg/AmdSevDxe/AmdSevDxe.c
>> +++ b/OvmfPkg/AmdSevDxe/AmdSevDxe.c
>> @@ -25,6 +25,8 @@
>>  #include <Library/UefiBootServicesTableLib.h>
>>  #include <Library/DxeServicesTableLib.h>
>>  #include <Library/MemEncryptSevLib.h>
>> +#include <Register/SmramSaveStateMap.h>
>> +#include <Register/QemuSmramSaveStateMap.h>
>>  
>>  EFI_STATUS
>>  EFIAPI
>> @@ -71,5 +73,38 @@ AmdSevDxeEntryPoint (
>>      FreePool (AllDescMap);
>>    }
>>  
>> +  //
>> +  // When SMM is enabled, clear the C-bit from SMM Saved State Area
>> +  //
>> +  // NOTES: The SavedStateArea address cleared here is before SMBASE
>> +  // relocation. Currently, we do not clear the SavedStateArea address after
>> +  // SMBASE is relocated due to the following reasons:
>> +  //
>> +  // 1) Guest BIOS never access the relocated SavedStateArea.
>> +  //
>> +  // 2) The C-bit works on page-aligned address, but the SavedStateArea
>> +  // address is not a page-aligned. Theoretically, we could roundup the address
>> +  // and clear the C-bit of aligned address but looking carefully we found
>> +  // that some portion of the page contains code -- which will causes a bigger
>> +  // issues for SEV guest. When SEV is enabled, all the code must be encrypted
>> +  // otherwise hardware will cause trap.
>> +  //
>> +  // We restore the C-bit for this SMM Saved State Area after SMBASE relocation
>> +  // is completed (See OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.c).
>> +  //
>> +  if (FeaturePcdGet (PcdSmmSmramRequire)) {
>> +    EFI_PHYSICAL_ADDRESS  SmmSavedStateAreaAddress;
>> +
>> +    SmmSavedStateAreaAddress = SMM_DEFAULT_SMBASE + SMRAM_SAVE_STATE_MAP_OFFSET;
>> +
>> +    Status = MemEncryptSevClearPageEncMask (
>> +               0,
>> +               SmmSavedStateAreaAddress,
>> +               EFI_SIZE_TO_PAGES (sizeof(QEMU_SMRAM_SAVE_STATE_MAP)),
>> +               FALSE
>> +               );
>> +    ASSERT_EFI_ERROR (Status);
>> +  }
>> +
>>    return EFI_SUCCESS;
>>  }
>> diff --git a/OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.c b/OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.c
>> index a307f64c9c61..946294701c62 100644
>> --- a/OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.c
>> +++ b/OvmfPkg/Library/SmmCpuFeaturesLib/SmmCpuFeaturesLib.c
>> @@ -20,6 +20,7 @@ WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED.
>>  #include <Library/MemoryAllocationLib.h>
>>  #include <Library/SmmServicesTableLib.h>
>>  #include <Library/DebugLib.h>
>> +#include <Library/MemEncryptSevLib.h>
>>  #include <Register/QemuSmramSaveStateMap.h>
>>  
>>  //
>> @@ -183,6 +184,26 @@ SmmCpuFeaturesSmmRelocationComplete (
>>    VOID
>>    )
>>  {
>> +  EFI_STATUS Status;
>> +  EFI_PHYSICAL_ADDRESS  SmmSavedStateAreaAddress;
>> +
>> +  //
>> +  // When SEV is enabled, the SMM SavedState is mapped with C=0
>> +  // (See OvmfPkg/AmdSevDxe/AmdSevDxe.c). Now the SMBASE is relocated hence we
>> +  // remap the address with C=1.
>> +  //
>> +  if (!MemEncryptSevIsEnabled ()) {
>> +    return;
>> +  }
>> +
>> +  SmmSavedStateAreaAddress = SMM_DEFAULT_SMBASE + SMRAM_SAVE_STATE_MAP_OFFSET;
>> +  Status = MemEncryptSevSetPageEncMask (
>> +             0,
>> +             SmmSavedStateAreaAddress,
>> +             EFI_SIZE_TO_PAGES (sizeof(QEMU_SMRAM_SAVE_STATE_MAP)),
>> +             FALSE
>> +             );
>> +  ASSERT_EFI_ERROR (Status);
>>  }
>>  
>>  /**
>>