Re: [edk2-devel] [RFC PATCH v3 32/43] OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Bypass flash detection with SEV-ES is enabled

["Lendacky, Thomas" <thomas.lendacky@amd.com>]

On 11/21/19 6:31 AM, Laszlo Ersek via Groups.Io wrote:
> On 11/20/19 21:06, Lendacky, Thomas wrote:
>> The flash detection routine will attempt to determine how the flash
>> device behaves (e.g. ROM, RAM, Flash). But when SEV-ES is enabled and
>> the flash device behaves as a ROM device (meaning it is marked read-only
>> by the hypervisor), this check may result in an infinite nested page fault
>> because of the attempted write. Since the instruction cannot be emulated
>> when SEV-ES is enabled, the RIP is never advanced, resulting in repeated
>> nested page faults.
>>
>> When SEV-ES is enabled, exit the flash detection early and assume that
>> the FD behaves as Flash. This will result in QemuFlashWrite() being called
>> to store EFI variables, which will also result in an infinite nested page
>> fault when the write is performed. In this case, update QemuFlashWrite()
>> to use the VmgMmioWrite function from the VmgExitLib library to have the
>> hypervisor perform the write without having to emulate the instruction.
> 
> (1) Please include the Bugzilla link in the commit message:
> 
> https://bugzilla.tianocore.org/show_bug.cgi?id=2198

Yup, will do. I'm also missing the Cc:'s, so I'll add those as well.

> 
>>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>>  OvmfPkg/OvmfPkgIa32.dsc                       |  1 +
>>  OvmfPkg/OvmfPkgIa32X64.dsc                    |  1 +
>>  OvmfPkg/OvmfPkgX64.dsc                        |  1 +
>>  .../FvbServicesRuntimeDxe.inf                 |  2 +
>>  .../QemuFlash.c                               | 38 +++++++++++++++++--
>>  5 files changed, 40 insertions(+), 3 deletions(-)
>>
>> diff --git a/OvmfPkg/OvmfPkgIa32.dsc b/OvmfPkg/OvmfPkgIa32.dsc
>> index 56670eefde6b..ff2814c6246e 100644
>> --- a/OvmfPkg/OvmfPkgIa32.dsc
>> +++ b/OvmfPkg/OvmfPkgIa32.dsc
>> @@ -320,6 +320,7 @@ [LibraryClasses.common.DXE_RUNTIME_DRIVER]
>>    BaseCryptLib|CryptoPkg/Library/BaseCryptLib/RuntimeCryptLib.inf
>>    PciLib|OvmfPkg/Library/DxePciLibI440FxQ35/DxePciLibI440FxQ35.inf
>>    QemuFwCfgS3Lib|OvmfPkg/Library/QemuFwCfgS3Lib/DxeQemuFwCfgS3LibFwCfg.inf
>> +  VmgExitLib|UefiCpuPkg/Library/VmgExitLib/VmgExitLib.inf
>>  
>>  [LibraryClasses.common.UEFI_DRIVER]
>>    PcdLib|MdePkg/Library/DxePcdLib/DxePcdLib.inf
>> diff --git a/OvmfPkg/OvmfPkgIa32X64.dsc b/OvmfPkg/OvmfPkgIa32X64.dsc
>> index 9897e6889573..212952cfaacd 100644
>> --- a/OvmfPkg/OvmfPkgIa32X64.dsc
>> +++ b/OvmfPkg/OvmfPkgIa32X64.dsc
>> @@ -325,6 +325,7 @@ [LibraryClasses.common.DXE_RUNTIME_DRIVER]
>>    BaseCryptLib|CryptoPkg/Library/BaseCryptLib/RuntimeCryptLib.inf
>>    PciLib|OvmfPkg/Library/DxePciLibI440FxQ35/DxePciLibI440FxQ35.inf
>>    QemuFwCfgS3Lib|OvmfPkg/Library/QemuFwCfgS3Lib/DxeQemuFwCfgS3LibFwCfg.inf
>> +  VmgExitLib|UefiCpuPkg/Library/VmgExitLib/VmgExitLib.inf
>>  
>>  [LibraryClasses.common.UEFI_DRIVER]
>>    PcdLib|MdePkg/Library/DxePcdLib/DxePcdLib.inf
>> diff --git a/OvmfPkg/OvmfPkgX64.dsc b/OvmfPkg/OvmfPkgX64.dsc
>> index 59c4f9207fc3..8331fc0b663e 100644
>> --- a/OvmfPkg/OvmfPkgX64.dsc
>> +++ b/OvmfPkg/OvmfPkgX64.dsc
>> @@ -325,6 +325,7 @@ [LibraryClasses.common.DXE_RUNTIME_DRIVER]
>>    BaseCryptLib|CryptoPkg/Library/BaseCryptLib/RuntimeCryptLib.inf
>>    PciLib|OvmfPkg/Library/DxePciLibI440FxQ35/DxePciLibI440FxQ35.inf
>>    QemuFwCfgS3Lib|OvmfPkg/Library/QemuFwCfgS3Lib/DxeQemuFwCfgS3LibFwCfg.inf
>> +  VmgExitLib|UefiCpuPkg/Library/VmgExitLib/VmgExitLib.inf
>>  
>>  [LibraryClasses.common.UEFI_DRIVER]
>>    PcdLib|MdePkg/Library/DxePcdLib/DxePcdLib.inf
>> diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf
>> index ca6326e833ed..0b7741ac07f8 100644
>> --- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf
>> +++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/FvbServicesRuntimeDxe.inf
>> @@ -38,6 +38,7 @@ [Sources]
>>  [Packages]
>>    MdePkg/MdePkg.dec
>>    MdeModulePkg/MdeModulePkg.dec
>> +  UefiCpuPkg/UefiCpuPkg.dec
>>    OvmfPkg/OvmfPkg.dec
>>  
>>  [LibraryClasses]
>> @@ -52,6 +53,7 @@ [LibraryClasses]
>>    UefiBootServicesTableLib
>>    UefiDriverEntryPoint
>>    UefiRuntimeLib
>> +  VmgExitLib
>>  
>>  [Guids]
>>    gEfiEventVirtualAddressChangeGuid   # ALWAYS_CONSUMED
> 
> These hunks look good.
> 
>> diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c
>> index c81c58972bf2..4f3bf690fcad 100644
>> --- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c
>> +++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c
>> @@ -9,7 +9,9 @@
>>  
>>  #include <Library/BaseMemoryLib.h>
>>  #include <Library/DebugLib.h>
>> +#include <Library/MemEncryptSevLib.h>
>>  #include <Library/PcdLib.h>
>> +#include <Library/VmgExitLib.h>
>>  
>>  #include "QemuFlash.h"
>>  
>> @@ -80,6 +82,20 @@ QemuFlashDetected (
>>  
>>    DEBUG ((EFI_D_INFO, "QEMU Flash: Attempting flash detection at %p\n", Ptr));
>>  
>> +  if (MemEncryptSevEsIsEnabled()) {
>> +    //
>> +    // When SEV-ES is enabled, the check below can result in an infinite
>> +    // loop with respect to a nested page fault. When the FD behaves as
>> +    // a ROM, the nested page table entry is read-only. The check below
> 
> (2) I suggest a slight change to the wording above. Please replace
> 
>   when the FD behaves as a ROM
> 
> with
> 
>   when the memslot is mapped read-only
> 
> (It's OK to make qemu / kvm references in this code: the name of the
> file includes "Qemu".)

Ok.

> 
>> +    // will cause a nested page fault that cannot be emulated, causing
>> +    // the instruction to retried over and over. For SEV-ES, assume that
>> +    // the FD does not behave as FLASH.
> 
> (3) I would recommend
> 
>   For SEV-ES, acknowledge that the FD appears as ROM and not as FLASH,
>   but report FLASH anyway.

How about if I expand on this to add "because FLASH behavior can be
simulated using VMGEXIT.", too.

> 
>> +    //
>> +    DEBUG ((DEBUG_INFO,
>> +      "QEMU Flash: SEV-ES enabled, assuming FD behaves as FLASH\n"));
>> +    return TRUE;
>> +  }
>> +
>>    OriginalUint8 = *Ptr;
>>    *Ptr = CLEAR_STATUS_CMD;
>>    ProbeUint8 = *Ptr;
>> @@ -147,6 +163,21 @@ QemuFlashRead (
>>  }
>>  
>>  
>> +STATIC
>> +VOID
>> +QemuFlashPtrWrite (
>> +  IN        volatile UINT8                      *Ptr,
>> +  IN        UINT8                               Value
>> +  )
>> +{
>> +  if (MemEncryptSevEsIsEnabled()) {
>> +    VmgMmioWrite ((UINT8 *) Ptr, &Value, 1);
>> +  } else {
>> +    *Ptr = Value;
>> +  }
>> +}
>> +
>> +
>>  /**
>>    Write to QEMU Flash
>>  
> 
> (4) This change will break the "FvbServicesSmm.inf" build of this module.
> 
> (Because, you have -- correctly -- added the VmgExitLib class to
> "FvbServicesRuntimeDxe.inf" only.)
> 
> Please:
> 
> - create two implementations of the QemuFlashPtrWrite() function, one in
> "QemuFlashDxe.c", and another in "QemuFlashSmm.c".
> 
> - the former should be the function shown above, the latter should only
> perform the direct assignment.
> 
> - the '#include <Library/VmgExitLib.h>' directive should also be moved
> to "QemuFlashDxe.c".
> 
> - the prototype for QemuFlashPtrWrite() should be added to "QemuFlash.h"
> please.

Ah, nice catch.  I'll do that.

Thanks,
Tom

> 
>> @@ -181,8 +212,9 @@ QemuFlashWrite (
>>    //
>>    Ptr = QemuFlashPtr (Lba, Offset);
>>    for (Loop = 0; Loop < *NumBytes; Loop++) {
>> -    *Ptr = WRITE_BYTE_CMD;
>> -    *Ptr = Buffer[Loop];
>> +    QemuFlashPtrWrite (Ptr, WRITE_BYTE_CMD);
>> +    QemuFlashPtrWrite (Ptr, Buffer[Loop]);
>> +
>>      Ptr++;
>>    }
>>  
>> @@ -190,7 +222,7 @@ QemuFlashWrite (
>>    // Restore flash to read mode
>>    //
>>    if (*NumBytes > 0) {
>> -    *(Ptr - 1) = READ_ARRAY_CMD;
>> +    QemuFlashPtrWrite (Ptr - 1, READ_ARRAY_CMD);
>>    }
>>  
>>    return EFI_SUCCESS;
>>
> 
> With (1) and (4) fixed, and with (2) and (3) optionally fixed (i.e., if
> you agree):
> 
> Reviewed-by: Laszlo Ersek <lersek@redhat.com>
> 
> Thanks
> Laszlo
> 
> 
> 
>