Re: [edk2-devel] [PATCH v5 36/42] OvmfPkg/QemuFlashFvbServicesRuntimeDxe: Bypass flash detection with SEV-ES is enabled

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

On 3/3/20 6:33 AM, Laszlo Ersek wrote:
> Hi Tom,
> 
> On 03/03/20 00:07, Lendacky, Thomas wrote:
>> BZ: https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbugzilla.tianocore.org%2Fshow_bug.cgi%3Fid%3D2198&data=02%7C01%7Cthomas.lendacky%40amd.com%7Cec51d6f2d119454bdbc508d7bf6f127a%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637188356100831031&sdata=h7GUpHnIH8kilQY9wvXwvIujG6YU5PgYl5wYK2rP2Eg%3D&reserved=0
>>
>> 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.
>>
>> Cc: Jordan Justen <jordan.l.justen@intel.com>
>> Cc: Laszlo Ersek <lersek@redhat.com>
>> Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
>> Reviewed-by: Laszlo Ersek <lersek@redhat.com>
>> Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
>> ---
>>  OvmfPkg/OvmfPkgIa32.dsc                       |  1 +
>>  OvmfPkg/OvmfPkgIa32X64.dsc                    |  1 +
>>  OvmfPkg/OvmfPkgX64.dsc                        |  1 +
> 
> I asked for these lib class resolutions to be dropped, under v4.

Hmmm... I thought I had when I consolidated the library references, but
obviously I didn't. I'll fix that up.

Thanks,
Tom

> 
> https://nam11.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmid.mail-archive.com%2F53e0bc61-5105-1597-7add-86e038015e15%40redhat.com&amp;data=02%7C01%7Cthomas.lendacky%40amd.com%7Cec51d6f2d119454bdbc508d7bf6f127a%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C637188356100841024&amp;sdata=QrbXlgrKDVhspRRKaXPUSaD5KJ5u7vZ9q6yQ70SKSWM%3D&amp;reserved=0
> 
> Laszlo
> 
>>  .../FvbServicesRuntimeDxe.inf                 |  2 ++
>>  .../QemuFlash.h                               |  6 +++++
>>  .../QemuFlash.c                               | 23 ++++++++++++++++---
>>  .../QemuFlashDxe.c                            | 15 ++++++++++++
>>  .../QemuFlashSmm.c                            |  9 ++++++++
>>  8 files changed, 55 insertions(+), 3 deletions(-)
>>
>> diff --git a/OvmfPkg/OvmfPkgIa32.dsc b/OvmfPkg/OvmfPkgIa32.dsc
>> index 51d7acafdda3..2531b7edccf5 100644
>> --- a/OvmfPkg/OvmfPkgIa32.dsc
>> +++ b/OvmfPkg/OvmfPkgIa32.dsc
>> @@ -318,6 +318,7 @@ [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 764fa8c287a0..629725ef2b44 100644
>> --- a/OvmfPkg/OvmfPkgIa32X64.dsc
>> +++ b/OvmfPkg/OvmfPkgIa32X64.dsc
>> @@ -323,6 +323,7 @@ [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 b6e29e09db97..74076cbe7692 100644
>> --- a/OvmfPkg/OvmfPkgX64.dsc
>> +++ b/OvmfPkg/OvmfPkgX64.dsc
>> @@ -323,6 +323,7 @@ [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 8125fd0735a1..3ce19d1bfa8e 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
>> diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.h b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.h
>> index f1afabcbe6ae..19ac1f733279 100644
>> --- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.h
>> +++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.h
>> @@ -89,5 +89,11 @@ QemuFlashBeforeProbe (
>>    IN  UINTN                   FdBlockCount
>>    );
>>  
>> +VOID
>> +QemuFlashPtrWrite (
>> +  IN        volatile UINT8    *Ptr,
>> +  IN        UINT8             Value
>> +  );
>> +
>>  #endif
>>  
>> diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c
>> index c81c58972bf2..ccf5ad7f7afb 100644
>> --- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c
>> +++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlash.c
>> @@ -9,6 +9,7 @@
>>  
>>  #include <Library/BaseMemoryLib.h>
>>  #include <Library/DebugLib.h>
>> +#include <Library/MemEncryptSevLib.h>
>>  #include <Library/PcdLib.h>
>>  
>>  #include "QemuFlash.h"
>> @@ -80,6 +81,21 @@ 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 memslot is mapped
>> +    // read-only, the nested page table entry is read-only. The check below
>> +    // will cause a nested page fault that cannot be emulated, causing
>> +    // the instruction to retried over and over. For SEV-ES, acknowledge that
>> +    // the FD appears as ROM and not as FLASH, but report FLASH anyway because
>> +    // FLASH behavior can be simulated using VMGEXIT.
>> +    //
>> +    DEBUG ((DEBUG_INFO,
>> +      "QEMU Flash: SEV-ES enabled, assuming FD behaves as FLASH\n"));
>> +    return TRUE;
>> +  }
>> +
>>    OriginalUint8 = *Ptr;
>>    *Ptr = CLEAR_STATUS_CMD;
>>    ProbeUint8 = *Ptr;
>> @@ -181,8 +197,9 @@ 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 +207,7 @@ QemuFlashWrite (
>>    // Restore flash to read mode
>>    //
>>    if (*NumBytes > 0) {
>> -    *(Ptr - 1) = READ_ARRAY_CMD;
>> +    QemuFlashPtrWrite (Ptr - 1, READ_ARRAY_CMD);
>>    }
>>  
>>    return EFI_SUCCESS;
>> diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
>> index 5aabe9d7b59c..939463a8e17c 100644
>> --- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
>> +++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashDxe.c
>> @@ -10,6 +10,8 @@
>>  **/
>>  
>>  #include <Library/UefiRuntimeLib.h>
>> +#include <Library/MemEncryptSevLib.h>
>> +#include <Library/VmgExitLib.h>
>>  
>>  #include "QemuFlash.h"
>>  
>> @@ -32,3 +34,16 @@ QemuFlashBeforeProbe (
>>    // Do nothing
>>    //
>>  }
>> +
>> +VOID
>> +QemuFlashPtrWrite (
>> +  IN        volatile UINT8    *Ptr,
>> +  IN        UINT8             Value
>> +  )
>> +{
>> +  if (MemEncryptSevEsIsEnabled()) {
>> +    VmgMmioWrite ((UINT8 *) Ptr, &Value, 1);
>> +  } else {
>> +    *Ptr = Value;
>> +  }
>> +}
>> diff --git a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashSmm.c b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashSmm.c
>> index 7eb426e03855..eff40ae28032 100644
>> --- a/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashSmm.c
>> +++ b/OvmfPkg/QemuFlashFvbServicesRuntimeDxe/QemuFlashSmm.c
>> @@ -46,3 +46,12 @@ QemuFlashBeforeProbe (
>>               );
>>    ASSERT_EFI_ERROR (Status);
>>  }
>> +
>> +VOID
>> +QemuFlashPtrWrite (
>> +  IN        volatile UINT8    *Ptr,
>> +  IN        UINT8             Value
>> +  )
>> +{
>> +  *Ptr = Value;
>> +}
>>
>