SMM callout vulnerability in SMM driver on multiple HP devices (SMM arbitrary code execution).

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Vulnerability exists in the software System Management Interrupt (SWSMI) handler located at offset 0x1574 in the module 0701.efi (code example given from S05_02020000.bin firmware).

The SWSMI handler with number 0xAE dereferences gRT (EFI_RUNTIME_SERVICES) pointer to call a GetVariable service, which is located outside of SMRAM. Hence, this can result in a code execution in SMM (escalating privilege from ring 0 to ring -2).

Below is the decompiled code of the vulnerable handler:

EFI_STATUS __fastcall SwSmiHandler_1574(EFI_HANDLE DispatchHandle)
{
  EFI_STATUS result;
  UINT32 Attributes;
  UINTN DataSize;
  EFI_SMM_SW_DISPATCH2_PROTOCOL *EfiSmmSwDispatch2;
  __int64 Data[6];

  Attributes = 6;
  DataSize = 32i64;
  if ( (gRT_20F8->GetVariable(L"AmdMemContextData", &VendorGuid, &Attributes, &DataSize, Data) & 0x8000000000000000ui64) == 0i64 )
    x_AmdMemContextHandler(Data);
  result = gSmst_20F0->SmmLocateProtocol(&EFI_SMM_SW_DISPATCH_PROTOCOL_GUID_2050, 0i64, &EfiSmmSwDispatch2);
  if ( (result & 0x8000000000000000ui64) == 0i64 )
    return (EfiSmmSwDispatch2->UnRegister)(EfiSmmSwDispatch2, DispatchHandle);
  return result;
}

Due to the fact this handler unregisters itself by calling EfiSmmSwDispatch2->UnRegister, the vulnerability cannot be exploited from the operating system. However, using EFI_BOOT_SERVICES and EFI_RUNTIME_SERVICES is unsafe inside a code intended to run in SMM (from SMRAM) because an attacker capable of executing code in DXE phase could exploit this vulnerability to escalate privileges to SMM (ring -2). This can be achieved even after EfiSmmReadyToLock event since no gSmst->InSmram() check is applied.

To exploit this vulnerability from DXE environment it is enough to:

• overwrite the GetVariable service address in the EFI_RUNTIME_SERVICES table with the shellcode address

• trigger the handler in the same way as in the DXE driver with the name 0701 from the firmware S05_02020000.bin:

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  EFI_STATUS __fastcall EfiEventReadyToBootNotifier(EFI_EVENT Event)
  {
    EFI_STATUS status;
    EFI_SMM_CONTROL2_PROTOCOL *EfiSmmControl2Protocol;
    char DataPort;
    char CommandPort;
  
    status = gBS_2098->LocateProtocol(&EFI_SMM_CONTROL2_PROTOCOL_GUID_2030, 0i64, &EfiSmmControl2Protocol);
    if ( (status & 0x8000000000000000ui64) == 0i64 )
    {
      CommandPort = 0xAE;
      DataPort = 1;
      (EfiSmmControl2Protocol->Trigger)(EfiSmmControl2Protocol, &CommandPort, &DataPort, 0i64, 0i64);
      return gBS_2098->CloseEvent(Event);
    }
    return status;
  }
  

To fix this vulnerability, it is essential to replace the GetVariable service from EFI_RUNTIME_SERVICES with EfiSmmVariableProtocol->SmmSetVariable.

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This bug is subject to a 90 day disclosure deadline. After 90 days elapsed or a patch has been made broadly available (whichever is earlier), the bug report will become visible to the public.

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BINARLY efiXplorer team

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• EDK II Secure Code Review Guide: SMM Callout
• Code Check(Mate) in SMM

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