[PATCH] MdeModulePkg/UefiBootManagerLib: fix crash on uninitialized ExitData

["Ard Biesheuvel" <ard.biesheuvel@linaro.org>]

As reported by Gary, the recent LoadImage/StartImage changes to
accommodate dispatching PE/COFF images built for foreign architectures
may result in a crash when loading an IA32 option ROM into a X64 VM
running OVMF:

  Loading driver at 0x0007E537000 EntryPoint=0x0007E53C06D 8086100e.efi
  InstallProtocolInterface: BC62157E-3E33-4FEC-9920-2D3B36D750DF 7F003B98
  ProtectUefiImageCommon - 0x7F002BC0
    - 0x000000007E537000 - 0x000000000009F900
  Image type IA32 can't be started on X64 UEFI system.
  ASSERT MdeModulePkg/Core/Dxe/Mem/Pool.c(698): Head->Signature == ((('p') |
              ('h' << 8)) | ((('d') | ('0' << 8)) << 16)) || Head->Signature
              == ((('p') | ('h' << 8)) | ((('d') | ('1' << 8)) << 16))

This turns out to be caused by the deferred image loading code in BDS,
which doesn't check the result code of gBS->StartImage(), and ends up
trying to free an uninitialized pointer. So ensure ExitData is initialized
before the call.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 MdeModulePkg/Library/UefiBootManagerLib/BmMisc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/MdeModulePkg/Library/UefiBootManagerLib/BmMisc.c b/MdeModulePkg/Library/UefiBootManagerLib/BmMisc.c
index fc8775dfa419..cf99de5b924a 100644
--- a/MdeModulePkg/Library/UefiBootManagerLib/BmMisc.c
+++ b/MdeModulePkg/Library/UefiBootManagerLib/BmMisc.c
@@ -502,6 +502,7 @@ EfiBootManagerDispatchDeferredImages (
         // a 5 Minute period
         //
         gBS->SetWatchdogTimer (5 * 60, 0x0000, 0x00, NULL);
+        ExitData = NULL;
         Status = gBS->StartImage (ImageHandle, &ExitDataSize, &ExitData);
         if (ExitData != NULL) {
           FreePool (ExitData);
-- 
2.17.1