StandaloneMmPkg/Core: Fix potential memory leak issue

In MmCoreFfsFindMmDriver(),
- ScratchBuffer is not freed in the error return path that DstBuffer page
allocation fails. Free ScratchBuffer before return with error.
- If the decoded buffer is identical to the data in InputSection,
ExtractGuidedSectionDecode() will change the value of DstBuffer rather
than changing the contents of the buffer that DstBuffer points at, in
which case freeing DstBuffer is wrong. Introduce a local variable
AllocatedDstBuffer for buffer free, free AllocatedDstBuffer immediately
if it is not used.

Cc: Laszlo Ersek <lersek@redhat.com>
Cc: Ard Biesheuvel <ardb+tianocore@kernel.org>
Cc: Sami Mujawar <sami.mujawar@arm.com>
Cc: Ray Ni <ray.ni@intel.com>
Signed-off-by: Wei6 Xu <wei6.xu@intel.com>
Reviewed-by: Ray Ni <ray.ni@intel.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

StandaloneMmPkg/Core/FwVol.c

@@ -84,6 +84,7 @@ MmCoreFfsFindMmDriver (
   UINT32                      DstBufferSize;
   VOID                        *ScratchBuffer;
   UINT32                      ScratchBufferSize;
+  VOID                        *AllocatedDstBuffer;
   VOID                        *DstBuffer;
   UINT16                      SectionAttribute;
   UINT32                      AuthenticationStatus;
@@ -148,25 +149,35 @@ MmCoreFfsFindMmDriver (
     //
     // Allocate destination buffer, extra one page for adjustment
     //
-    DstBuffer = (VOID *)(UINTN)AllocatePages (EFI_SIZE_TO_PAGES (DstBufferSize));
-    if (DstBuffer == NULL) {
+    AllocatedDstBuffer = (VOID *)(UINTN)AllocatePages (EFI_SIZE_TO_PAGES (DstBufferSize));
+    if (AllocatedDstBuffer == NULL) {
+      FreePages (ScratchBuffer, EFI_SIZE_TO_PAGES (ScratchBufferSize));
       return EFI_OUT_OF_RESOURCES;
     }
 
     //
     // Call decompress function
     //
-    Status = ExtractGuidedSectionDecode (
-               Section,
-               &DstBuffer,
-               ScratchBuffer,
-               &AuthenticationStatus
-               );
+    DstBuffer = AllocatedDstBuffer;
+    Status    = ExtractGuidedSectionDecode (
+                  Section,
+                  &DstBuffer,
+                  ScratchBuffer,
+                  &AuthenticationStatus
+                  );
     FreePages (ScratchBuffer, EFI_SIZE_TO_PAGES (ScratchBufferSize));
     if (EFI_ERROR (Status)) {
       goto FreeDstBuffer;
     }
 
+    //
+    // Free allocated DstBuffer if it is not used
+    //
+    if (DstBuffer != AllocatedDstBuffer) {
+      FreePages (AllocatedDstBuffer, EFI_SIZE_TO_PAGES (DstBufferSize));
+      AllocatedDstBuffer = NULL;
+    }
+
     DEBUG ((
       DEBUG_INFO,
       "Processing compressed firmware volume (AuthenticationStatus == %x)\n",
@@ -210,7 +221,9 @@ MmCoreFfsFindMmDriver (
   return EFI_SUCCESS;
 
 FreeDstBuffer:
-  FreePages (DstBuffer, EFI_SIZE_TO_PAGES (DstBufferSize));
+  if (AllocatedDstBuffer != NULL) {
+    FreePages (AllocatedDstBuffer, EFI_SIZE_TO_PAGES (DstBufferSize));
+  }
 
   return Status;
 }