1 files changed, 53 insertions, 9 deletions
diff --git a/FatPkg/EnhancedFatDxe/Init.c b/FatPkg/EnhancedFatDxe/Init.c
index 9c51ed5b7b..f020cf703d 100644
--- a/FatPkg/EnhancedFatDxe/Init.c
+++ b/FatPkg/EnhancedFatDxe/Init.c
@@ -96,14 +96,6 @@ FatAllocateVolume (
   }
 
   //
-  // Initialize cache
-  //
-  Status = FatInitializeDiskCache (Volume);
-  if (EFI_ERROR (Status)) {
-    goto Done;
-  }
-
-  //
   // Install our protocol interfaces on the device's handle
   //
   Status = gBS->InstallMultipleProtocolInterfaces (
@@ -237,6 +229,7 @@ FatOpenDevice (
   UINTN                 SectorsPerFat;
   UINT8                 SectorsPerClusterAlignment;
   UINT8                 BlockAlignment;
+  UINTN                 ReservedFatEntry;
 
   //
   // Read the FAT_BOOT_SECTOR BPB info
@@ -423,7 +416,58 @@ FatOpenDevice (
   // We are now defining FAT Type
   //
   Volume->FatType = FatType;
-  ASSERT (FatType != FatUndefined);
+
+  //
+  // Initialize cache before we use the helper functions that hit the cache
+  //
+  Status = FatInitializeDiskCache (Volume);
+  if (EFI_ERROR (Status)) {
+    return Status;
+  }
+
+  //
+  // Check the reserved FAT entries to ensure they contain valid values
+  //
+  ReservedFatEntry = FatGetFatEntry (Volume, 0);
+  if (Volume->FatEntryBuffer == MAX_UINT32) {
+    return EFI_VOLUME_CORRUPTED;
+  }
+
+  // Reserved FAT entry 0 should contain the BPB_MEDIA byte value in the low 8 bits with all other bits set to 1
+  switch (FatType) {
+    case Fat12:
+      if ((ReservedFatEntry & FAT_CLUSTER_MASK_FAT12) != ((UINTN)FatBs.FatBsb.Media | 0xF00)) {
+        return EFI_VOLUME_CORRUPTED;
+      }
+
+      break;
+
+    case Fat16:
+      if ((ReservedFatEntry & FAT_CLUSTER_MASK_FAT16) != ((UINTN)FatBs.FatBsb.Media | 0xFF00)) {
+        return EFI_VOLUME_CORRUPTED;
+      }
+
+      break;
+
+    case Fat32:
+      // the upper 4 bits of a FAT32 entry are reserved, so are unchecked here
+      if ((ReservedFatEntry & FAT_CLUSTER_MASK_FAT32) != ((UINTN)FatBs.FatBsb.Media | 0x0FFFFF00)) {
+        return EFI_VOLUME_CORRUPTED;
+      }
+
+      break;
+
+    default:
+      return EFI_VOLUME_CORRUPTED;
+  }
+
+  // Reserved FAT entry 1 should contain the end of chain mark. On FAT16 and FAT32, the high 2 bits may be used as
+  // dirty and hardware error bits, so are ignored in this check, but FatGetFatEntry already ignores them to unify the
+  // logic across FAT types
+  ReservedFatEntry = FatGetFatEntry (Volume, 1);
+  if ((Volume->FatEntryBuffer == MAX_UINT32) || !FAT_END_OF_FAT_CHAIN (ReservedFatEntry)) {
+    return EFI_VOLUME_CORRUPTED;
+  }
 
   return EFI_SUCCESS;
 }