[PATCH 1/1] FatPkg/EnhancedFatDxe Fix Double Cluster Allocation

["Robinson, Herbie" <Herbie.Robinson@stratus.com>]

This is a fix for a double cluster allocation when the disk is full.  The double allocation happens because FatGrowEof calls FatAllocateCluster without immediately marking the each returned cluster as allocated.  The fix is to move the FatSetFatEntry call inside the loop.  I've also include some improvements to the sanity checks that I added while tracking this down.  They are optional.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by:Herbie Robinson <Herbie.Robinson@stratus.com>
---
diff --git a/FatPkg/EnhancedFatDxe/FileSpace.c b/FatPkg/EnhancedFatDxe/FileSpace.c
index 1254cd6..e17d3b6 100644
--- a/FatPkg/EnhancedFatDxe/FileSpace.c
+++ b/FatPkg/EnhancedFatDxe/FileSpace.c
@@ -467,7 +467,7 @@ FatGrowEof (
       ClusterCount  = 0;

       while (!FAT_END_OF_FAT_CHAIN (Cluster)) {
-        if (Cluster == FAT_CLUSTER_FREE || Cluster >= FAT_CLUSTER_SPECIAL) {
+        if (Cluster < FAT_MIN_CLUSTER || Cluster > Volume->MaxCluster + 1) {

           DEBUG (
             (EFI_D_INIT | EFI_D_ERROR,
@@ -509,6 +509,11 @@ FatGrowEof (
         goto Done;
       }

+      if (NewCluster < FAT_MIN_CLUSTER || NewCluster > Volume->MaxCluster + 1) {
+        Status = EFI_VOLUME_CORRUPTED;
+        goto Done;
+      }
+
       if (LastCluster != 0) {
         FatSetFatEntry (Volume, LastCluster, NewCluster);
       } else {
@@ -518,12 +523,21 @@ FatGrowEof (

       LastCluster = NewCluster;
       CurSize += 1;
+
+      //
+      // Terminate the cluster list
+      //
+      // Note that we must do this EVERY time we allocate a cluster, because
+      // FatAllocateCluster scans the FAT looking for a free cluster and
+      // "LastCluster" is no longer free!  Usually, FatAllocateCluster will
+      // start looking with the cluster after "LastCluster"; however, when
+      // there is only one free cluster left, it will find "LastCluster"
+      // a second time.  There are other, less predictable scenarios
+      // where this could happen, as well.
+      //
+      FatSetFatEntry (Volume, LastCluster, (UINTN) FAT_CLUSTER_LAST);
+      OFile->FileLastCluster = LastCluster;
     }
-    //
-    // Terminate the cluster list
-    //
-    FatSetFatEntry (Volume, LastCluster, (UINTN) FAT_CLUSTER_LAST);
-    OFile->FileLastCluster = LastCluster;
   }

   OFile->FileSize = (UINTN) NewSizeInBytes;
@@ -603,7 +617,7 @@ FatOFilePosition (
       Cluster = FatGetFatEntry (Volume, Cluster);
     }

-    if (Cluster < FAT_MIN_CLUSTER) {
+    if (Cluster < FAT_MIN_CLUSTER || Cluster > Volume->MaxCluster + 1) {
       return EFI_VOLUME_CORRUPTED;
     }