Re: [PATCH v3 1/3] ArmPkg/ArmMmuLib AARCH64: limit recursion when freeing page tables

["Ashish Singhal" <ashishsingha@nvidia.com>]

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Reviewed-by: Ashish Singhal <ashishsingha@nvidia.com>
Tested-by: Ashish Singhal <ashishsingha@nvidia.com>
________________________________
From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Sent: Wednesday, March 25, 2020 9:29 AM
To: devel@edk2.groups.io <devel@edk2.groups.io>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>; Laszlo Ersek <lersek@redhat.com>; Leif Lindholm <leif@nuviainc.com>; Ashish Singhal <ashishsingha@nvidia.com>
Subject: [PATCH v3 1/3] ArmPkg/ArmMmuLib AARCH64: limit recursion when freeing page tables

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FreePageTablesRecursive () traverses the page table tree depth first
to free all pages that it finds, without taking into account the
level at which it is operating.

Since TT_TYPE_TABLE_ENTRY aliases TT_TYPE_BLOCK_ENTRY_LEVEL3, we cannot
distinguish table entries from block entries unless we take the level
into account, and so we may be dereferencing garbage if we happen to
try and free a hierarchy of page tables that has level 3 pages in it.

Let's fix this by passing the level into FreePageTablesRecursive (),
and limit the recursion to levels < 3.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
---
 ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
index a43d468b73ca..d78918cf7ba8 100644
--- a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
+++ b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
@@ -142,15 +142,21 @@ ReplaceTableEntry (
 STATIC
 VOID
 FreePageTablesRecursive (
-  IN  UINT64  *TranslationTable
+  IN  UINT64  *TranslationTable,
+  IN  UINTN   Level
   )
 {
   UINTN   Index;

-  for (Index = 0; Index < TT_ENTRY_COUNT; Index++) {
-    if ((TranslationTable[Index] & TT_TYPE_MASK) == TT_TYPE_TABLE_ENTRY) {
-      FreePageTablesRecursive ((VOID *)(UINTN)(TranslationTable[Index] &
-                                               TT_ADDRESS_MASK_BLOCK_ENTRY));
+  ASSERT (Level <= 3);
+
+  if (Level < 3) {
+    for (Index = 0; Index < TT_ENTRY_COUNT; Index++) {
+      if ((TranslationTable[Index] & TT_TYPE_MASK) == TT_TYPE_TABLE_ENTRY) {
+        FreePageTablesRecursive ((VOID *)(UINTN)(TranslationTable[Index] &
+                                                 TT_ADDRESS_MASK_BLOCK_ENTRY),
+                                 Level + 1);
+      }
     }
   }
   FreePages (TranslationTable, 1);
@@ -254,7 +260,7 @@ UpdateRegionMappingRecursive (
           // possible for existing table entries, since we cannot revert the
           // modifications we made to the subhierarchy it represents.)
           //
-          FreePageTablesRecursive (TranslationTable);
+          FreePageTablesRecursive (TranslationTable, Level + 1);
         }
         return Status;
       }
--
2.17.1


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