[edk2] [PATCH 3/5] ArmPkg/ArmMmuLib AARCH64: implement support for EFI_MEMORY_RP permissions

[Ard Biesheuvel]

Wire up the access flag (AF) page table attribute to the EFI_MEMORY_RP
permission attribute, so that attempts to read from such a region will
trigger an access flag fault.

Note that this is a stronger notion than just read protection, since
it now implies that any write or execute attempt is trapped as well.
However, this does not really matter in practice since we never assume
that a read protected page is writable or executable, and StackGuard
and HeapGuard (which are the primary users of this facility) certainly
don't care.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Ard Biesheuvel <ard.biesheu...@linaro.org>
---
 ArmPkg/Drivers/CpuDxe/AArch64/Mmu.c              |  5 +++--
 ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c | 14 +++++++++++---
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/ArmPkg/Drivers/CpuDxe/AArch64/Mmu.c 
b/ArmPkg/Drivers/CpuDxe/AArch64/Mmu.c
index 3e216c7cb235..e62e3fa87112 100644
--- a/ArmPkg/Drivers/CpuDxe/AArch64/Mmu.c
+++ b/ArmPkg/Drivers/CpuDxe/AArch64/Mmu.c
@@ -223,8 +223,9 @@ EfiAttributeToArmAttribute (
     ArmAttributes = TT_ATTR_INDX_MASK;
   }
 
-  // Set the access flag to match the block attributes
-  ArmAttributes |= TT_AF;
+  if ((EfiAttributes & EFI_MEMORY_RP) == 0) {
+    ArmAttributes |= TT_AF;
+  }
 
   // Determine protection attributes
   if (EfiAttributes & EFI_MEMORY_RO) {
diff --git a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c 
b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
index e1fabfcbea14..b59c081a7e49 100644
--- a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
+++ b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
@@ -102,6 +102,10 @@ PageAttributeToGcdAttribute (
     GcdAttributes |= EFI_MEMORY_XP;
   }
 
+  if ((PageAttributes & TT_AF) == 0) {
+    GcdAttributes |= EFI_MEMORY_RP;
+  }
+
   return GcdAttributes;
 }
 
@@ -451,7 +455,11 @@ GcdAttributeToPageAttribute (
     PageAttributes |= TT_AP_RO_RO;
   }
 
-  return PageAttributes | TT_AF;
+  if ((GcdAttributes & EFI_MEMORY_RP) == 0) {
+    PageAttributes |= TT_AF;
+  }
+
+  return PageAttributes;
 }
 
 EFI_STATUS
@@ -474,9 +482,9 @@ ArmSetMemoryAttributes (
     // No memory type was set in Attributes, so we are going to update the
     // permissions only.
     //
-    PageAttributes &= TT_AP_MASK | TT_UXN_MASK | TT_PXN_MASK;
+    PageAttributes &= TT_AP_MASK | TT_UXN_MASK | TT_PXN_MASK | TT_AF;
     PageAttributeMask = ~(TT_ADDRESS_MASK_BLOCK_ENTRY | TT_AP_MASK |
-                          TT_PXN_MASK | TT_XN_MASK);
+                          TT_PXN_MASK | TT_XN_MASK | TT_AF);
   }
 
   TranslationTable = ArmGetTTBR0BaseAddress ();
-- 
2.20.1

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