[PATCH v4 05/11] ArmPkg/ArmMmuLib ARM: Clear individual permission bits

["Ard Biesheuvel" <ardb@kernel.org>]

Currently, the MMU code that is supposed to clear the RO or XP
attributes from a region just clears both unconditionally. This
approximates the desired behavior to some extent, but it does mean that
setting the RO bit first on a code region, and then clearing the XP bit
results both RO and XP being cleared, and we end up with writable code,
and avoiding that is the point of all these protections.

Once we introduce RP support, this will only get worse, so let's fix
this up, by reshuffling the attribute update code to take the entry mask
from the caller, and use the mask to preserve other attributes when
clearing RO or XP.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c | 94 +++++++++++++++++---
 1 file changed, 81 insertions(+), 13 deletions(-)

diff --git a/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c b/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c
index 484c67476619..23f613f5dbb0 100644
--- a/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c
+++ b/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c
@@ -81,12 +81,12 @@ UpdatePageEntries (
   IN  EFI_PHYSICAL_ADDRESS  BaseAddress,
   IN  UINT64                Length,
   IN  UINT64                Attributes,
+  IN  UINT32                EntryMask,
   OUT BOOLEAN               *FlushTlbs OPTIONAL
   )
 {
   EFI_STATUS  Status;
   UINT32      EntryValue;
-  UINT32      EntryMask;
   UINT32      FirstLevelIdx;
   UINT32      Offset;
   UINT32      NumPageEntries;
@@ -104,7 +104,6 @@ UpdatePageEntries (
 
   // EntryMask: bitmask of values to change (1 = change this value, 0 = leave alone)
   // EntryValue: values at bit positions specified by EntryMask
-  EntryMask = TT_DESCRIPTOR_PAGE_TYPE_MASK | TT_DESCRIPTOR_PAGE_AP_MASK | TT_DESCRIPTOR_PAGE_XN_MASK | TT_DESCRIPTOR_PAGE_AF;
   EntryValue = TT_DESCRIPTOR_PAGE_TYPE_PAGE;
 
   // Although the PI spec is unclear on this, the GCD guarantees that only
@@ -220,11 +219,11 @@ EFI_STATUS
 UpdateSectionEntries (
   IN EFI_PHYSICAL_ADDRESS  BaseAddress,
   IN UINT64                Length,
-  IN UINT64                Attributes
+  IN UINT64                Attributes,
+  IN UINT32                EntryMask
   )
 {
   EFI_STATUS                           Status;
-  UINT32                               EntryMask;
   UINT32                               EntryValue;
   UINT32                               FirstLevelIdx;
   UINT32                               NumSections;
@@ -240,8 +239,6 @@ UpdateSectionEntries (
   // EntryValue: values at bit positions specified by EntryMask
 
   // Make sure we handle a section range that is unmapped
-  EntryMask = TT_DESCRIPTOR_SECTION_TYPE_MASK | TT_DESCRIPTOR_SECTION_XN_MASK |
-              TT_DESCRIPTOR_SECTION_AP_MASK | TT_DESCRIPTOR_SECTION_AF;
   EntryValue = TT_DESCRIPTOR_SECTION_TYPE_SECTION;
 
   // Although the PI spec is unclear on this, the GCD guarantees that only
@@ -310,6 +307,7 @@ UpdateSectionEntries (
                  (FirstLevelIdx + i) << TT_DESCRIPTOR_SECTION_BASE_SHIFT,
                  TT_DESCRIPTOR_SECTION_SIZE,
                  Attributes,
+                 ConvertSectionAttributesToPageAttributes (EntryMask),
                  NULL
                  );
     } else {
@@ -340,11 +338,26 @@ UpdateSectionEntries (
   return Status;
 }
 
+/**
+  Update the permission or memory type attributes on a range of memory.
+
+  @param  BaseAddress           The start of the region.
+  @param  Length                The size of the region.
+  @param  Attributes            A mask of EFI_MEMORY_xx constants.
+  @param  SectionMask           A mask of short descriptor section attributes
+                                describing which descriptor bits to update.
+
+  @retval EFI_SUCCESS           The attributes were set successfully.
+  @retval EFI_OUT_OF_RESOURCES  The operation failed due to insufficient memory.
+
+**/
+STATIC
 EFI_STATUS
-ArmSetMemoryAttributes (
+SetMemoryAttributes (
   IN EFI_PHYSICAL_ADDRESS  BaseAddress,
   IN UINT64                Length,
-  IN UINT64                Attributes
+  IN UINT64                Attributes,
+  IN UINT32                SectionMask
   )
 {
   EFI_STATUS  Status;
@@ -375,7 +388,12 @@ ArmSetMemoryAttributes (
         Attributes
         ));
 
-      Status = UpdateSectionEntries (BaseAddress, ChunkLength, Attributes);
+      Status = UpdateSectionEntries (
+                 BaseAddress,
+                 ChunkLength,
+                 Attributes,
+                 SectionMask
+                 );
 
       FlushTlbs = TRUE;
     } else {
@@ -401,6 +419,7 @@ ArmSetMemoryAttributes (
                  BaseAddress,
                  ChunkLength,
                  Attributes,
+                 ConvertSectionAttributesToPageAttributes (SectionMask),
                  &FlushTlbs
                  );
     }
@@ -420,13 +439,47 @@ ArmSetMemoryAttributes (
   return Status;
 }
 
+/**
+  Update the permission or memory type attributes on a range of memory.
+
+  @param  BaseAddress           The start of the region.
+  @param  Length                The size of the region.
+  @param  Attributes            A mask of EFI_MEMORY_xx constants.
+
+  @retval EFI_SUCCESS           The attributes were set successfully.
+  @retval EFI_OUT_OF_RESOURCES  The operation failed due to insufficient memory.
+
+**/
+EFI_STATUS
+ArmSetMemoryAttributes (
+  IN EFI_PHYSICAL_ADDRESS  BaseAddress,
+  IN UINT64                Length,
+  IN UINT64                Attributes
+  )
+{
+  return SetMemoryAttributes (
+           BaseAddress,
+           Length,
+           Attributes,
+           TT_DESCRIPTOR_SECTION_TYPE_MASK |
+           TT_DESCRIPTOR_SECTION_XN_MASK |
+           TT_DESCRIPTOR_SECTION_AP_MASK |
+           TT_DESCRIPTOR_SECTION_AF
+           );
+}
+
 EFI_STATUS
 ArmSetMemoryRegionNoExec (
   IN  EFI_PHYSICAL_ADDRESS  BaseAddress,
   IN  UINT64                Length
   )
 {
-  return ArmSetMemoryAttributes (BaseAddress, Length, EFI_MEMORY_XP);
+  return SetMemoryAttributes (
+           BaseAddress,
+           Length,
+           EFI_MEMORY_XP,
+           TT_DESCRIPTOR_SECTION_XN_MASK
+           );
 }
 
 EFI_STATUS
@@ -435,7 +488,12 @@ ArmClearMemoryRegionNoExec (
   IN  UINT64                Length
   )
 {
-  return ArmSetMemoryAttributes (BaseAddress, Length, __EFI_MEMORY_RWX);
+  return SetMemoryAttributes (
+           BaseAddress,
+           Length,
+           0,
+           TT_DESCRIPTOR_SECTION_XN_MASK
+           );
 }
 
 EFI_STATUS
@@ -444,7 +502,12 @@ ArmSetMemoryRegionReadOnly (
   IN  UINT64                Length
   )
 {
-  return ArmSetMemoryAttributes (BaseAddress, Length, EFI_MEMORY_RO);
+  return SetMemoryAttributes (
+           BaseAddress,
+           Length,
+           EFI_MEMORY_RO,
+           TT_DESCRIPTOR_SECTION_AP_MASK
+           );
 }
 
 EFI_STATUS
@@ -453,5 +516,10 @@ ArmClearMemoryRegionReadOnly (
   IN  UINT64                Length
   )
 {
-  return ArmSetMemoryAttributes (BaseAddress, Length, __EFI_MEMORY_RWX);
+  return SetMemoryAttributes (
+           BaseAddress,
+           Length,
+           0,
+           TT_DESCRIPTOR_SECTION_AP_MASK
+           );
 }
-- 
2.39.1