[PATCH v5 08/38] ArmPkg/ArmMmuLib: Avoid splitting block entries if possible

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

Currently, the ARM MMU page table logic will break down any block entry
that overlaps with the region being mapped, even if the block entry in
question is using the same attributes as the new region.

This means that creating a non-executable mapping inside a region that
is already mapped non-executable at a coarser granularity may trigger a
call to AllocatePages (), which may recurse back into the page table
code to update the attributes on the newly allocated page tables.

Let's avoid this, by preserving the block entry if it already covers the
region being mapped with the correct attributes.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c | 10 ++++++++++
 ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c   | 11 +++++++++++
 2 files changed, 21 insertions(+)

diff --git a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
index 6d21a2e41dd1..1ce200c43c72 100644
--- a/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
+++ b/ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
@@ -251,6 +251,16 @@ UpdateRegionMappingRecursive (
       ASSERT (Level < 3);
 
       if (!IsTableEntry (*Entry, Level)) {
+        //
+        // If the region we are trying to map is already covered by a block
+        // entry with the right attributes, don't bother splitting it up.
+        //
+        if (IsBlockEntry (*Entry, Level) &&
+            ((*Entry & TT_ATTRIBUTES_MASK & ~AttributeClearMask) == AttributeSetMask))
+        {
+          continue;
+        }
+
         //
         // No table entry exists yet, so we need to allocate a page table
         // for the next level.
diff --git a/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c b/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c
index 247cf87bf3d3..299d38ad07e8 100644
--- a/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c
+++ b/ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c
@@ -170,6 +170,17 @@ UpdatePageEntries (
 
     // Does this descriptor need to be converted from section entry to 4K pages?
     if (!TT_DESCRIPTOR_SECTION_TYPE_IS_PAGE_TABLE (Descriptor)) {
+      //
+      // If the section mapping covers the requested region with the expected
+      // attributes, splitting it is unnecessary, and should be avoided as it
+      // may result in unbounded recursion when using a strict NX policy.
+      //
+      if ((EntryValue & ~TT_DESCRIPTOR_PAGE_TYPE_MASK & EntryMask) ==
+          (ConvertSectionAttributesToPageAttributes (Descriptor) & EntryMask))
+      {
+        continue;
+      }
+
       Status = ConvertSectionToPages (FirstLevelIdx << TT_DESCRIPTOR_SECTION_BASE_SHIFT);
       if (EFI_ERROR (Status)) {
         // Exit for loop
-- 
2.39.2