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ArmPkg/ArmMmuLib: Avoid splitting block entries if possible 

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>
Reviewed-by: Leif Lindholm <quic_llindhol@quicinc.com>
This commit is contained in:
Ard Biesheuvel 2023-02-08 18:19:36 +01:00 committed by mergify[bot]
parent f07a9df9af
commit ae2c904c3d
2 changed files with 21 additions and 0 deletions
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10
ArmPkg/Library/ArmMmuLib/AArch64/ArmMmuLibCore.c
View File

@ -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.

11
ArmPkg/Library/ArmMmuLib/Arm/ArmMmuLibUpdate.c
View File

@ -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