[PATCH] MdeModulePkg/DxeCore: Use correct type for alignment mask

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

The page allocator code in CoreFindFreePagesI() uses a mask derived from
its UINTN Alignment argument to align the descriptor end address of a
MEMORY_MAP entry to the requested alignment, in order to check whether
the descriptor covers enough sufficiently aligned area to satisfy the
request.

However, on 32-bit architectures, 'Alignment' is a 32-bit type, whereas
DescEnd is a 64-bit type, and so the resulting operation performed on
the end address comes down to masking with 0xfffff000 instead of the
intended 0xffffffff_fffff000. Given the -1 at the end of the expression,
the resulting address is 0xffffffff_fffffffff for any descriptor that
ends on a 4G aligned boundary, and this is certainly not what was
intended.

So cast Alignment to UINT64 to ensure that the mask has the right size.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
---
 MdeModulePkg/Core/Dxe/Mem/Page.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/MdeModulePkg/Core/Dxe/Mem/Page.c b/MdeModulePkg/Core/Dxe/Mem/Page.c
index 160289c1f9ec..5903ce7ab525 100644
--- a/MdeModulePkg/Core/Dxe/Mem/Page.c
+++ b/MdeModulePkg/Core/Dxe/Mem/Page.c
@@ -1097,7 +1097,7 @@ CoreFindFreePagesI (
       DescEnd = MaxAddress;
     }
 
-    DescEnd = ((DescEnd + 1) & (~(Alignment - 1))) - 1;
+    DescEnd = ((DescEnd + 1) & (~((UINT64)Alignment - 1))) - 1;
 
     // Skip if DescEnd is less than DescStart after alignment clipping
     if (DescEnd < DescStart) {
-- 
2.35.1