arm64/kernel: Always use level 2 or higher for early mappings

The page table population code in map_range() uses a recursive algorithm
to create the early mappings of the kernel, the DTB and the ID mapped
text and data pages, and this fails to take into account that the way
these page tables may be constructed is not precisely the same at each
level. In particular, block mappings are not permitted at each level,
and the code as it exists today might inadvertently create such a
forbidden block mapping if it were used to map a region of the
appropriate size and alignment.

This never happens in practice, given the limited size of the assets
being mapped by the early boot code. Nonetheless, it would be better if
this code would behave correctly in all circumstances.

So only permit block mappings at level 2, and page mappings at level 3,
for any page size, and use table mappings exclusively at all other
levels. This change should have no impact in practice, but it makes the
code more robust.

Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Reported-by: Ryan Roberts <ryan.roberts@arm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Link: https://lore.kernel.org/r/20250311073043.96795-2-ardb+git@google.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

Author: ardbiesheuvel

arch/arm64/kernel/pi/map_range.c

@@ -45,12 +45,12 @@ void __init map_range(u64 *pte, u64 start, u64 end, u64 pa, pgprot_t prot,
 	 * clearing the mapping
 	 */
 	if (protval)
-		protval |= (level < 3) ? PMD_TYPE_SECT : PTE_TYPE_PAGE;
+		protval |= (level == 2) ? PMD_TYPE_SECT : PTE_TYPE_PAGE;
 
 	while (start < end) {
 		u64 next = min((start | lmask) + 1, PAGE_ALIGN(end));
 
-		if (level < 3 && (start | next | pa) & lmask) {
+		if (level < 2 || (level == 2 && (start | next | pa) & lmask)) {
 			/*
 			 * This chunk needs a finer grained mapping. Create a
 			 * table mapping if necessary and recurse.