[PATCH 6.6.y] arm64: mm: clear extra idmap level before use

[Karl Mehltretter <kmehltretter@gmail.com>]

The 6.6.y adaptation of commit 0e9df1c905d8 ("arm64: mm: Don't remap
pgtables for allocate vs populate") removes the clearing performed by
early_pgtable_alloc().  Its replacement clears allocations made by the
generic page-table walkers, but 6.6's create_idmap() still allocates an
extra root level directly when a sub-48-bit VA kernel is loaded
sufficiently high in physical memory.

memblock_phys_alloc_range() does not zero the returned memory.  The direct
caller therefore publishes an uncleared root page and passes it to
__create_pgd_mapping().  A stale nonzero entry can trip the bad-descriptor
BUG_ON or be followed as a page-table descriptor, preventing the kernel
from booting.

Clear the direct allocation through its linear alias before publishing it.
init_clear_pgtable() also supplies the barrier required before the table
descriptor becomes visible.

Mainline is not affected because commit e6128a8e523c ("arm64: mm: Use
48-bit virtual addressing for the permanent ID map") removed the dynamic
extra level before page-table initialization moved out of the allocator.

Fixes: 54322d95309d ("arm64: mm: Don't remap pgtables for allocate vs populate")
---
A deterministic QEMU A/B used 16 KiB pages, a 36-bit VA, a 48-bit PA, an
Image loaded at 0x1000200000, and test-only instrumentation that filled the
extra root with 0x02.  The affected kernel hit the expected bad-descriptor
BUG_ON.  With this patch, the same poisoned allocation survived
create_idmap() before the test stopped deliberately.

The injected contents make the allocator's permitted nonzero return
deterministic; they do not estimate real-world incidence.

Tested on:
  6.6.y a4a971135a2ff64382ae4235b3ae60503bb1036a (Linux 6.6.155)

 arch/arm64/mm/mmu.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index e075792d722575d38488e8352df8947c6aaddd2b..4e9e997e08daf543e5c09799563f991a9f8a7cc0 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -777,9 +777,10 @@ static void __init create_idmap(void)
 	/* check if we need an additional level of translation */
 	if (VA_BITS < 48 && idmap_t0sz < (64 - VA_BITS_MIN)) {
 		pgd_phys = early_pgtable_alloc(PAGE_SHIFT);
+		pgd = __va(pgd_phys);
+		init_clear_pgtable(pgd);
 		set_pgd(&idmap_pg_dir[start >> VA_BITS],
 			__pgd(pgd_phys | P4D_TYPE_TABLE));
-		pgd = __va(pgd_phys);
 	}
 	__create_pgd_mapping(pgd, start, start, size, PAGE_KERNEL_ROX,
 			     early_pgtable_alloc, 0);
-- 
2.39.5 (Apple Git-154)