powerpc/32s: map kasan zero shadow with PAGE_READONLY instead of PAGE_KERNEL_RO

For hash32, the zero shadow page gets mapped with PAGE_READONLY instead
of PAGE_KERNEL_RO, because the PP bits don't provide a RO kernel, so
PAGE_KERNEL_RO is equivalent to PAGE_KERNEL. By using PAGE_READONLY,
the page is RO for both kernel and user, but this is not a security issue
as it contains only zeroes.
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/kasan/kasan_init_32.c

@@ -39,6 +39,9 @@ static int kasan_init_shadow_page_tables(unsigned long k_start, unsigned long k_
 
 		if (!new)
 			return -ENOMEM;
+		if (early_mmu_has_feature(MMU_FTR_HPTE_TABLE))
+			kasan_populate_pte(new, PAGE_READONLY);
+		else
 			kasan_populate_pte(new, PAGE_KERNEL_RO);
 		pmd_populate_kernel(&init_mm, pmd, new);
 	}
@@ -84,6 +87,9 @@ static int __ref kasan_init_region(void *start, size_t size)
 
 static void __init kasan_remap_early_shadow_ro(void)
 {
+	if (early_mmu_has_feature(MMU_FTR_HPTE_TABLE))
+		kasan_populate_pte(kasan_early_shadow_pte, PAGE_READONLY);
+	else
 		kasan_populate_pte(kasan_early_shadow_pte, PAGE_KERNEL_RO);
 
 	flush_tlb_kernel_range(KASAN_SHADOW_START, KASAN_SHADOW_END);