arm64: Make sure permission updates happen for pmd/pud

Commit 15122ee2c515 ("arm64: Enforce BBM for huge IO/VMAP mappings")
disallowed block mappings for ioremap since that code does not honor
break-before-make. The same APIs are also used for permission updating
though and the extra checks prevent the permission updates from happening,
even though this should be permitted. This results in read-only permissions
not being fully applied. Visibly, this can occasionaly be seen as a failure
on the built in rodata test when the test data ends up in a section or
as an odd RW gap on the page table dump. Fix this by using
pgattr_change_is_safe instead of p*d_present for determining if the
change is permitted.

Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Peter Robinson <pbrobinson@gmail.com>
Reported-by: Peter Robinson <pbrobinson@gmail.com>
Fixes: 15122ee2c515 ("arm64: Enforce BBM for huge IO/VMAP mappings")
Signed-off-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>

arch/arm64/mm/mmu.c

@@ -933,13 +933,15 @@ int pud_set_huge(pud_t *pudp, phys_addr_t phys, pgprot_t prot)
 {
 	pgprot_t sect_prot = __pgprot(PUD_TYPE_SECT |
 					pgprot_val(mk_sect_prot(prot)));
+	pud_t new_pud = pfn_pud(__phys_to_pfn(phys), sect_prot);
 
-	/* ioremap_page_range doesn't honour BBM */
-	if (pud_present(READ_ONCE(*pudp)))
+	/* Only allow permission changes for now */
+	if (!pgattr_change_is_safe(READ_ONCE(pud_val(*pudp)),
+				   pud_val(new_pud)))
 		return 0;
 
 	BUG_ON(phys & ~PUD_MASK);
-	set_pud(pudp, pfn_pud(__phys_to_pfn(phys), sect_prot));
+	set_pud(pudp, new_pud);
 	return 1;
 }
 
@@ -947,13 +949,15 @@ int pmd_set_huge(pmd_t *pmdp, phys_addr_t phys, pgprot_t prot)
 {
 	pgprot_t sect_prot = __pgprot(PMD_TYPE_SECT |
 					pgprot_val(mk_sect_prot(prot)));
+	pmd_t new_pmd = pfn_pmd(__phys_to_pfn(phys), sect_prot);
 
-	/* ioremap_page_range doesn't honour BBM */
-	if (pmd_present(READ_ONCE(*pmdp)))
+	/* Only allow permission changes for now */
+	if (!pgattr_change_is_safe(READ_ONCE(pmd_val(*pmdp)),
+				   pmd_val(new_pmd)))
 		return 0;
 
 	BUG_ON(phys & ~PMD_MASK);
-	set_pmd(pmdp, pfn_pmd(__phys_to_pfn(phys), sect_prot));
+	set_pmd(pmdp, new_pmd);
 	return 1;
 }