arm64: mm: use XN table mapping attributes for user/kernel mappings

As the kernel and user space page tables are strictly mutually exclusive
when it comes to executable permissions, we can set the UXN table attribute
on all table entries that are created while creating kernel mappings in the
swapper page tables, and the PXN table attribute on all table entries that
are created while creating user space mappings in user space page tables.

While at it, get rid of a redundant comment.

Reviewed-by: Anshuman Khandual <anshuman.khandual@arm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20210310104942.174584-4-ardb@kernel.org
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/include/asm/pgalloc.h

@@ -27,7 +27,10 @@ static inline void __pud_populate(pud_t *pudp, phys_addr_t pmdp, pudval_t prot)
 
 static inline void pud_populate(struct mm_struct *mm, pud_t *pudp, pmd_t *pmdp)
 {
-	__pud_populate(pudp, __pa(pmdp), PUD_TYPE_TABLE);
+	pudval_t pudval = PUD_TYPE_TABLE;
+
+	pudval |= (mm == &init_mm) ? PUD_TABLE_UXN : PUD_TABLE_PXN;
+	__pud_populate(pudp, __pa(pmdp), pudval);
 }
 #else
 static inline void __pud_populate(pud_t *pudp, phys_addr_t pmdp, pudval_t prot)
@@ -45,7 +48,10 @@ static inline void __p4d_populate(p4d_t *p4dp, phys_addr_t pudp, p4dval_t prot)
 
 static inline void p4d_populate(struct mm_struct *mm, p4d_t *p4dp, pud_t *pudp)
 {
-	__p4d_populate(p4dp, __pa(pudp), P4D_TYPE_TABLE);
+	p4dval_t p4dval = P4D_TYPE_TABLE;
+
+	p4dval |= (mm == &init_mm) ? P4D_TABLE_UXN : P4D_TABLE_PXN;
+	__p4d_populate(p4dp, __pa(pudp), p4dval);
 }
 #else
 static inline void __p4d_populate(p4d_t *p4dp, phys_addr_t pudp, p4dval_t prot)
@@ -70,16 +76,15 @@ static inline void __pmd_populate(pmd_t *pmdp, phys_addr_t ptep,
 static inline void
 pmd_populate_kernel(struct mm_struct *mm, pmd_t *pmdp, pte_t *ptep)
 {
-	/*
-	 * The pmd must be loaded with the physical address of the PTE table
-	 */
-	__pmd_populate(pmdp, __pa(ptep), PMD_TYPE_TABLE);
+	VM_BUG_ON(mm != &init_mm);
+	__pmd_populate(pmdp, __pa(ptep), PMD_TYPE_TABLE | PMD_TABLE_UXN);
 }
 
 static inline void
 pmd_populate(struct mm_struct *mm, pmd_t *pmdp, pgtable_t ptep)
 {
-	__pmd_populate(pmdp, page_to_phys(ptep), PMD_TYPE_TABLE);
+	VM_BUG_ON(mm == &init_mm);
+	__pmd_populate(pmdp, page_to_phys(ptep), PMD_TYPE_TABLE | PMD_TABLE_PXN);
 }
 #define pmd_pgtable(pmd) pmd_page(pmd)