arm64: Implement arch-specific pte_access_permitted()

The generic pte_access_permitted() implementation only checks for
pte_present() (together with the write permission where applicable).
However, for both kernel ptes and PROT_NONE mappings pte_present() also
returns true on arm64 even though such mappings are not user accessible.
Additionally, arm64 now supports execute-only user permission
(PROT_EXEC) which is implemented by clearing the PTE_USER bit.

With this patch the arm64 implementation of pte_access_permitted()
checks for the PTE_VALID and PTE_USER bits together with writable access
if applicable.

Cc: <[email protected]>
Reported-by: Al Viro <[email protected]>
Signed-off-by: Catalin Marinas <[email protected]>
Signed-off-by: Will Deacon <[email protected]>

arch/arm64/include/asm/pgtable.h

@@ -98,6 +98,8 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
 	((pte_val(pte) & (PTE_VALID | PTE_USER | PTE_UXN)) == (PTE_VALID | PTE_UXN))
 #define pte_valid_young(pte) \
 	((pte_val(pte) & (PTE_VALID | PTE_AF)) == (PTE_VALID | PTE_AF))
+#define pte_valid_user(pte) \
+	((pte_val(pte) & (PTE_VALID | PTE_USER)) == (PTE_VALID | PTE_USER))
 
 /*
  * Could the pte be present in the TLB? We must check mm_tlb_flush_pending
@@ -107,6 +109,18 @@ extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
 #define pte_accessible(mm, pte)	\
 	(mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid_young(pte))
 
+/*
+ * p??_access_permitted() is true for valid user mappings (subject to the
+ * write permission check) other than user execute-only which do not have the
+ * PTE_USER bit set. PROT_NONE mappings do not have the PTE_VALID bit set.
+ */
+#define pte_access_permitted(pte, write) \
+	(pte_valid_user(pte) && (!(write) || pte_write(pte)))
+#define pmd_access_permitted(pmd, write) \
+	(pte_access_permitted(pmd_pte(pmd), (write)))
+#define pud_access_permitted(pud, write) \
+	(pte_access_permitted(pud_pte(pud), (write)))
+
 static inline pte_t clear_pte_bit(pte_t pte, pgprot_t prot)
 {
 	pte_val(pte) &= ~pgprot_val(prot);