arm64: Make !dirty ptes read-only

The AArch64 Linux port relies on the mm code to wrprotect clean ptes.
This however is not the case with newly created ptes and
PAGE_SHARED(_EXEC) is writable but !dirty.
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>
Cc: <stable@vger.kernel.org>

arch/arm64/include/asm/pgtable.h

@@ -159,6 +159,8 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
 {
 	if (pte_present_exec_user(pte))
 		__sync_icache_dcache(pte, addr);
+	if (!pte_dirty(pte))
+		pte = pte_wrprotect(pte);
 	set_pte(ptep, pte);
 }