x86: fix NX bit handling in change_page_attr()

This patch fixes a bug of change_page_attr/change_page_attr_addr on
Intel i386/x86_64 CPUs.  After changing page attribute to be
executable with these functions, the page remains un-executable on
Intel i386/x86_64 CPU.  Because on Intel i386/x86_64 CPU, only if the
"NX" bits of all three level page tables are cleared (PAE is enabled),
the corresponding page is executable (refer to section 4.13.2 of Intel
64 and IA-32 Architectures Software Developer's Manual).  So, the bug
is fixed through clearing the "NX" bit of PMD when splitting the huge
PMD.
Signed-off-by: NHuang Ying <ying.huang@intel.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>

arch/x86/mm/pageattr.c

@@ -130,8 +130,13 @@ static int split_large_page(pte_t *kpte, unsigned long address)
 		set_pte(&pbase[i], pfn_pte(addr >> PAGE_SHIFT, ref_prot));
 
 	/*
-	 * Install the new, split up pagetable:
+	 * Install the new, split up pagetable. Important detail here:
+	 *
+	 * On Intel the NX bit of all levels must be cleared to make a
+	 * page executable. See section 4.13.2 of Intel 64 and IA-32
+	 * Architectures Software Developer's Manual).
 	 */
+	ref_prot = pte_pgprot(pte_mkexec(pte_clrhuge(*kpte)));
 	__set_pmd_pte(kpte, address, mk_pte(base, ref_prot));
 	base = NULL;