mprotect: prevent alteration of the PAT bits

There is a defect in mprotect, which lets the user change the page cache
type bits by-passing the kernel reserve_memtype and free_memtype
wrappers.  Fix the problem by not letting mprotect change the PAT bits.
Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-x86/pgtable.h

@@ -57,7 +57,8 @@
 #define _KERNPG_TABLE	(_PAGE_PRESENT | _PAGE_RW | _PAGE_ACCESSED |	\
 			 _PAGE_DIRTY)
 
-#define _PAGE_CHG_MASK	(PTE_MASK | _PAGE_ACCESSED | _PAGE_DIRTY)
+#define _PAGE_CHG_MASK	(PTE_MASK | _PAGE_PCD | _PAGE_PWT |		\
+			 _PAGE_ACCESSED | _PAGE_DIRTY)
 
 #define _PAGE_CACHE_MASK	(_PAGE_PCD | _PAGE_PWT)
 #define _PAGE_CACHE_WB		(0)
@@ -288,12 +289,21 @@ static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
 	 * Chop off the NX bit (if present), and add the NX portion of
 	 * the newprot (if present):
 	 */
-	val &= _PAGE_CHG_MASK & ~_PAGE_NX;
-	val |= pgprot_val(newprot) & __supported_pte_mask;
+	val &= _PAGE_CHG_MASK;
+	val |= pgprot_val(newprot) & (~_PAGE_CHG_MASK) & __supported_pte_mask;
 
 	return __pte(val);
 }
 
+/* mprotect needs to preserve PAT bits when updating vm_page_prot */
+#define pgprot_modify pgprot_modify
+static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
+{
+	pgprotval_t preservebits = pgprot_val(oldprot) & _PAGE_CHG_MASK;
+	pgprotval_t addbits = pgprot_val(newprot);
+	return __pgprot(preservebits | addbits);
+}
+
 #define pte_pgprot(x) __pgprot(pte_val(x) & (0xfff | _PAGE_NX))
 
 #define canon_pgprot(p) __pgprot(pgprot_val(p) & __supported_pte_mask)

mm/mprotect.c

@@ -26,6 +26,13 @@
 #include <asm/cacheflush.h>
 #include <asm/tlbflush.h>
 
+#ifndef pgprot_modify
+static inline pgprot_t pgprot_modify(pgprot_t oldprot, pgprot_t newprot)
+{
+	return newprot;
+}
+#endif
+
 static void change_pte_range(struct mm_struct *mm, pmd_t *pmd,
 		unsigned long addr, unsigned long end, pgprot_t newprot,
 		int dirty_accountable)
@@ -192,7 +199,9 @@ mprotect_fixup(struct vm_area_struct *vma, struct vm_area_struct **pprev,
 	 * held in write mode.
 	 */
 	vma->vm_flags = newflags;
-	vma->vm_page_prot = vm_get_page_prot(newflags);
+	vma->vm_page_prot = pgprot_modify(vma->vm_page_prot,
+					  vm_get_page_prot(newflags));
+
 	if (vma_wants_writenotify(vma)) {
 		vma->vm_page_prot = vm_get_page_prot(newflags & ~VM_SHARED);
 		dirty_accountable = 1;