Re: [PATCH v21 08/26] x86/mm: Introduce _PAGE_COW

On 3/1/2021 7:52 AM, Borislav Petkov wrote:

On Wed, Feb 17, 2021 at 02:27:12PM -0800, Yu-cheng Yu wrote:

quoted

@@ -182,7 +206,7 @@ static inline int pud_young(pud_t pud)
  
  static inline int pte_write(pte_t pte)
  {
-	return pte_flags(pte) & _PAGE_RW;

Put here a comment along the lines of:

	/*
	 * Shadow stack pages are always writable - but not by normal
	 * instructions but only by shadow stack operations. Therefore, the
	 * W=0, D=1 test.
	 */

to make it clear what this means.

quoted

+	return (pte_flags(pte) & _PAGE_RW) || pte_shstk(pte);
  }
  
  static inline int pte_huge(pte_t pte)

@@ -314,6 +338,24 @@ static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear)
  	return native_make_pte(v & ~clear);
  }
  
+static inline pte_t pte_make_cow(pte_t pte)

pte_mkcow like the rest of the "pte_mkX" functions.

And below too, for the other newly added pXd_make_* helpers.

quoted

  static inline pmd_t pmd_mkdirty(pmd_t pmd)
  {
-	return pmd_set_flags(pmd, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
+	pmdval_t dirty = _PAGE_DIRTY;
+
+	/* Avoid creating (HW)Dirty=1, Write=0 PMDs */
+	if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !(pmd_flags(pmd) & _PAGE_RW))

						      !(pmd_write(pmd))

quoted

+		dirty = _PAGE_COW;
+
+	return pmd_set_flags(pmd, dirty | _PAGE_SOFT_DIRTY);
+}

...

quoted

  static inline pud_t pud_mkdirty(pud_t pud)
  {
-	return pud_set_flags(pud, _PAGE_DIRTY | _PAGE_SOFT_DIRTY);
+	pudval_t dirty = _PAGE_DIRTY;
+
+	/* Avoid creating (HW)Dirty=1, Write=0 PUDs */
+	if (cpu_feature_enabled(X86_FEATURE_SHSTK) && !(pud_flags(pud) & _PAGE_RW))

						      !(pud_write(pud))

I will update.  Thanks!

--
Yu-cheng