Re: [PATCH v17 08/26] x86/mm: Introduce _PAGE_COW

[Yu, Yu-cheng]

On 1/21/2021 10:44 AM, Borislav Petkov wrote:

On Tue, Dec 29, 2020 at 01:30:35PM -0800, Yu-cheng Yu wrote:

[...]

@@ -343,6 +349,16 @@ static inline pte_t pte_mkold(pte_t pte)

static inline pte_t pte_wrprotect(pte_t pte)

  {
+       /*
+        * Blindly clearing _PAGE_RW might accidentally create
+        * a shadow stack PTE (RW=0, Dirty=1).  Move the hardware
+        * dirty value to the software bit.
+        */
+       if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+               pte.pte |= (pte.pte & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << 
_PAGE_BIT_COW;

Why the unreadable shifting when you can simply do:

                 if (pte.pte & _PAGE_DIRTY)
                         pte.pte |= _PAGE_COW;

?

It clears _PAGE_DIRTY and sets _PAGE_COW.  That is,

if (pte.pte & _PAGE_DIRTY) {
        pte.pte &= ~_PAGE_DIRTY;
        pte.pte |= _PAGE_COW;
}

So, shifting makes resulting code more efficient.

@@ -434,16 +469,40 @@ static inline pmd_t pmd_mkold(pmd_t pmd)

static inline pmd_t pmd_mkclean(pmd_t pmd)

  {
-       return pmd_clear_flags(pmd, _PAGE_DIRTY);
+       return pmd_clear_flags(pmd, _PAGE_DIRTY_BITS);
  }

static inline pmd_t pmd_wrprotect(pmd_t pmd)

  {
+       /*
+        * Blindly clearing _PAGE_RW might accidentally create
+        * a shadow stack PMD (RW=0, Dirty=1).  Move the hardware
+        * dirty value to the software bit.
+        */
+       if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+               pmdval_t v = native_pmd_val(pmd);
+
+               v |= (v & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << _PAGE_BIT_COW;

As above.

@@ -488,17 +554,35 @@ static inline pud_t pud_mkold(pud_t pud)

static inline pud_t pud_mkclean(pud_t pud)

  {
-       return pud_clear_flags(pud, _PAGE_DIRTY);
+       return pud_clear_flags(pud, _PAGE_DIRTY_BITS);
  }

static inline pud_t pud_wrprotect(pud_t pud)

  {
+       /*
+        * Blindly clearing _PAGE_RW might accidentally create
+        * a shadow stack PUD (RW=0, Dirty=1).  Move the hardware
+        * dirty value to the software bit.
+        */
+       if (cpu_feature_enabled(X86_FEATURE_SHSTK)) {
+               pudval_t v = native_pud_val(pud);
+
+               v |= (v & _PAGE_DIRTY) >> _PAGE_BIT_DIRTY << _PAGE_BIT_COW;

Ditto.

@@ -1131,6 +1222,12 @@ extern int pmdp_clear_flush_young(struct vm_area_struct 
*vma,
  #define pmd_write pmd_write
  static inline int pmd_write(pmd_t pmd)
  {
+       /*
+        * If _PAGE_DIRTY is set, then the PMD must either have _PAGE_RW or
+        * be a shadow stack PMD, which is logically writable.
+        */
+       if (cpu_feature_enabled(X86_FEATURE_SHSTK))
+               return pmd_flags(pmd) & (_PAGE_RW | _PAGE_DIRTY);

        else

        return pmd_flags(pmd) & _PAGE_RW;
  }