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;
> >   }
> >