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

[Borislav Petkov]

On Tue, Dec 29, 2020 at 01:30:35PM -0800, Yu-cheng Yu wrote:
> @@ -182,6 +182,12 @@ static inline int pud_young(pud_t pud)
>  
>  static inline int pte_write(pte_t pte)
>  {
> +	/*
> +	 * If _PAGE_DIRTY is set, the PTE must either have _PAGE_RW or be
> +	 * a shadow stack PTE, which is logically writable.
> +	 */
> +	if (cpu_feature_enabled(X86_FEATURE_SHSTK))
> +		return pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY);
>  	return pte_flags(pte) & _PAGE_RW;

        if (cpu_feature_enabled(X86_FEATURE_SHSTK))
                return pte_flags(pte) & (_PAGE_RW | _PAGE_DIRTY);
        else
                return pte_flags(pte) & _PAGE_RW;

The else makes it ballanced and easier to read.


> @@ -333,7 +339,7 @@ static inline pte_t pte_clear_uffd_wp(pte_t pte)
>  
>  static inline pte_t pte_mkclean(pte_t pte)
>  {
> -	return pte_clear_flags(pte, _PAGE_DIRTY);
> +	return pte_clear_flags(pte, _PAGE_DIRTY_BITS);
>  }
>  
>  static inline pte_t pte_mkold(pte_t pte)
> @@ -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;

?

> @@ -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;
>  }
>  
-- 
Regards/Gruss,
    Boris.

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