Re: [PATCH 6/9] x86/mm: Introduce ptep_set_wrprotect_flush and related functions

Andy Lutomirski
2018-06-07T16:24:22Z

On Thu, Jun 7, 2018 at 7:40 AM Yu-cheng Yu <yu-cheng.yu@intel.com> wrote:
>
> The function ptep_set_wrprotect()/huge_ptep_set_wrprotect() is
> used by copy_page_range()/copy_hugetlb_page_range() to copy
> PTEs.
>
> On x86, when the shadow stack is enabled, only a shadow stack
> PTE has the read-only and _PAGE_DIRTY_HW combination. Upon
> making a dirty PTE read-only, we move its _PAGE_DIRTY_HW to
> _PAGE_DIRTY_SW.
>
> When ptep_set_wrprotect() moves _PAGE_DIRTY_HW to _PAGE_DIRTY_SW,
> if the PTE is writable and the mm is shared, another task could
> race to set _PAGE_DIRTY_HW again.
>
> Introduce ptep_set_wrprotect_flush(), pmdp_set_wrprotect_flush(),
> and huge_ptep_set_wrprotect_flush() to make sure this does not
> happen.
>

This patch adds flushes where they didn't previously exist.

> +static inline void ptep_set_wrprotect_flush(struct vm_area_struct *vma,
> + unsigned long addr, pte_t *ptep)
> +{
> + bool rw;
> +
> + rw = test_and_clear_bit(_PAGE_BIT_RW, (unsigned long *)&ptep->pte);
> + if (IS_ENABLED(CONFIG_X86_INTEL_SHADOW_STACK_USER)) {
> + struct mm_struct *mm = vma->vm_mm;
> + pte_t pte;
> +
> + if (rw && (atomic_read(&mm->mm_users) > 1))
> + pte = ptep_clear_flush(vma, addr, ptep);

Why are you clearing the pte?

> -#define __HAVE_ARCH_PMDP_SET_WRPROTECT
> -static inline void pmdp_set_wrprotect(struct mm_struct *mm,
> - unsigned long addr, pmd_t *pmdp)
> +#define __HAVE_ARCH_HUGE_PTEP_SET_WRPROTECT_FLUSH
> +static inline void huge_ptep_set_wrprotect_flush(struct vm_area_struct *vma,
> + unsigned long addr, pte_t *ptep)
> {
> - clear_bit(_PAGE_BIT_RW, (unsigned long *)pmdp);
> + ptep_set_wrprotect_flush(vma, addr, ptep);

Maybe I'm just missing something, but you're changed the semantics of
this function significantly.