Re: [RFC PATCH 2/4] mm: add PTE_MARKER_GUARD PTE marker

[Jann Horn]

On Fri, Sep 27, 2024 at 2:51 PM Lorenzo Stoakes
<lorenzo.stoakes@xxxxxxxxxx> wrote:
> Add a new PTE marker that results in any access causing the accessing
> process to segfault.
[...]
>  static inline int is_poisoned_swp_entry(swp_entry_t entry)
> +{
> +       /*
> +        * We treat guard pages as poisoned too as these have the same semantics
> +        * as poisoned ranges, only with different fault handling.
> +        */
> +       return is_pte_marker_entry(entry) &&
> +               (pte_marker_get(entry) &
> +                (PTE_MARKER_POISONED | PTE_MARKER_GUARD));
> +}

This means MADV_FREE will also clear guard PTEs, right?

> diff --git a/mm/memory.c b/mm/memory.c
> index 5c6486e33e63..6c413c3d72fd 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -1457,7 +1457,7 @@ static inline bool should_zap_folio(struct zap_details *details,
>         return !folio_test_anon(folio);
>  }
>
> -static inline bool zap_drop_file_uffd_wp(struct zap_details *details)
> +static inline bool zap_drop_markers(struct zap_details *details)
>  {
>         if (!details)
>                 return false;
> @@ -1478,7 +1478,7 @@ zap_install_uffd_wp_if_needed(struct vm_area_struct *vma,
>         if (vma_is_anonymous(vma))
>                 return;
>
> -       if (zap_drop_file_uffd_wp(details))
> +       if (zap_drop_markers(details))
>                 return;
>
>         for (;;) {
> @@ -1673,7 +1673,15 @@ static unsigned long zap_pte_range(struct mmu_gather *tlb,
>                          * drop the marker if explicitly requested.
>                          */
>                         if (!vma_is_anonymous(vma) &&
> -                           !zap_drop_file_uffd_wp(details))
> +                           !zap_drop_markers(details))
> +                               continue;
> +               } else if (is_guard_swp_entry(entry)) {
> +                       /*
> +                        * Ordinary zapping should not remove guard PTE
> +                        * markers. Only do so if we should remove PTE markers
> +                        * in general.
> +                        */
> +                       if (!zap_drop_markers(details))
>                                 continue;

Just a comment: It's nice that the feature is restricted to anonymous
VMAs, otherwise we'd have to figure out here what to do about
unmap_mapping_folio() (which sets ZAP_FLAG_DROP_MARKER together with
details.single_folio)...


>                 } else if (is_hwpoison_entry(entry) ||
>                            is_poisoned_swp_entry(entry)) {
> @@ -4005,6 +4013,10 @@ static vm_fault_t handle_pte_marker(struct vm_fault *vmf)
>         if (marker & PTE_MARKER_POISONED)
>                 return VM_FAULT_HWPOISON;
>
> +       /* Hitting a guard page is always a fatal condition. */
> +       if (marker & PTE_MARKER_GUARD)
> +               return VM_FAULT_SIGSEGV;
> +
>         if (pte_marker_entry_uffd_wp(entry))
>                 return pte_marker_handle_uffd_wp(vmf);
>
> --
> 2.46.2
>