Re: [PATCH] mm/page_table_check: Support userfault wr-protect entries

[Pasha Tatashin]

Hi Peter,

Thanks for this patch, I like this extra checking logic, my comments below:

On Mon, Apr 15, 2024 at 4:53 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> Allow page_table_check hooks to check over userfaultfd wr-protect criteria
> upon pgtable updates.  The rule is no co-existance allowed for any writable
> flag against userfault wr-protect flag.
>
> This should be better than c2da319c2e, where we used to only sanitize such
> issues during a pgtable walk, but when hitting such issue we don't have a
> good chance to know where does that writable bit came from [1], so that
> even the pgtable walk exposes a kernel bug (which is still helpful on
> triaging) but not easy to track and debug.
>
> Now we switch to track the source.  It's much easier too with the recent
> introduction of page table check.
>
> There are some limitations with using the page table check here for
> userfaultfd wr-protect purpose:
>
>   - It is only enabled with explicit enablement of page table check configs
>   and/or boot parameters, but should be good enough to track at least
>   syzbot issues, as syzbot should enable PAGE_TABLE_CHECK[_ENFORCED] for
>   x86 [1].  We used to have DEBUG_VM but it's now off for most distros,
>   while distros also normally not enable PAGE_TABLE_CHECK[_ENFORCED], which
>   is similar.
>
>   - It conditionally works with the ptep_modify_prot API.  It will be
>   bypassed when e.g. XEN PV is enabled, however still work for most of the
>   rest scenarios, which should be the common cases so should be good
>   enough.
>
>   - Hugetlb check is a bit hairy, as the page table check cannot identify
>   hugetlb pte or normal pte via trapping at set_pte_at(), because of the
>   current design where hugetlb maps every layers to pte_t... For example,
>   the default set_huge_pte_at() can invoke set_pte_at() directly and lose
>   the hugetlb context, treating it the same as a normal pte_t. So far it's
>   fine because we have huge_pte_uffd_wp() always equals to pte_uffd_wp() as
>   long as supported (x86 only).  It'll be a bigger problem when we'll
>   define _PAGE_UFFD_WP differently at various pgtable levels, because then
>   one huge_pte_uffd_wp() per-arch will stop making sense first.. as of now
>   we can leave this for later too.
>
> This patch also removes commit c2da319c2e altogether, as we have something
> better now.
>
> [1] https://lore.kernel.org/all/000000000000dce0530615c89210@xxxxxxxxxx/
>
> Cc: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/pgtable.h | 18 +-----------------
>  mm/page_table_check.c          | 32 +++++++++++++++++++++++++++++++-

Please add the new logic to: Documentation/mm/page_table_check.rst

>  2 files changed, 32 insertions(+), 18 deletions(-)
>
> diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
> index 273f7557218c..65b8e5bb902c 100644
> --- a/arch/x86/include/asm/pgtable.h
> +++ b/arch/x86/include/asm/pgtable.h
> @@ -388,23 +388,7 @@ static inline pte_t pte_wrprotect(pte_t pte)
>  #ifdef CONFIG_HAVE_ARCH_USERFAULTFD_WP
>  static inline int pte_uffd_wp(pte_t pte)
>  {
> -       bool wp = pte_flags(pte) & _PAGE_UFFD_WP;
> -
> -#ifdef CONFIG_DEBUG_VM
> -       /*
> -        * Having write bit for wr-protect-marked present ptes is fatal,
> -        * because it means the uffd-wp bit will be ignored and write will
> -        * just go through.
> -        *
> -        * Use any chance of pgtable walking to verify this (e.g., when
> -        * page swapped out or being migrated for all purposes). It means
> -        * something is already wrong.  Tell the admin even before the
> -        * process crashes. We also nail it with wrong pgtable setup.
> -        */
> -       WARN_ON_ONCE(wp && pte_write(pte));
> -#endif
> -
> -       return wp;
> +       return pte_flags(pte) & _PAGE_UFFD_WP;
>  }
>
>  static inline pte_t pte_mkuffd_wp(pte_t pte)
> diff --git a/mm/page_table_check.c b/mm/page_table_check.c
> index af69c3c8f7c2..d4eb1212f0f5 100644
> --- a/mm/page_table_check.c
> +++ b/mm/page_table_check.c
> @@ -7,6 +7,8 @@
>  #include <linux/kstrtox.h>
>  #include <linux/mm.h>
>  #include <linux/page_table_check.h>
> +#include <linux/swap.h>
> +#include <linux/swapops.h>
>
>  #undef pr_fmt
>  #define pr_fmt(fmt)    "page_table_check: " fmt
> @@ -182,6 +184,23 @@ void __page_table_check_pud_clear(struct mm_struct *mm, pud_t pud)
>  }
>  EXPORT_SYMBOL(__page_table_check_pud_clear);
>
> +/* Whether the swap entry cached writable information */
> +static inline bool swap_cached_writable(swp_entry_t entry)
> +{
> +       unsigned type = swp_type(entry);
> +
> +       return type == SWP_DEVICE_EXCLUSIVE_WRITE ||
> +           type == SWP_MIGRATION_WRITE;
> +}
> +
> +static inline void __page_table_check_pte(pte_t pte)

may be something like:
page_table_check_new_pte() ? Naming is starting to get confusing. The
idea for this function is to check the pte that is about to be set
into the page table.

> +{
> +       if (pte_present(pte) && pte_uffd_wp(pte))
> +               WARN_ON_ONCE(pte_write(pte));
> +       else if (is_swap_pte(pte) && pte_swp_uffd_wp(pte))
> +               WARN_ON_ONCE(swap_cached_writable(pte_to_swp_entry(pte)));
> +}
> +
>  void __page_table_check_ptes_set(struct mm_struct *mm, pte_t *ptep, pte_t pte,
>                 unsigned int nr)
>  {
> @@ -190,18 +209,29 @@ void __page_table_check_ptes_set(struct mm_struct *mm, pte_t *ptep, pte_t pte,
>         if (&init_mm == mm)
>                 return;
>
> -       for (i = 0; i < nr; i++)
> +       for (i = 0; i < nr; i++) {
> +               __page_table_check_pte(pte);

This should really be called only once after this loop.

>                 __page_table_check_pte_clear(mm, ptep_get(ptep + i));
> +       }
>         if (pte_user_accessible_page(pte))
>                 page_table_check_set(pte_pfn(pte), nr, pte_write(pte));
>  }
>  EXPORT_SYMBOL(__page_table_check_ptes_set);
>
> +static inline void __page_table_check_pmd(pmd_t pmd)

page_table_check_new_pmd() ?

> +{
> +       if (pmd_present(pmd) && pmd_uffd_wp(pmd))
> +               WARN_ON_ONCE(pmd_write(pmd));
> +       else if (is_swap_pmd(pmd) && pmd_swp_uffd_wp(pmd))
> +               WARN_ON_ONCE(swap_cached_writable(pmd_to_swp_entry(pmd)));
> +}
> +
>  void __page_table_check_pmd_set(struct mm_struct *mm, pmd_t *pmdp, pmd_t pmd)
>  {
>         if (&init_mm == mm)
>                 return;
>
> +       __page_table_check_pmd(pmd);
>         __page_table_check_pmd_clear(mm, *pmdp);
>         if (pmd_user_accessible_page(pmd)) {
>                 page_table_check_set(pmd_pfn(pmd), PMD_SIZE >> PAGE_SHIFT,
> --
> 2.44.0
>

Thanks,
Pasha