Re: [PATCH v3] mm/page_table_check: Support userfault wr-protect entries

[Pasha Tatashin]

On Wed, Apr 17, 2024 at 3:26 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>
> ---
> v2:
> - Rename __page_table_check_pxx() to page_table_check_pxx_flags(),
>   meanwhile move the pte check out of the loop [Pasha]
> - Fix build issues reported from the bot, also added SWP_DEVICE_WRITE which
>   was overlooked before
> v3:
> - Add missing doc update [Pasha]
> ---
>  Documentation/mm/page_table_check.rst |  9 ++++++-
>  arch/x86/include/asm/pgtable.h        | 18 +------------
>  mm/page_table_check.c                 | 39 +++++++++++++++++++++++++++
>  3 files changed, 48 insertions(+), 18 deletions(-)
>
> diff --git a/Documentation/mm/page_table_check.rst b/Documentation/mm/page_table_check.rst
> index c12838ce6b8d..5bd1d987d76d 100644
> --- a/Documentation/mm/page_table_check.rst
> +++ b/Documentation/mm/page_table_check.rst
> @@ -14,7 +14,7 @@ Page table check performs extra verifications at the time when new pages become
>  accessible from the userspace by getting their page table entries (PTEs PMDs
>  etc.) added into the table.
>
> -In case of detected corruption, the kernel is crashed. There is a small
> +In case of most detected corruption, the kernel is crashed. There is a small
>  performance and memory overhead associated with the page table check. Therefore,
>  it is disabled by default, but can be optionally enabled on systems where the
>  extra hardening outweighs the performance costs. Also, because page table check
> @@ -22,6 +22,13 @@ is synchronous, it can help with debugging double map memory corruption issues,
>  by crashing kernel at the time wrong mapping occurs instead of later which is
>  often the case with memory corruptions bugs.
>
> +It can also be used to do page table entry checks over various flags, dump
> +warnings when illegal combinations of entry flags are detected.  Currently,
> +userfaultfd is the only user of such to sanity check wr-protect bit against
> +any writable flags.  Illegal flag combinations will not directly cause data
> +corruption in this case immediately, but that will cause read-only data to
> +be writable, causing data corrupt when the page content is later modified.

I would replace: "causing data corrupt ..." to "leading to corruption ..."

> +
>  Double mapping detection logic
>  ==============================
>
> 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..388bcf60d8b5 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 +184,31 @@ 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);
> +
> +#ifdef CONFIG_DEVICE_PRIVATE
> +       if (type == SWP_DEVICE_EXCLUSIVE_WRITE || type == SWP_DEVICE_WRITE)
> +               return true;
> +#endif
> +#ifdef CONFIG_MIGRATION
> +       if (type == SWP_MIGRATION_WRITE)
> +               return true;
> +#endif
> +
> +       return false;
> +}

This should be re-written like this:

static inline bool swap_cached_writable(swp_entry_t entry)
{
        return is_writable_device_exclusive_entry(entry) ||
                is_writable_device_private_entry(entry) ||
                is_writable_migration_entry(entry);
}

Otherwise the patch looks good.

Pasha