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
