On Wed, Apr 17, 2024 at 5:25 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] > v4: > - Fix wordings in doc, use more elegant swap helpers [Pasha] > --- > Documentation/mm/page_table_check.rst | 9 +++++++- > arch/x86/include/asm/pgtable.h | 18 +--------------- > mm/page_table_check.c | 30 +++++++++++++++++++++++++++ > 3 files changed, 39 insertions(+), 18 deletions(-) Reviewed-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx> Thanks, Pasha
