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

[Pasha Tatashin]

On Wed, Apr 17, 2024 at 1:17 PM Pasha Tatashin
<pasha.tatashin@xxxxxxxxxx> wrote:
>
> On Wed, Apr 17, 2024 at 12:53 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
> >
> > On Tue, Apr 16, 2024 at 05:34:50PM -0400, Pasha Tatashin wrote:
> > > Hi Peter,
> >
> > Pasha,
> >
> > >
> > > Thanks for this patch, I like this extra checking logic, my comments below:
> >
> > Thanks for taking a look.
> >
> > >
> > > 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@googlecom/
> > > >
> > > > 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
> >
> > Will do.
> >
> > >
> > > >  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;

This breaks linux-next build:

mm/page_table_check.c: In function ‘swap_cached_writable’:
mm/page_table_check.c:192:24: error: ‘SWP_DEVICE_EXCLUSIVE_WRITE’
undeclared (first use in this function)
  192 |         return type == SWP_DEVICE_EXCLUSIVE_WRITE ||
      |                        ^~~~~~~~~~~~~~~~~~~~~~~~~~
mm/page_table_check.c:192:24: note: each undeclared identifier is
reported only once for each function it appears in
mm/page_table_check.c:194:1: error: control reaches end of non-void
function [-Werror=return-type]
  194 | }

Looks like there is a dependence on CONFIG_DEVICE_PRIVATE.

Pasha