Re: [PATCH v2 2/2] arm64/mm: fix incorrect file_map_count for invalid pmd/pud

[Will Deacon]

On Mon, Nov 21, 2022 at 11:15:49AM +0800, Liu Shixin wrote:
> On 2022/11/18 22:34, Will Deacon wrote:
> > On Thu, Nov 17, 2022 at 03:56:02PM +0800, Liu Shixin wrote:
> >> The page table check trigger BUG_ON() unexpectedly when split hugepage:
> >>
> >>  ------------[ cut here ]------------
> >>  kernel BUG at mm/page_table_check.c:119!
> >>  Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
> >>  Dumping ftrace buffer:
> >>     (ftrace buffer empty)
> >>  Modules linked in:
> >>  CPU: 7 PID: 210 Comm: transhuge-stres Not tainted 6.1.0-rc3+ #748
> >>  Hardware name: linux,dummy-virt (DT)
> >>  pstate: 20000005 (nzCv daif -PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> >>  pc : page_table_check_set.isra.0+0x398/0x468
> >>  lr : page_table_check_set.isra.0+0x1c0/0x468
> >> [...]
> >>  Call trace:
> >>   page_table_check_set.isra.0+0x398/0x468
> >>   __page_table_check_pte_set+0x160/0x1c0
> >>   __split_huge_pmd_locked+0x900/0x1648
> >>   __split_huge_pmd+0x28c/0x3b8
> >>   unmap_page_range+0x428/0x858
> >>   unmap_single_vma+0xf4/0x1c8
> >>   zap_page_range+0x2b0/0x410
> >>   madvise_vma_behavior+0xc44/0xe78
> >>   do_madvise+0x280/0x698
> >>   __arm64_sys_madvise+0x90/0xe8
> >>   invoke_syscall.constprop.0+0xdc/0x1d8
> >>   do_el0_svc+0xf4/0x3f8
> >>   el0_svc+0x58/0x120
> >>   el0t_64_sync_handler+0xb8/0xc0
> >>   el0t_64_sync+0x19c/0x1a0
> >> [...]
> >>
> >> On arm64, pmd_leaf() will return true even if the pmd is invalid due to
> >> pmd_present_invalid() check. So in pmdp_invalidate() the file_map_count
> >> will not only decrease once but also increase once. Then in set_pte_at(),
> >> the file_map_count increase again, and so trigger BUG_ON() unexpectedly.
> >>
> >> Fix this problem by adding pmd_valid() in pmd_user_accessible_page().
> >> Moreover, add pud_valid() for pud_user_accessible_page() too.
> >>
> >> Fixes: 42b2547137f5 ("arm64/mm: enable ARCH_SUPPORTS_PAGE_TABLE_CHECK")
> >> Reported-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
> >> Signed-off-by: Liu Shixin <liushixin2@xxxxxxxxxx>
> >> Acked-by: Pasha Tatashin <pasha.tatashin@xxxxxxxxxx>
> >> ---
> >>  arch/arm64/include/asm/pgtable.h | 4 ++--
> >>  1 file changed, 2 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> >> index edf6625ce965..3bc64199aa2e 100644
> >> --- a/arch/arm64/include/asm/pgtable.h
> >> +++ b/arch/arm64/include/asm/pgtable.h
> >> @@ -863,12 +863,12 @@ static inline bool pte_user_accessible_page(pte_t pte)
> >>  
> >>  static inline bool pmd_user_accessible_page(pmd_t pmd)
> >>  {
> >> -	return pmd_leaf(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));
> >> +	return pmd_valid(pmd) && pmd_leaf(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd));
> > Hmm, doesn't this have a funny interaction with PROT_NONE where the pmd is
> > invalid but present? If you don't care about PROT_NONE, then you could just
> > do:
> >
> >   pmd_valid(pmd) && !pmd_table(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd))
> >
> > but if you do care then you could do:
> >
> >   pmd_leaf(pmd) && !pmd_present_invalid(pmd) && (pmd_user(pmd) || pmd_user_exec(pmd))
> I prefer the latter. I will fix and resend later.
> >>  static inline bool pud_user_accessible_page(pud_t pud)
> >>  {
> >> -	return pud_leaf(pud) && pud_user(pud);
> >> +	return pud_valid(pud) && pud_leaf(pud) && pud_user(pud);
> > Not caused by this patch, but why don't we have something like a
> > pud_user_exec() check here like we do for the pte and pmd levels?
> As far as I know, there is no user use the user executable pud on arm64, so didn't define pud_user_exec().

I can believe they don't get exposed to userspace at all, but exposing only
as non-executable doesn't sound right. So I would have thought that either
pud_user_accessible_page() would always return false or it would need to
check for the executable case too.

Will