Re: [PATCH -next v4 3/4] arm64: mm: add support for page table check

[Pasha Tatashin <pasha.tatashin@soleen.com>]

On Mon, Apr 18, 2022 at 11:47 AM Tong Tiangen <tongtiangen@huawei.com> wrote:
>
>
>
> 在 2022/4/18 17:28, Anshuman Khandual 写道:
> > On 4/18/22 09:14, Tong Tiangen wrote:
> >> From: Kefeng Wang <wangkefeng.wang@huawei.com>
> >>
> [...]
> >>   #endif
> >
> > Ran this series on arm64 platform after enabling
> >
> > - CONFIG_PAGE_TABLE_CHECK
> > - CONFIG_PAGE_TABLE_CHECK_ENFORCED (avoiding kernel command line option)
> >
> > After some time, the following error came up
> >
> > [   23.266013] ------------[ cut here ]------------
> > [   23.266807] kernel BUG at mm/page_table_check.c:90!
> > [   23.267609] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
> > [   23.268503] Modules linked in:
> > [   23.269012] CPU: 1 PID: 30 Comm: khugepaged Not tainted 5.18.0-rc3-00004-g60aa8e363a91 #2
> > [   23.270383] Hardware name: linux,dummy-virt (DT)
> > [   23.271210] pstate: 40400005 (nZcv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
> > [   23.272445] pc : page_table_check_clear.isra.6+0x114/0x148
> > [   23.273429] lr : page_table_check_clear.isra.6+0x64/0x148
> > [   23.274395] sp : ffff80000afb3ca0
> > [   23.274994] x29: ffff80000afb3ca0 x28: fffffc00022558e8 x27: ffff80000a27f628
> > [   23.276260] x26: ffff800009f9f2b0 x25: ffff00008a8d5000 x24: ffff800009f09fa0
> > [   23.277527] x23: 0000ffff89e00000 x22: ffff800009f09fb8 x21: ffff000089414cc0
> > [   23.278798] x20: 0000000000000200 x19: fffffc00022a0000 x18: 0000000000000001
> > [   23.280066] x17: 0000000000000001 x16: 0000000000000000 x15: 0000000000000003
> > [   23.281331] x14: 0000000000000068 x13: 00000000000000c0 x12: 0000000000000010
> > [   23.282602] x11: fffffc0002320008 x10: fffffc0002320000 x9 : ffff800009fa1000
> > [   23.283868] x8 : 00000000ffffffff x7 : 0000000000000001 x6 : ffff800009fa1f08
> > [   23.285135] x5 : 0000000000000000 x4 : 0000000000000000 x3 : 0000000000000000
> > [   23.286406] x2 : 00000000ffffffff x1 : ffff000080f2800c x0 : ffff000080f28000
> > [   23.287673] Call trace:
> > [   23.288123]  page_table_check_clear.isra.6+0x114/0x148
> > [   23.289043]  __page_table_check_pmd_clear+0x3c/0x50
> > [   23.289918]  pmdp_collapse_flush+0x114/0x370
> > [   23.290692]  khugepaged+0x1170/0x19e0
> > [   23.291356]  kthread+0x110/0x120
> > [   23.291945]  ret_from_fork+0x10/0x20
> > [   23.292596] Code: 91001041 b8e80024 51000482 36fffd62 (d4210000)
> > [   23.293678] ---[ end trace 0000000000000000 ]---
> > [   23.294511] note: khugepaged[30] exited with preempt_count 2
> >
> > Looking into file mm/page_table_check.c where this problem occured.
> >
> > /*
> >   * An enty is removed from the page table, decrement the counters for that page
> >   * verify that it is of correct type and counters do not become negative.
> >   */
> > static void page_table_check_clear(struct mm_struct *mm, unsigned long addr,
> >                                     unsigned long pfn, unsigned long pgcnt)
> > {
> >          struct page_ext *page_ext;
> >          struct page *page;
> >          unsigned long i;
> >          bool anon;
> >
> >          if (!pfn_valid(pfn))
> >                  return;
> >
> >          page = pfn_to_page(pfn);
> >          page_ext = lookup_page_ext(page);
> >          anon = PageAnon(page);
> >
> >          for (i = 0; i < pgcnt; i++) {
> >                  struct page_table_check *ptc = get_page_table_check(page_ext);
> >
> >                  if (anon) {
> >                          BUG_ON(atomic_read(&ptc->file_map_count));
> >                          BUG_ON(atomic_dec_return(&ptc->anon_map_count) < 0);
> >                  } else {
> >                          BUG_ON(atomic_read(&ptc->anon_map_count));
> >   Triggered here ====>>  BUG_ON(atomic_dec_return(&ptc->file_map_count) < 0);
> >                  }
> >                  page_ext = page_ext_next(page_ext);
> >          }
> > }
> >
> > Could you explain what was expected during pmdp_collapse_flush() which when
> > failed, triggered this BUG_ON() ? This counter seems to be page table check
> > specific, could it just go wrong ? I have not looked into the details about
> > page table check mechanism.
> >
> > - Anshuman
> > .
>
> Hi Anshuman:
>
> Thanks for your job.
>
> Let me briefly explain the principle of page table check(PTC).
>
> PTC introduces the following struct for page mapping type count:
> struct page_table_check {
>          atomic_t anon_map_count;
>          atomic_t file_map_count;
> };
> This structure can be obtained by "lookup_page_ext(page)"
>
> When page table entries are set(pud/pmd/pte), page_table_check_set()  is
> called to increase the page mapping count, Also check for errors (eg:if
> a page is used for anonymous mapping, then the page cannot be used for
> file mapping at the same time).
>
> When page table entries are clear(pud/pmd/pte), page_table_check_clear()
>   is called to decrease the page mapping count, Also check for errors.
>
> The error check rules are described in the following documents:
> Documentation/vm/page_table_check.rst
>
> The setting and clearing of page table entries are symmetrical.
>
> Here __page_table_check_pmd_clear() trigger BUGON which indicates that
> the pmd entry file mapping count has become negative.
>
> I guess if PTC didn't detect this exception, would there have been any
> problems?

It is hard to tell what sort of problem has been detected. More
debugging is needed in order to understand it. A huge file entry is
being removed from the page table. However, at least one sub page of
that entry does not have a record that it was added as a file entry to
the page table. At Google we found a few internal security bugs using
PTCs. However, this being new on ARM64, it is possible that the bug is
in PTC/khugepaged itself.

Anshuman is it possible to repro your scenario in QEMU?

Thank you,
Pasha

>
> Thanks,
> Tong.