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From:   Pasha Tatashin <pasha.tatashin@soleen.com>
Date:   Mon, 31 Jan 2022 15:16:15 -0500
Message-ID: <CA+CK2bDYhzQ7wP0Ef3n6YGDo2=C0cdhYnEq9XkbOGzPKBp4SSQ@mail.gmail.com>
Subject: Re: [PATCH v4 4/4] mm/page_table_check: check entries at pmd levels
To:     Wei Xu <weixugc@google.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Paul Turner <pjt@google.com>, Greg Thelen <gthelen@google.com>,
        Ingo Molnar <mingo@redhat.com>, Will Deacon <will@kernel.org>,
        Mike Rapoport <rppt@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>,
        Jiri Slaby <jirislaby@kernel.org>,
        Muchun Song <songmuchun@bytedance.com>,
        Fusion Future <qydwhotmail@gmail.com>,
        Hugh Dickins <hughd@google.com>, Zi Yan <ziy@nvidia.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>
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On Fri, Jan 28, 2022 at 6:39 PM Wei Xu <weixugc@google.com> wrote:
>
> On Wed, Jan 26, 2022 at 10:36 AM Pasha Tatashin
> <pasha.tatashin@soleen.com> wrote:
> >
> > syzbot detected a case where the page table counters were not properly
> > updated.
> >
> > syzkaller login:  ------------[ cut here ]------------
> > kernel BUG at mm/page_table_check.c:162!
> > invalid opcode: 0000 [#1] PREEMPT SMP KASAN
> > CPU: 0 PID: 3099 Comm: pasha Not tainted 5.16.0+ #48
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIO4
> > RIP: 0010:__page_table_check_zero+0x159/0x1a0
> > Code: 7d 3a b2 ff 45 39 f5 74 2a e8 43 38 b2 ff 4d 85 e4 01
> > RSP: 0018:ffff888010667418 EFLAGS: 00010293
> > RAX: 0000000000000000 RBX: 0000000000000001 RCX: 0000000000
> > RDX: ffff88800cea8680 RSI: ffffffff81becaf9 RDI: 0000000003
> > RBP: ffff888010667450 R08: 0000000000000001 R09: 0000000000
> > R10: ffffffff81becaab R11: 0000000000000001 R12: ffff888008
> > R13: 0000000000000001 R14: 0000000000000200 R15: dffffc0000
> > FS:  0000000000000000(0000) GS:ffff888035e00000(0000) knlG0
> > CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> > CR2: 00007ffd875cad00 CR3: 00000000094ce000 CR4: 0000000000
> > DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000
> > DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000
> > Call Trace:
> >  <TASK>
> >  free_pcp_prepare+0x3be/0xaa0
> >  free_unref_page+0x1c/0x650
> >  ? trace_hardirqs_on+0x6a/0x1d0
> >  free_compound_page+0xec/0x130
> >  free_transhuge_page+0x1be/0x260
> >  __put_compound_page+0x90/0xd0
> >  release_pages+0x54c/0x1060
> >  ? filemap_remove_folio+0x161/0x210
> >  ? lock_downgrade+0x720/0x720
> >  ? __put_page+0x150/0x150
> >  ? filemap_free_folio+0x164/0x350
> >  __pagevec_release+0x7c/0x110
> >  shmem_undo_range+0x85e/0x1250
> > ...
> >
> > The repro involved having a huge page that is split due to uprobe event
> > temporarily replacing one of the pages in the huge page. Later the huge
> > page was combined again, but the counters were off, as the PTE level
> > was not properly updated.
> >
> > Make sure that when PMD is cleared and prior to freeing the level the
> > PTEs are updated.
> >
> > Fixes: df4e817b7108 ("mm: page table check")
> >
> > Signed-off-by: Pasha Tatashin <pasha.tatashin@soleen.com>
> > ---
> >  include/linux/page_table_check.h | 18 ++++++++++++++++++
> >  mm/khugepaged.c                  |  3 +++
> >  mm/page_table_check.c            | 21 +++++++++++++++++++++
> >  3 files changed, 42 insertions(+)
> >
> > diff --git a/include/linux/page_table_check.h b/include/linux/page_table_check.h
> > index 38cace1da7b6..e88bbe37727b 100644
> > --- a/include/linux/page_table_check.h
> > +++ b/include/linux/page_table_check.h
> > @@ -26,6 +26,8 @@ void __page_table_check_pmd_set(struct mm_struct *mm, unsigned long addr,
> >                                 pmd_t *pmdp, pmd_t pmd);
> >  void __page_table_check_pud_set(struct mm_struct *mm, unsigned long addr,
> >                                 pud_t *pudp, pud_t pud);
> > +void __page_table_check_pmd_clear_full(struct mm_struct *mm, unsigned long addr,
> > +                                      pmd_t pmd);
> >
> >  static inline void page_table_check_alloc(struct page *page, unsigned int order)
> >  {
> > @@ -100,6 +102,16 @@ static inline void page_table_check_pud_set(struct mm_struct *mm,
> >         __page_table_check_pud_set(mm, addr, pudp, pud);
> >  }
> >
> > +static inline void page_table_check_pmd_clear_full(struct mm_struct *mm,
> > +                                                  unsigned long addr,
> > +                                                  pmd_t pmd)
> > +{
> > +       if (static_branch_likely(&page_table_check_disabled))
> > +               return;
> > +
> > +       __page_table_check_pmd_clear_full(mm, addr, pmd);
> > +}
> > +
> >  #else
> >
> >  static inline void page_table_check_alloc(struct page *page, unsigned int order)
> > @@ -143,5 +155,11 @@ static inline void page_table_check_pud_set(struct mm_struct *mm,
> >  {
> >  }
> >
> > +static inline void page_table_check_pmd_clear_full(struct mm_struct *mm,
> > +                                                  unsigned long addr,
> > +                                                  pmd_t pmd)
> > +{
> > +}
> > +
> >  #endif /* CONFIG_PAGE_TABLE_CHECK */
> >  #endif /* __LINUX_PAGE_TABLE_CHECK_H */
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 30e59e4af272..d84977c6dc0d 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -16,6 +16,7 @@
> >  #include <linux/hashtable.h>
> >  #include <linux/userfaultfd_k.h>
> >  #include <linux/page_idle.h>
> > +#include <linux/page_table_check.h>
> >  #include <linux/swapops.h>
> >  #include <linux/shmem_fs.h>
> >
> > @@ -1422,10 +1423,12 @@ static void collapse_and_free_pmd(struct mm_struct *mm, struct vm_area_struct *v
> >         spinlock_t *ptl;
> >         pmd_t pmd;
> >
> > +       mmap_assert_write_locked(mm);
> >         ptl = pmd_lock(vma->vm_mm, pmdp);
> >         pmd = pmdp_collapse_flush(vma, addr, pmdp);
> >         spin_unlock(ptl);
> >         mm_dec_nr_ptes(mm);
> > +       page_table_check_pmd_clear_full(mm, addr, pmd);
>

Hi Wei,

Thank you for your feedback,

> pmdp_collapse_flush() already calls page_table_check_pmd_clear() via
> pmdp_huge_get_and_clean().  Both pmdp_table_check_pmd_clear() and
> page_table_check_pmd_clear_full() can call
> __page_table_check_pmd_clear(). If that happens, then the page table
> check counters can be messed up.  Certainly, there is no bug here
> because the pmd is not huge and __page_table_check_pmd_clear() should
> be skipped in both calls. But it would be better to avoid this
> unnecessary subtlety by renaming page_table_check_pmd_clear_full() to
> page_table_check_clear_pte_range() and not calling
> __page_table_check_pmd_clear() there.  To make the code even more

Makes sense, I will rename page_table_check_pmd_clear_full() to
page_table_check_clear_pte_range()
and remove the call to __page_table_check_pmd_clear().

> clear, __page_table_check_pmd_clear() can also be renamed as
> __page_table_check_huge_pmd_clear() (similar for its callers).

Let's keep the current names for now as this does not affect the bug
fix. Perhaps, I can rename it later when working on ARM64 support.

Pasha