Re: [PATCH] mm: khugepaged: Fix kernel BUG in hpage_collapse_scan_file

[Yang Shi <shy828301@gmail.com>]

On Wed, Mar 29, 2023 at 2:53 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Wed, 29 Mar 2023 18:53:30 +0400 Ivan Orlov <ivan.orlov0322@gmail.com> wrote:
>
> > Syzkaller reported the following issue:
> >
> > ...
> >
> > The 'xas_store' call during page cache scanning can potentially
> > translate 'xas' into the error state (with the reproducer provided
> > by the syzkaller the error code is -ENOMEM). However, there are no
> > further checks after the 'xas_store', and the next call of 'xas_next'
> > at the start of the scanning cycle doesn't increase the xa_index,
> > and the issue occurs.
> >
> > This patch will add the xarray state error checking after the
> > 'xas_store' and the corresponding result error code.
> >
> > Tested via syzbot.
> >
> > Reported-by: syzbot+9578faa5475acb35fa50@syzkaller.appspotmail.com
> > Link: https://syzkaller.appspot.com/bug?id=7d6bb3760e026ece7524500fe44fb024a0e959fc
> > Signed-off-by: Ivan Orlov <ivan.orlov0322@gmail.com>
> > ---
> >  mm/khugepaged.c | 10 ++++++++++
> >  1 file changed, 10 insertions(+)
> >
> > diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> > index 92e6f56a932d..4d9850d9ea7f 100644
> > --- a/mm/khugepaged.c
> > +++ b/mm/khugepaged.c
> > @@ -55,6 +55,7 @@ enum scan_result {
> >       SCAN_CGROUP_CHARGE_FAIL,
> >       SCAN_TRUNCATED,
> >       SCAN_PAGE_HAS_PRIVATE,
> > +     SCAN_STORE_FAILED,
> >  };
> >
> >  #define CREATE_TRACE_POINTS
> > @@ -1840,6 +1841,15 @@ static int collapse_file(struct mm_struct *mm, unsigned long addr,
> >                                       goto xa_locked;
> >                               }
> >                               xas_store(&xas, hpage);
> > +                             if (xas_error(&xas)) {
> > +                                     /* revert shmem_charge performed
> > +                                      * in the previous condition
> > +                                      */
> > +                                     mapping->nrpages--;
> > +                                     shmem_uncharge(mapping->host, 1);
> > +                                     result = SCAN_STORE_FAILED;
> > +                                     goto xa_locked;
> > +                             }
> >                               nr_none++;
> >                               continue;
> >                       }
>
> Needs this, I assume.
>
> --- a/include/trace/events/huge_memory.h~mm-khugepaged-fix-kernel-bug-in-hpage_collapse_scan_file-fix
> +++ a/include/trace/events/huge_memory.h
> @@ -36,7 +36,8 @@
>         EM( SCAN_ALLOC_HUGE_PAGE_FAIL,  "alloc_huge_page_failed")       \
>         EM( SCAN_CGROUP_CHARGE_FAIL,    "ccgroup_charge_failed")        \
>         EM( SCAN_TRUNCATED,             "truncated")                    \
> -       EMe(SCAN_PAGE_HAS_PRIVATE,      "page_has_private")             \
> +       EM( SCAN_PAGE_HAS_PRIVATE,      "page_has_private")             \
> +       EMe(SCAN_STORE_FAILED,          "store_failed")

I'm a little bit reluctant to make the error code list longer, can we
just return SCAN_FAIL? IIUC this issue should happen very rarely,
maybe not worth a new error code.

Basically the rollback approach makes sense to me. IIRC Zach was
looking into the same problem, loop him in. He may share some
thoughts.

>
>  #undef EM
>  #undef EMe
> _
>
>