Re: [PATCH 3/4] mm: swap: fix potensial buffer overflow in setup_clusters()

[Baoquan He <bhe@redhat.com>]

On 05/26/25 at 02:44am, Kairui Song wrote:
> On Thu, May 22, 2025 at 11:32 AM Kemeng Shi <shikemeng@huaweicloud.com> wrote:
> >
> > In setup_swap_map(), we only ensure badpages are in range (0, last_page].
> > As maxpages might be < last_page, setup_clusters() will encounter a
> > buffer overflow when a badpage is >= maxpages.
> > Only call inc_cluster_info_page() for badpage which is < maxpages to
> > fix the issue.
> >
> > Fixes: b843786b0bd01 ("mm: swapfile: fix SSD detection with swapfile on btrfs")
> > Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
> > ---
> >  mm/swapfile.c | 10 +++++++---
> >  1 file changed, 7 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/swapfile.c b/mm/swapfile.c
> > index a82f4ebefca3..63ab9f14b2c6 100644
> > --- a/mm/swapfile.c
> > +++ b/mm/swapfile.c
> > @@ -3208,9 +3208,13 @@ static struct swap_cluster_info *setup_clusters(struct swap_info_struct *si,
> >          * and the EOF part of the last cluster.
> >          */
> >         inc_cluster_info_page(si, cluster_info, 0);
> > -       for (i = 0; i < swap_header->info.nr_badpages; i++)
> > -               inc_cluster_info_page(si, cluster_info,
> > -                                     swap_header->info.badpages[i]);
> > +       for (i = 0; i < swap_header->info.nr_badpages; i++) {
> > +               unsigned int page_nr = swap_header->info.badpages[i];
> > +
> > +               if (page_nr >= maxpages)
> > +                       continue;
> > +               inc_cluster_info_page(si, cluster_info, page_nr);
> 
> I think we might need a pr_err or pr_warn here, this means mkswap
> marked the wrong region as a bad block? Or some fs side things went
> wrong.

There's aready warning in read_swap_header():
static unsigned long read_swap_header(struct swap_info_struct *si,
                                        union swap_header *swap_header,
                                        struct inode *inode)
{
	......
	if (last_page > maxpages) {
                pr_warn("Truncating oversized swap area, only using %luk out of %luk\n",
                        K(maxpages), K(last_page));
        }
	...
}

And if we add pr_err|warn here, we also need add it in setup_swap_map()
when filling swap_map.

> 
> 
> > +       }
> >         for (i = maxpages; i < round_up(maxpages, SWAPFILE_CLUSTER); i++)
> >                 inc_cluster_info_page(si, cluster_info, i);
> >
> > --
> > 2.30.0
> >
> >
>