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

[Kemeng Shi <shikemeng@huaweicloud.com>]

on 5/26/2025 2:44 AM, 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.
As Baoquan metioned that there is already warning in read_swap_header().
Besides, I think last_page in swap_header already indicates the range
which is acceptable to fs and we only need to explicitly handle page_nr
which is > last_page.
> 
> 
>> +       }
>>         for (i = maxpages; i < round_up(maxpages, SWAPFILE_CLUSTER); i++)
>>                 inc_cluster_info_page(si, cluster_info, i);
>>
>> --
>> 2.30.0
>>
>>
>