Re: [PATCH 2/6] mm: swap: use correct step in loop to wait all clusters in wait_for_allocation()

[Kairui Song]

On Sat, Feb 22, 2025 at 3:12 PM Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx> wrote:
>
> Use correct step in loop to wait all clusters in wait_for_allocation().
> If we miss some cluster in wait_for_allocation(), use after free may
> occurs as following:
> shmem_writepage                  swapoff
>  folio_alloc_swap
>   get_swap_pages
>    scan_swap_map_slots
>     cluster_alloc_swap_entry
>      alloc_swap_scan_cluster
>       cluster_alloc_range
>        /* SWP_WRITEOK is valid */
>        if (!(si->flags & SWP_WRITEOK))
>
>                                   ...
>                                   del_from_avail_list(p, true);
>                                   ...
>                                   /* miss the cluster in shmem_writepage */
>                                   wait_for_allocation()
>                                   ...
>                                   try_to_unuse()
>
>        memset(si->swap_map + start, usage, nr_pages);
>        swap_range_alloc(si, nr_pages);
>        ci->count += nr_pages;
>        /* return a valid entry */
>
>                                   ...
>                                   exit_swap_address_space(p->type);
>                                   ...
>
>  ...
>  add_to_swap_cache
>   /* dereference swap_address_space(entry) which is NULL */
>   xas_lock_irq(&xas);
>
> Fixes: e47bd46eab97e ("mm, swap: hold a reference during scan and cleanup flag usage")
> Signed-off-by: Kemeng Shi <shikemeng@xxxxxxxxxxxxxxx>
> ---
>  mm/swapfile.c | 1 -
>  1 file changed, 1 deletion(-)
>
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index e5f58ab86329..425126c0a07d 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -2627,7 +2627,6 @@ static void wait_for_allocation(struct swap_info_struct *si)
>         for (offset = 0; offset < end; offset += SWAPFILE_CLUSTER) {
>                 ci = lock_cluster(si, offset);
>                 unlock_cluster(ci);
> -               offset += SWAPFILE_CLUSTER;
>         }
>  }
>

Thanks, good catch.

Reviewed-by: Kairui Song <kasong@xxxxxxxxxxx>