Re: [PATCH] mm: zswap: fix potential memory corruption on duplicate store

[Nhat Pham]

On Fri, Sep 22, 2023 at 10:22 AM Domenico Cerasuolo
<cerasuolodomenico@xxxxxxxxx> wrote:
>
> While stress-testing zswap a memory corruption was happening when writing
> back pages. __frontswap_store used to check for duplicate entries before
> attempting to store a page in zswap, this was because if the store fails
> the old entry isn't removed from the tree. This change removes duplicate
> entries in zswap_store before the actual attempt.
>
> Based on commit ce9ecca0238b ("Linux 6.6-rc2")
>
> Fixes: 42c06a0e8ebe ("mm: kill frontswap")
> Signed-off-by: Domenico Cerasuolo <cerasuolodomenico@xxxxxxxxx>
Acked-by: Nhat Pham <nphamcs@xxxxxxxxx>
> ---
>  mm/zswap.c | 13 +++++++++++++
>  1 file changed, 13 insertions(+)
>
> diff --git a/mm/zswap.c b/mm/zswap.c
> index 412b1409a0d7..9146f9f19061 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -1218,6 +1218,19 @@ bool zswap_store(struct folio *folio)
>         if (!zswap_enabled || !tree)
>                 return false;
>
> +       /*
> +        * If this is a duplicate, it must be removed before attempting to store
> +        * it, otherwise, if the store fails the old page won't be removed from
> +        * the tree, and it might be written back overriding the new data.
> +        */
> +       spin_lock(&tree->lock);
> +       dupentry = zswap_rb_search(&tree->rbroot, offset);
> +       if (dupentry) {
> +               zswap_duplicate_entry++;
> +               zswap_invalidate_entry(tree, dupentry);
> +       }
> +       spin_unlock(&tree->lock);
> +
>         /*
>          * XXX: zswap reclaim does not work with cgroups yet. Without a
>          * cgroup-aware entry LRU, we will push out entries system-wide based on
> --
> 2.34.1
>