On Mon 07-11-16 14:07:36, Johannes Weiner wrote:
> The radix tree counts valid entries in each tree node. Entries stored
> in the tree cannot be removed by simpling storing NULL in the slot or
> the internal counters will be off and the node never gets freed again.
>
> When collapsing a shmem page fails, restore the holes that were filled
> with radix_tree_insert() with a proper radix tree deletion.
>
> Fixes: f3f0e1d2150b ("khugepaged: add support of collapse for tmpfs/shmem pages")
> Reported-by: Jan Kara <jack@xxxxxxx>
> Signed-off-by: Johannes Weiner <hannes@xxxxxxxxxxx>
> ---
> mm/khugepaged.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/mm/khugepaged.c b/mm/khugepaged.c
> index 728d7790dc2d..eac6f0580e26 100644
> --- a/mm/khugepaged.c
> +++ b/mm/khugepaged.c
> @@ -1520,7 +1520,8 @@ static void collapse_shmem(struct mm_struct *mm,
> if (!nr_none)
> break;
> /* Put holes back where they were */
> - radix_tree_replace_slot(slot, NULL);
> + radix_tree_delete(&mapping->page_tree,
> + iter.index);
Hum, but this is inside radix_tree_for_each_slot() iteration. And
radix_tree_delete() may end up freeing nodes resulting in invalidating
current slot pointer and the iteration code will do use-after-free.
Honza
--
Jan Kara <jack@xxxxxxxx>
SUSE Labs, CR
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