linux-kernel - Re: [PATCH 1/6] mm: khugepaged: fix radix tree node leak in shmem collapse error path

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Date:   Tue, 8 Nov 2016 10:53:52 +0100
From:   Jan Kara <jack@...e.cz>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        "Kirill A. Shutemov" <kirill@...temov.name>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH 1/6] mm: khugepaged: fix radix tree node leak in shmem
 collapse error path

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@...e.cz>
> Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> ---
>  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@...e.com>
SUSE Labs, CR