Re: [PATCH] mm/ksm: fix possible UAF of stable_node

[David Hildenbrand <david@redhat.com>]

On 13.05.24 05:07, Chengming Zhou wrote:
> The commit 2c653d0ee2ae ("ksm: introduce ksm_max_page_sharing per page
> deduplication limit") introduced a possible failure case in the
> stable_tree_insert(), where we may free the new allocated stable_node_dup
> if we fail to prepare the missing chain node.
> 
> Then that kfolio return and unlock with a freed stable_node set... And
> any MM activities can come in to access kfolio->mapping, so UAF.
> 
> Fix it by moving folio_set_stable_node() to the end after stable_node
> is inserted successfully.
> 
> Fixes: 2c653d0ee2ae ("ksm: introduce ksm_max_page_sharing per page deduplication limit")
> Signed-off-by: Chengming Zhou <chengming.zhou@linux.dev>
> ---
>   mm/ksm.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/ksm.c b/mm/ksm.c
> index e1034bf1c937..a8b76af5cf64 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -2153,7 +2153,6 @@ static struct ksm_stable_node *stable_tree_insert(struct folio *kfolio)
>   
>   	INIT_HLIST_HEAD(&stable_node_dup->hlist);
>   	stable_node_dup->kpfn = kpfn;
> -	folio_set_stable_node(kfolio, stable_node_dup);
>   	stable_node_dup->rmap_hlist_len = 0;
>   	DO_NUMA(stable_node_dup->nid = nid);
>   	if (!need_chain) {
> @@ -2172,6 +2171,8 @@ static struct ksm_stable_node *stable_tree_insert(struct folio *kfolio)
>   		stable_node_chain_add_dup(stable_node_dup, stable_node);
>   	}
>   
> +	folio_set_stable_node(kfolio, stable_node_dup);
> +
>   	return stable_node_dup;

Looks correct to me.

We might now link the node before the folio->mapping is set up. Do we 
care? Don't think so.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb