Re: [PATCH 38/46] mm/ksm: reset the unstable trees of all nodes

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Tue, Oct 27, 2026 at 12:17:58AM +0100, David Hildenbrand (Arm) wrote:
> Each full scan resets ksm_nr_node_ids unstable tree roots. Enabling
> merge_across_nodes resets this count to 1, even if per-node roots were
> previously allocated.
>
> So when toggling "merge_across_nodes" 0 -> 1 -> 0, a scan that starts
> while the setting is 1 and continues after switching back to 0 can
> encounter stale unstable tree roots, resulting in UAF.
>
> Only root is allowed to change "merge_across_nodes", and the documented
> sequence is: "merge_across_nodes setting can be changed only when there
> are no ksm shared pages in the system: set run 2 to unmerge pages first,
> then to 1 after changing merge_across_nodes, to remerge according to the
> new setting."
>
> Let's fix it by tracking in ksm_nr_node_ids the actual number of
> allocated tree roots.
>
> remove_all_stable_nodes() and ksm_check_stable_tree() will now scan
> more tree roots (skipping the empty ones), which is harmless.
>
> Fixes: ef53d16cded7 ("ksm: allocate roots when needed")
> Cc: stable@vger.kernel.org
> Reported-by: Josef Bacik <josef@toxicpanda.com>
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>

Acked-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>

> ---
>  mm/ksm.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/mm/ksm.c b/mm/ksm.c
> index dbfb8f98b1892..9c51925825cbf 100644
> --- a/mm/ksm.c
> +++ b/mm/ksm.c
> @@ -475,6 +475,7 @@ static void advisor_stop_scan(void)
>  #ifdef CONFIG_NUMA
>  /* Zeroed when merging across nodes is not allowed */
>  static unsigned int ksm_merge_across_nodes = 1;
> +/* Number of allocated stable and unstable tree roots. */
>  static int ksm_nr_node_ids = 1;
>  #else
>  #define ksm_merge_across_nodes	1U
> @@ -3636,14 +3637,13 @@ static ssize_t merge_across_nodes_store(struct kobject *kobj,
>  			else {
>  				root_stable_tree = buf;
>  				root_unstable_tree = buf + nr_node_ids;
> +				ksm_nr_node_ids = nr_node_ids;
>  				/* Stable tree is empty but not the unstable */
>  				root_unstable_tree[0] = one_unstable_tree[0];
>  			}
>  		}
> -		if (!err) {
> +		if (!err)
>  			ksm_merge_across_nodes = knob;
> -			ksm_nr_node_ids = knob ? 1 : nr_node_ids;
> -		}
>  	}
>  	mutex_unlock(&ksm_thread_mutex);
>
>
> --
> 2.43.0
>

--
Cheers, Lorenzo