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

[David Hildenbrand (Arm)]

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 at vger.kernel.org
Reported-by: Josef Bacik <josef at toxicpanda.com>
Signed-off-by: David Hildenbrand (Arm) <david at 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