mm/ksm: fix possible UAF of stable_node

The commit 2c653d0 ("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.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 2c653d0 ("ksm: introduce ksm_max_page_sharing per page deduplication limit")
Signed-off-by: Chengming Zhou <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Stefan Roesch <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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;
 }