[PATCH 2/2] maple_tree: Fix a potential memory leak, OOB access, or other unpredictable bug

[Peng Zhang <zhangpeng.00@bytedance.com>]

In mas_alloc_nodes(), there is such a piece of code:
while (requested) {
	...
	node->node_count = 0;
	...
}
"node->node_count = 0" means to initialize the node_count field of the
new node, but the node may not be a new node. It may be a node that
existed before and node_count has a value, setting it to 0 will cause a
memory leak. At this time, mas->alloc->total will be greater than the
actual number of nodes in the linked list, which may cause many other
errors. For example, out-of-bounds access in mas_pop_node(), and
mas_pop_node() may return addresses that should not be used.
Fix it by initializing node_count only for new nodes.

Fixes: 54a611b60590 ("Maple Tree: add new data structure")
Signed-off-by: Peng Zhang <zhangpeng.00@bytedance.com>
Cc: <stable@vger.kernel.org>
---
 lib/maple_tree.c | 16 ++++------------
 1 file changed, 4 insertions(+), 12 deletions(-)

diff --git a/lib/maple_tree.c b/lib/maple_tree.c
index 65fd861b30e1..9e25b3215803 100644
--- a/lib/maple_tree.c
+++ b/lib/maple_tree.c
@@ -1249,26 +1249,18 @@ static inline void mas_alloc_nodes(struct ma_state *mas, gfp_t gfp)
 	node = mas->alloc;
 	node->request_count = 0;
 	while (requested) {
-		max_req = MAPLE_ALLOC_SLOTS;
-		if (node->node_count) {
-			unsigned int offset = node->node_count;
-
-			slots = (void **)&node->slot[offset];
-			max_req -= offset;
-		} else {
-			slots = (void **)&node->slot;
-		}
-
+		max_req = MAPLE_ALLOC_SLOTS - node->node_count;
+		slots = (void **)&node->slot[node->node_count];
 		max_req = min(requested, max_req);
 		count = mt_alloc_bulk(gfp, max_req, slots);
 		if (!count)
 			goto nomem_bulk;
 
+		if (node->node_count == 0)
+			node->slot[0]->node_count = 0;
 		node->node_count += count;
 		allocated += count;
 		node = node->slot[0];
-		node->node_count = 0;
-		node->request_count = 0;
 		requested -= count;
 	}
 	mas->alloc->total = allocated;
-- 
2.20.1