Bluetooth: Fix UAF in hci_conn_hash_flush again

Commit 06149746 ("Bluetooth: hci_conn: Add support for linking
multiple hcon") reintroduced a previously fixed bug [1] ("KASAN:
slab-use-after-free Read in hci_conn_hash_flush"). This bug was
originally fixed by commit 5dc7d23e ("Bluetooth: hci_conn: Fix
possible UAF").

The hci_conn_unlink function was added to avoid invalidating the link
traversal caused by successive hci_conn_del operations releasing extra
connections. However, currently hci_conn_unlink itself also releases
extra connections, resulted in the reintroduced bug.

This patch follows a more robust solution for cleaning up all
connections, by repeatedly removing the first connection until there are
none left. This approach does not rely on the inner workings of
hci_conn_del and ensures proper cleanup of all connections.

Meanwhile, we need to make sure that hci_conn_del never fails. Indeed it
doesn't, as it now always returns zero. To make this a bit clearer, this
patch also changes its return type to void.

Reported-by: syzbot+8bb72f86fc823817bc5d@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/linux-bluetooth/000000000000aa920505f60d25ad@google.com/
Fixes: 06149746 ("Bluetooth: hci_conn: Add support for linking multiple hcon")
Signed-off-by: Ruihan Li <lrh2000@pku.edu.cn>
Co-developed-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>

include/net/bluetooth/hci_core.h

@@ -1327,7 +1327,7 @@ int hci_le_create_cis(struct hci_conn *conn);
 
 struct hci_conn *hci_conn_add(struct hci_dev *hdev, int type, bdaddr_t *dst,
 			      u8 role);
-int hci_conn_del(struct hci_conn *conn);
+void hci_conn_del(struct hci_conn *conn);
 void hci_conn_hash_flush(struct hci_dev *hdev);
 void hci_conn_check_pending(struct hci_dev *hdev);
 

net/bluetooth/hci_conn.c

@@ -1088,6 +1088,14 @@ static void hci_conn_unlink(struct hci_conn *conn)
 
 			hci_conn_unlink(child);
 
+			/* If hdev is down it means
+			 * hci_dev_close_sync/hci_conn_hash_flush is in progress
+			 * and links don't need to be cleanup as all connections
+			 * would be cleanup.
+			 */
+			if (!test_bit(HCI_UP, &hdev->flags))
+				continue;
+
 			/* Due to race, SCO connection might be not established
 			 * yet at this point. Delete it now, otherwise it is
 			 * possible for it to be stuck and can't be deleted.
@@ -1112,7 +1120,7 @@ static void hci_conn_unlink(struct hci_conn *conn)
 	conn->link = NULL;
 }
 
-int hci_conn_del(struct hci_conn *conn)
+void hci_conn_del(struct hci_conn *conn)
 {
 	struct hci_dev *hdev = conn->hdev;
 
@@ -1163,8 +1171,6 @@ int hci_conn_del(struct hci_conn *conn)
 	 * rest of hci_conn_del.
 	 */
 	hci_conn_cleanup(conn);
-
-	return 0;
 }
 
 struct hci_dev *hci_get_route(bdaddr_t *dst, bdaddr_t *src, uint8_t src_type)
@@ -2465,22 +2471,27 @@ void hci_conn_enter_active_mode(struct hci_conn *conn, __u8 force_active)
 /* Drop all connection on the device */
 void hci_conn_hash_flush(struct hci_dev *hdev)
 {
-	struct hci_conn_hash *h = &hdev->conn_hash;
-	struct hci_conn *c, *n;
+	struct list_head *head = &hdev->conn_hash.list;
+	struct hci_conn *conn;
 
 	BT_DBG("hdev %s", hdev->name);
 
-	list_for_each_entry_safe(c, n, &h->list, list) {
-		c->state = BT_CLOSED;
-
-		hci_disconn_cfm(c, HCI_ERROR_LOCAL_HOST_TERM);
+	/* We should not traverse the list here, because hci_conn_del
+	 * can remove extra links, which may cause the list traversal
+	 * to hit items that have already been released.
+	 */
+	while ((conn = list_first_entry_or_null(head,
+						struct hci_conn,
+						list)) != NULL) {
+		conn->state = BT_CLOSED;
+		hci_disconn_cfm(conn, HCI_ERROR_LOCAL_HOST_TERM);
 
 		/* Unlink before deleting otherwise it is possible that
 		 * hci_conn_del removes the link which may cause the list to
 		 * contain items already freed.
 		 */
-		hci_conn_unlink(c);
-		hci_conn_del(c);
+		hci_conn_unlink(conn);
+		hci_conn_del(conn);
 	}
 }