[PATCH] tipc: fix use-after-free in netns teardown

["Jérémy Jean" <Jeremy.Jean@oss.cyber.gouv.fr>]

conn_put() decreases the reference count of a connection, and when it
reaches zero, tipc_conn_kref_release() removes the connection from
conn_idr and frees it. Removing the entry is protected by idr_lock,
but decreasing the count is not.

During netns dismantle, it may happen that a connection gets its count
dropped to zero by conn_put() while tipc_topsrv_stop() still holds the
lock idr_lock. tipc_topsrv_stop() calls conn_get() on that connection
whose count is already zero, and then releases idr_lock.
tipc_conn_kref_release() can then free the connection before
tipc_conn_close() uses it. KASAN reports the UAF as:

  BUG: KASAN: slab-use-after-free in tipc_conn_close (net/tipc/topsrv.c:158)
  Read of size 8 at addr ff1100000cf4f408 by task kworker/u16:3/70

Stop accepting connections before walking conn_idr. Use
kref_get_unless_zero() to avoid taking references when the count is zero.
Retry from the first entry, releasing idr_lock and calling cond_resched()
between iterations so pending work can finish.

Fixes: 667eeab4999e ("tipc: Fix use-after-free in tipc_conn_close().")
Cc: stable@vger.kernel.org
Assisted-by: LLM
Signed-off-by: Jérémy Jean <Jeremy.Jean@oss.cyber.gouv.fr>
---
 net/tipc/topsrv.c | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/net/tipc/topsrv.c b/net/tipc/topsrv.c
index af530c9ed8402..9ab2349f71190 100644
--- a/net/tipc/topsrv.c
+++ b/net/tipc/topsrv.c
@@ -700,16 +700,23 @@ static void tipc_topsrv_stop(struct net *net)
 	struct tipc_conn *con;
 	int id;
 
+	write_lock_bh(&lsock->sk->sk_callback_lock);
+	lsock->sk->sk_user_data = NULL;
+	write_unlock_bh(&lsock->sk->sk_callback_lock);
+	cancel_work_sync(&srv->awork);
+
 	spin_lock_bh(&srv->idr_lock);
-	for (id = 0; srv->idr_in_use; id++) {
-		con = idr_find(&srv->conn_idr, id);
+	for (id = 0; srv->idr_in_use; id = 0) {
+		con = idr_get_next(&srv->conn_idr, &id);
+		if (con && !kref_get_unless_zero(&con->kref))
+			con = NULL;
+		spin_unlock_bh(&srv->idr_lock);
 		if (con) {
-			conn_get(con);
-			spin_unlock_bh(&srv->idr_lock);
 			tipc_conn_close(con);
 			conn_put(con);
-			spin_lock_bh(&srv->idr_lock);
 		}
+		cond_resched();
+		spin_lock_bh(&srv->idr_lock);
 	}
 	__module_get(lsock->ops->owner);
 	__module_get(lsock->sk->sk_prot_creator->owner);
-- 
2.47.3