ksmbd: fix user-after-free from session log off

There is racy issue between smb2 session log off and smb2 session setup.
It will cause user-after-free from session log off.
This add session_lock when setting SMB2_SESSION_EXPIRED and referece
count to session struct not to free session while it is being used.

Cc: [email protected] # v5.15+
Reported-by: [email protected] # ZDI-CAN-25282
Signed-off-by: Namjae Jeon <[email protected]>
Signed-off-by: Steve French <[email protected]>

Author: namjaejeon

fs/smb/server/mgmt/user_session.c

@@ -177,9 +177,10 @@ static void ksmbd_expire_session(struct ksmbd_conn *conn)
 
 	down_write(&conn->session_lock);
 	xa_for_each(&conn->sessions, id, sess) {
-		if (sess->state != SMB2_SESSION_VALID ||
-		    time_after(jiffies,
-			       sess->last_active + SMB2_SESSION_TIMEOUT)) {
+		if (atomic_read(&sess->refcnt) == 0 &&
+		    (sess->state != SMB2_SESSION_VALID ||
+		     time_after(jiffies,
+			       sess->last_active + SMB2_SESSION_TIMEOUT))) {
 			xa_erase(&conn->sessions, sess->id);
 			hash_del(&sess->hlist);
 			ksmbd_session_destroy(sess);
@@ -269,8 +270,6 @@ struct ksmbd_session *ksmbd_session_lookup_slowpath(unsigned long long id)
 
 	down_read(&sessions_table_lock);
 	sess = __session_lookup(id);
-	if (sess)
-		sess->last_active = jiffies;
 	up_read(&sessions_table_lock);
 
 	return sess;
@@ -289,6 +288,22 @@ struct ksmbd_session *ksmbd_session_lookup_all(struct ksmbd_conn *conn,
 	return sess;
 }
 
+void ksmbd_user_session_get(struct ksmbd_session *sess)
+{
+	atomic_inc(&sess->refcnt);
+}
+
+void ksmbd_user_session_put(struct ksmbd_session *sess)
+{
+	if (!sess)
+		return;
+
+	if (atomic_read(&sess->refcnt) <= 0)
+		WARN_ON(1);
+	else
+		atomic_dec(&sess->refcnt);
+}
+
 struct preauth_session *ksmbd_preauth_session_alloc(struct ksmbd_conn *conn,
 						    u64 sess_id)
 {
@@ -393,6 +408,7 @@ static struct ksmbd_session *__session_create(int protocol)
 	xa_init(&sess->rpc_handle_list);
 	sess->sequence_number = 1;
 	rwlock_init(&sess->tree_conns_lock);
+	atomic_set(&sess->refcnt, 1);
 
 	ret = __init_smb2_session(sess);
 	if (ret)

fs/smb/server/mgmt/user_session.h

@@ -61,6 +61,8 @@ struct ksmbd_session {
 	struct ksmbd_file_table		file_table;
 	unsigned long			last_active;
 	rwlock_t			tree_conns_lock;
+
+	atomic_t			refcnt;
 };
 
 static inline int test_session_flag(struct ksmbd_session *sess, int bit)
@@ -104,4 +106,6 @@ void ksmbd_release_tree_conn_id(struct ksmbd_session *sess, int id);
 int ksmbd_session_rpc_open(struct ksmbd_session *sess, char *rpc_name);
 void ksmbd_session_rpc_close(struct ksmbd_session *sess, int id);
 int ksmbd_session_rpc_method(struct ksmbd_session *sess, int id);
+void ksmbd_user_session_get(struct ksmbd_session *sess);
+void ksmbd_user_session_put(struct ksmbd_session *sess);
 #endif /* __USER_SESSION_MANAGEMENT_H__ */

fs/smb/server/server.c

@@ -238,6 +238,8 @@ static void __handle_ksmbd_work(struct ksmbd_work *work,
 	} while (is_chained == true);
 
 send:
+	if (work->sess)
+		ksmbd_user_session_put(work->sess);
 	if (work->tcon)
 		ksmbd_tree_connect_put(work->tcon);
 	smb3_preauth_hash_rsp(work);

fs/smb/server/smb2pdu.c

@@ -605,8 +605,10 @@ int smb2_check_user_session(struct ksmbd_work *work)
 
 	/* Check for validity of user session */
 	work->sess = ksmbd_session_lookup_all(conn, sess_id);
-	if (work->sess)
+	if (work->sess) {
+		ksmbd_user_session_get(work->sess);
 		return 1;
+	}
 	ksmbd_debug(SMB, "Invalid user session, Uid %llu\n", sess_id);
 	return -ENOENT;
 }
@@ -1740,6 +1742,7 @@ int smb2_sess_setup(struct ksmbd_work *work)
 		}
 
 		conn->binding = true;
+		ksmbd_user_session_get(sess);
 	} else if ((conn->dialect < SMB30_PROT_ID ||
 		    server_conf.flags & KSMBD_GLOBAL_FLAG_SMB3_MULTICHANNEL) &&
 		   (req->Flags & SMB2_SESSION_REQ_FLAG_BINDING)) {
@@ -1766,6 +1769,7 @@ int smb2_sess_setup(struct ksmbd_work *work)
 		}
 
 		conn->binding = false;
+		ksmbd_user_session_get(sess);
 	}
 	work->sess = sess;
 
@@ -2228,7 +2232,9 @@ int smb2_session_logoff(struct ksmbd_work *work)
 	}
 
 	ksmbd_destroy_file_table(&sess->file_table);
+	down_write(&conn->session_lock);
 	sess->state = SMB2_SESSION_EXPIRED;
+	up_write(&conn->session_lock);
 
 	ksmbd_free_user(sess->user);
 	sess->user = NULL;