ksmbd: fix session use-after-free in multichannel connection

There is a race condition between session setup and
ksmbd_sessions_deregister. The session can be freed before the connection
is added to channel list of session.
This patch check reference count of session before freeing it.

Cc: stable@vger.kernel.org
Reported-by: Sean Heelan <seanheelan@gmail.com>
Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/smb/server/auth.c

@@ -1016,9 +1016,9 @@ static int ksmbd_get_encryption_key(struct ksmbd_work *work, __u64 ses_id,
 
 	ses_enc_key = enc ? sess->smb3encryptionkey :
 		sess->smb3decryptionkey;
-	if (enc)
-		ksmbd_user_session_get(sess);
 	memcpy(key, ses_enc_key, SMB3_ENC_DEC_KEY_SIZE);
+	if (!enc)
+		ksmbd_user_session_put(sess);
 
 	return 0;
 }

fs/smb/server/mgmt/user_session.c

@@ -181,7 +181,7 @@ static void ksmbd_expire_session(struct ksmbd_conn *conn)
 	down_write(&sessions_table_lock);
 	down_write(&conn->session_lock);
 	xa_for_each(&conn->sessions, id, sess) {
-		if (atomic_read(&sess->refcnt) == 0 &&
+		if (atomic_read(&sess->refcnt) <= 1 &&
 		    (sess->state != SMB2_SESSION_VALID ||
 		     time_after(jiffies,
 			       sess->last_active + SMB2_SESSION_TIMEOUT))) {
@@ -233,7 +233,8 @@ void ksmbd_sessions_deregister(struct ksmbd_conn *conn)
 				down_write(&conn->session_lock);
 				xa_erase(&conn->sessions, sess->id);
 				up_write(&conn->session_lock);
-				ksmbd_session_destroy(sess);
+				if (atomic_dec_and_test(&sess->refcnt))
+					ksmbd_session_destroy(sess);
 			}
 		}
 	}
@@ -252,7 +253,8 @@ void ksmbd_sessions_deregister(struct ksmbd_conn *conn)
 		if (xa_empty(&sess->ksmbd_chann_list)) {
 			xa_erase(&conn->sessions, sess->id);
 			hash_del(&sess->hlist);
-			ksmbd_session_destroy(sess);
+			if (atomic_dec_and_test(&sess->refcnt))
+				ksmbd_session_destroy(sess);
 		}
 	}
 	up_write(&conn->session_lock);
@@ -328,8 +330,8 @@ void ksmbd_user_session_put(struct ksmbd_session *sess)
 
 	if (atomic_read(&sess->refcnt) <= 0)
 		WARN_ON(1);
-	else
-		atomic_dec(&sess->refcnt);
+	else if (atomic_dec_and_test(&sess->refcnt))
+		ksmbd_session_destroy(sess);
 }
 
 struct preauth_session *ksmbd_preauth_session_alloc(struct ksmbd_conn *conn,
@@ -436,7 +438,7 @@ 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);
+	atomic_set(&sess->refcnt, 2);
 
 	ret = __init_smb2_session(sess);
 	if (ret)

fs/smb/server/smb2pdu.c

@@ -2235,13 +2235,14 @@ int smb2_session_logoff(struct ksmbd_work *work)
 		return -ENOENT;
 	}
 
-	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;
+	if (sess->user) {
+		ksmbd_free_user(sess->user);
+		sess->user = NULL;
+	}
 	ksmbd_all_conn_set_status(sess_id, KSMBD_SESS_NEED_NEGOTIATE);
 
 	rsp->StructureSize = cpu_to_le16(4);