Bluetooth: cmtp: fix file refcount when cmtp_attach_device fails

When cmtp_attach_device fails, cmtp_add_connection returns the error value
which leads to the caller to doing fput through sockfd_put. But
cmtp_session kthread, which is stopped in this path will also call fput,
leading to a potential refcount underflow or a use-after-free.

Add a refcount before we signal the kthread to stop. The kthread will try
to grab the cmtp_session_sem mutex before doing the fput, which is held
when get_file is called, so there should be no races there.

Reported-by: Ryota Shiga
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

diff --git a/net/bluetooth/cmtp/core.c b/net/bluetooth/cmtp/core.c
index 07cfa3249f83ae6226d06aae59f27ce6b41a84e8..0a2d78e811cf5bbea7d735bc345a1ddc9a46cbf8 100644 (file)
--- a/net/bluetooth/cmtp/core.c
+++ b/net/bluetooth/cmtp/core.c
@@ -392,6 +392,11 @@ int cmtp_add_connection(struct cmtp_connadd_req *req, struct socket *sock)
        if (!(session->flags & BIT(CMTP_LOOPBACK))) {
                err = cmtp_attach_device(session);
                if (err < 0) {
+                       /* Caller will call fput in case of failure, and so
+                        * will cmtp_session kthread.
+                        */
+                       get_file(session->sock->file);
+
                        atomic_inc(&session->terminate);
                        wake_up_interruptible(sk_sleep(session->sock->sk));
                        up_write(&cmtp_session_sem);