Bluetooth: cmtp: fix file refcount when cmtp_attach_device fails

commit 8da3a0b87f4f1c3a3bbc4bfb78cf68476e97d183 upstream.

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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/net/bluetooth/cmtp/core.c b/net/bluetooth/cmtp/core.c
index 7f26a5a19ff6d8fb87ef5404e32eb5e2ce0b5b53..9873684a9d8ff3e8ec2ef30389e14b1ecb1bc54c 100644 (file)
--- a/net/bluetooth/cmtp/core.c
+++ b/net/bluetooth/cmtp/core.c
@@ -391,6 +391,11 @@ 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);