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    Subject[PATCH 4.14 34/79] Bluetooth: cmtp: fix file refcount when cmtp_attach_device fails
    Date
    From: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

    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>
    ---
    net/bluetooth/cmtp/core.c | 5 +++++
    1 file changed, 5 insertions(+)

    --- a/net/bluetooth/cmtp/core.c
    +++ b/net/bluetooth/cmtp/core.c
    @@ -391,6 +391,11 @@ int cmtp_add_connection(struct cmtp_conn
    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);

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