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    Subject[ 143/221] staging: comedi: comedi_test: fix race when cancelling command
    Date
    3.7-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Ian Abbott <abbotti@mev.co.uk>

    commit c0729eeefdcd76db338f635162bf0739fd2c5f6f upstream.

    Éric Piel reported a kernel oops in the "comedi_test" module. It was a
    NULL pointer dereference within `waveform_ai_interrupt()` (actually a
    timer function) that sometimes occurred when a running asynchronous
    command is cancelled (either by the `COMEDI_CANCEL` ioctl or by closing
    the device file).

    This seems to be a race between the caller of `waveform_ai_cancel()`
    which on return from that function goes and tears down the running
    command, and the timer function which uses the command. In particular,
    `async->cmd.chanlist` gets freed (and the pointer set to NULL) by
    `do_become_nonbusy()` in "comedi_fops.c" but a previously scheduled
    `waveform_ai_interrupt()` timer function will dereference that pointer
    regardless, leading to the oops.

    Fix it by replacing the `del_timer()` call in `waveform_ai_cancel()`
    with `del_timer_sync()`.

    Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
    Reported-by: Éric Piel <piel@delmic.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/staging/comedi/drivers/comedi_test.c | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    --- a/drivers/staging/comedi/drivers/comedi_test.c
    +++ b/drivers/staging/comedi/drivers/comedi_test.c
    @@ -372,7 +372,7 @@ static int waveform_ai_cancel(struct com
    struct waveform_private *devpriv = dev->private;

    devpriv->timer_running = 0;
    - del_timer(&devpriv->timer);
    + del_timer_sync(&devpriv->timer);
    return 0;
    }


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