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    SubjectRe: [PATCH] s390: cio: fix two use-after-free bugs in device.c
    On Fri, 20 Nov 2020 15:48:49 +0800
    Qinglang Miao <miaoqinglang@huawei.com> wrote:

    > put_device calls release function which do kfree() inside.
    > So following use of sch&cdev would cause use-after-free bugs.
    >
    > Fix these by simply adjusting the position of put_device.
    >
    > Fixes: 37db8985b211 ("s390/cio: add basic protected virtualization support")
    > Fixes: 74bd0d859dc3 ("s390/cio: fix unlocked access of online member")
    > Reported-by: Hulk Robot <hulkci@huawei.com>
    > Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
    > ---
    > drivers/s390/cio/device.c | 6 +++---
    > 1 file changed, 3 insertions(+), 3 deletions(-)
    >
    > diff --git a/drivers/s390/cio/device.c b/drivers/s390/cio/device.c
    > index b29fe8d50..69492417b 100644
    > --- a/drivers/s390/cio/device.c
    > +++ b/drivers/s390/cio/device.c
    > @@ -1664,10 +1664,10 @@ void __init ccw_device_destroy_console(struct ccw_device *cdev)
    > struct io_subchannel_private *io_priv = to_io_private(sch);
    >
    > set_io_private(sch, NULL);
    > - put_device(&sch->dev);
    > - put_device(&cdev->dev);
    > dma_free_coherent(&sch->dev, sizeof(*io_priv->dma_area),
    > io_priv->dma_area, io_priv->dma_area_dma);
    > + put_device(&sch->dev);
    > + put_device(&cdev->dev);

    That change looks reasonable.

    > kfree(io_priv);
    > }
    >
    > @@ -1774,8 +1774,8 @@ static int ccw_device_remove(struct device *dev)
    > ret, cdev->private->dev_id.ssid,
    > cdev->private->dev_id.devno);
    > /* Give up reference obtained in ccw_device_set_online(). */
    > - put_device(&cdev->dev);
    > spin_lock_irq(cdev->ccwlock);
    > + put_device(&cdev->dev);

    As the comment above states, the put_device() gives up the reference
    obtained in ccw_device_set_online(). There's at least one more
    reference remaining (held by the caller of the remove function). Moving
    the put_device() does not fix anything here.

    > }
    > ccw_device_set_timeout(cdev, 0);
    > cdev->drv = NULL;

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