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    SubjectRe: [PATCH] PCI: fix use-after-free in pci_register_host_bridge
    On Fri, Nov 20, 2020 at 03:48:48PM +0800, Qinglang Miao wrote:
    > When put_device(&bridge->dev) being called, kfree(bridge) is inside
    > of release function, so the following device_del would cause a
    > use-after-free bug.
    >
    > Fixes: 37d6a0a6f470 ("PCI: Add pci_register_host_bridge() interface")

    That commit did have some problems, but this patch doesn't apply to that
    commit. See commits 1b54ae8327a4 and 9885440b16b8.

    > Reported-by: Hulk Robot <hulkci@huawei.com>
    > Signed-off-by: Qinglang Miao <miaoqinglang@huawei.com>
    > ---
    > drivers/pci/probe.c | 2 +-
    > 1 file changed, 1 insertion(+), 1 deletion(-)
    >
    > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
    > index 4289030b0..82292e87e 100644
    > --- a/drivers/pci/probe.c
    > +++ b/drivers/pci/probe.c
    > @@ -991,8 +991,8 @@ static int pci_register_host_bridge(struct pci_host_bridge *bridge)
    > return 0;
    >
    > unregister:
    > - put_device(&bridge->dev);
    > device_del(&bridge->dev);
    > + put_device(&bridge->dev);

    I don't think this is right.

    Let's look at pci_register_host_bridge() with only the relevant
    sections:

    static int pci_register_host_bridge(struct pci_host_bridge *bridge)
    {
    ...

    err = device_add(&bridge->dev);
    if (err) {
    put_device(&bridge->dev);
    goto free;
    }
    bus->bridge = get_device(&bridge->dev);

    ...
    if (err)
    goto unregister;
    ...

    return 0;

    unregister:
    put_device(&bridge->dev);
    device_del(&bridge->dev);

    free:
    kfree(bus);
    return err;
    }

    The documentation for device_add says this:
    * Rule of thumb is: if device_add() succeeds, you should call
    * device_del() when you want to get rid of it. If device_add() has
    * *not* succeeded, use *only* put_device() to drop the reference
    * count.

    The put_device at the end is to balance the get_device after device_add.
    It will *only* decrement the use count. Then we call device_del as the
    documentation says.

    Rob

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