linux-kernel - Re: [PATCH] PCI: fix use-after-free in pci_register_host

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Date:   Fri, 11 Dec 2020 09:46:52 -0600
From:   Rob Herring <robh@...nel.org>
To:     Qinglang Miao <miaoqinglang@...wei.com>
Cc:     Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [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@...wei.com>
> Signed-off-by: Qinglang Miao <miaoqinglang@...wei.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