Re: [PATCH] PCI: fix use-after-free in pci_register_host_bridge

[Qinglang Miao]

在 2020/12/11 23:46, Rob Herring 写道:
> 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@xxxxxxxxxx>
> > Signed-off-by: Qinglang Miao <miaoqinglang@xxxxxxxxxx>
> > ---
> >   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
> .
Hi, Rob

Your words make sence to me: the code is *logicly* correct here and 
won't raise a use-after-free bug. I do hold a misunderstanding of this 
one, sorry for that ~

But I still think this patch should be reconsidered:

The kdoc of device_unregister explicitly mentions the possibility that 
other refs might continue to exist after device_unregister was called, 
and *del_device* is first part of it.

By the way, 'del_device() called before put_device()' is everywhere in 
kernel code, like device_unregister(), pci_destroy_dev() or 
switchtec_pci_remove()

In fact, I can't find another place in kernel code looks like:
	put_device(x);
 	device_del(x);

So I guess put_device() ought to be the last time we touch the object 
(I don't find evidence strong enough in kdoc to prove this) and putting 
put_device after device_del is a more natural logic.

Qinglang
.

>