Messages in this thread | | | Subject | Re: [PATCH] PCI: fix use-after-free in pci_register_host_bridge | From | Qinglang Miao <> | Date | Mon, 14 Dec 2020 15:24:41 +0800 |
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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@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 > . 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 .
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