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
.
