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SubjectRe: [PATCH] scsi: zfcp: fix use-after-free in zfcp_unit_remove
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On 11/26/20 4:12 PM, Benjamin Block wrote:
> On Thu, Nov 26, 2020 at 08:07:32PM +0800, Qinglang Miao wrote:
>> 在 2020/11/26 17:42, Benjamin Block 写道:
>>> On Thu, Nov 26, 2020 at 09:13:53AM +0100, Cornelia Huck wrote:
>>>> On Thu, 26 Nov 2020 09:27:41 +0800
>>>> Qinglang Miao <miaoqinglang@huawei.com> wrote:
>>>>> 在 2020/11/26 1:06, Benjamin Block 写道:
>>>>>> On Fri, Nov 20, 2020 at 03:48:54PM +0800, Qinglang Miao wrote:
> ....
>>> Let's go by example. If we assume the reference count of `unit->dev` is
>>> R, and the function starts with R = 1 (otherwise the deivce would've
>>> been freed already), we get:
>>>
>>> int zfcp_unit_remove(struct zfcp_port *port, u64 fcp_lun)
>>> {
>>> struct zfcp_unit *unit;
>>> struct scsi_device *sdev;
>>> write_lock_irq(&port->unit_list_lock);
>>> // unit->dev (R = 1)
>>> unit = _zfcp_unit_find(port, fcp_lun);
>>> // get_device(&unit->dev)
>>> // unit->dev (R = 2)
>>> if (unit)
>>> list_del(&unit->list);
>>> write_unlock_irq(&port->unit_list_lock);
>>> if (!unit)
>>> return -EINVAL;
>>> sdev = zfcp_unit_sdev(unit);
>>> if (sdev) {
>>> scsi_remove_device(sdev);
>>> scsi_device_put(sdev);
>>> }
>>> // unit->dev (R = 2)
>>> put_device(&unit->dev);
>>> // unit->dev (R = 1)
>>> device_unregister(&unit->dev);
>>> // unit->dev (R = 0)
>>> return 0;
>>> }
>>>
>>> If we now apply this patch, we'd end up with R = 1 after
>>> `device_unregister()`, and the device would not be properly removed.
>>>
>>> If you still think that's wrong, then you'll need to better explain why.
>>>
>> Hi Banjamin and Cornelia,
>>
>> Your replies make me reliaze that I've been holding a mistake understanding
>> of put_device() as well as reference count.
>>
>> Thanks for you two's patient explanation !!
>>
>> BTW, should I send a v2 on these two patches to move the position of
>> put_device()?
>
> Feel free to do so.
>
> I think having the `put_device()` call after `device_unregister()` in
> both `zfcp_unit_remove()` and `zfcp_sysfs_port_remove_store()` is more
> natural, because it ought to be the last time we touch the object in
> both functions.

If you move put_device(), you could add a comment like we did here to explain
which (hidden) get_device is undone:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/s390/scsi?id=ef4021fe5fd77ced0323cede27979d80a56211ca
("scsi: zfcp: fix to prevent port_remove with pure auto scan LUNs (only sdevs)")
So in this patch it could be:
put_device(&unit->dev); /* undo _zfcp_unit_find() */
And in the other patch it could be:
put_device(&port->dev); /* undo zfcp_get_port_by_wwpn() */
Then it would be clearer next time somebody looks at the code.

Especially for the other patch on zfcp_sysfs_port_remove_store() moving the
put_device(&port->dev) to at least *after* the call of
zfcp_erp_port_shutdown(port, 0, "syprs_1") would make the code cleaner to me.
Along the idead of passing the port to zfcp_erp_port_shutdown with the
reference we got from zfcp_get_port_by_wwpn(). That said, the current code is
of course still correct as we currently have the port ref of the earlier
device_register so passing the port to zfcp_erp_port_shutdown() is safe.

If we wanted to make the gets and puts nicely nested, then we could move the
puts to just before the device_unregister, but that's bike shedding:
device_register() --+
get_device() --+ |
put_device() --+ |
device_unregister() --+

Benjamin's suggested move location works for me, too. After all, the kdoc of
device_unregister explicitly mentions the possibility that other refs might
continue to exist after device_unregister was called:
device_register() --+
get_device() ---------|--+
device_unregister() --+ |
put_device() ------------+

--
Mit freundlichen Gruessen / Kind regards
Steffen Maier

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