net: dsa: properly keep track of conduit reference

In the Linux kernel, the following vulnerability has been resolved:

net: dsa: properly keep track of conduit reference

Problem description

-------------------

DSA has a mumbo-jumbo of reference handling of the conduit net device

and its kobject which, sadly, is just wrong and doesn't make sense.

There are two distinct problems.

1. The OF path, which uses of_find_net_device_by_node(), never releases

the elevated refcount on the conduit's kobject. Nominally, the OF and

non-OF paths should result in objects having identical reference

counts taken, and it is already suspicious that

dsa_dev_to_net_device() has a put_device() call which is missing in

dsa_port_parse_of(), but we can actually even verify that an issue

exists. With CONFIG_DEBUG_KOBJECT_RELEASE=y, if we run this command

"before" and "after" applying this patch:

(unbind the conduit driver for net device eno2)

echo 0000:00:00.2 > /sys/bus/pci/drivers/fsl_enetc/unbind

we see these lines in the output diff which appear only with the patch

applied:

kobject: 'eno2' (ffff002009a3a6b8): kobject_release, parent 0000000000000000 (delayed 1000)

kobject: '109' (ffff0020099d59a0): kobject_release, parent 0000000000000000 (delayed 1000)

2. After we find the conduit interface one way (OF) or another (non-OF),

it can get unregistered at any time, and DSA remains with a long-lived,

but in this case stale, cpu_dp->conduit pointer. Holding the net

device's underlying kobject isn't actually of much help, it just

prevents it from being freed (but we never need that kobject

directly). What helps us to prevent the net device from being

unregistered is the parallel netdev reference mechanism (dev_hold()

and dev_put()).

Actually we actually use that netdev tracker mechanism implicitly on

user ports since commit 2f1e8ea726e9 ("net: dsa: link interfaces with

the DSA master to get rid of lockdep warnings"), via netdev_upper_dev_link().

But time still passes at DSA switch probe time between the initial

of_find_net_device_by_node() code and the user port creation time, time

during which the conduit could unregister itself and DSA wouldn't know

about it.

So we have to run of_find_net_device_by_node() under rtnl_lock() to

prevent that from happening, and release the lock only with the netdev

tracker having acquired the reference.

Do we need to keep the reference until dsa_unregister_switch() /

dsa_switch_shutdown()?

1: Maybe yes. A switch device will still be registered even if all user

ports failed to probe, see commit 86f8b1c01a0a ("net: dsa: Do not

make user port errors fatal"), and the cpu_dp->conduit pointers

remain valid. I haven't audited all call paths to see whether they

will actually use the conduit in lack of any user port, but if they

do, it seems safer to not rely on user ports for that reference.

2. Definitely yes. We support changing the conduit which a user port is

associated to, and we can get into a situation where we've moved all

user ports away from a conduit, thus no longer hold any reference to

it via the net device tracker. But we shouldn't let it go nonetheless

- see the next change in relation to dsa_tree_find_first_conduit()

and LAG conduits which disappear.

We have to be prepared to return to the physical conduit, so the CPU

port must explicitly keep another reference to it. This is also to

say: the user ports and their CPU ports may not always keep a

reference to the same conduit net device, and both are needed.

As for the conduit's kobject for the /sys/class/net/ entry, we don't

care about it, we can release it as soon as we hold the net device

object itself.

History and blame attribution

-----------------------------

The code has been refactored so many times, it is very difficult to

follow and properly attribute a blame, but I'll try to make a short

history which I hope to be correct.

We have two distinct probing paths:

- one for OF, introduced in 2016 i

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