Re: NULL pointer dereference in veth_stats_one

Ben Hutchings
2013-01-04T20:25:17Z

On Fri, 2013-01-04 at 11:23 -0800, Eric Dumazet wrote:
> On Fri, 2013-01-04 at 18:17 +0000, Ben Hutchings wrote:
>
> > This possibly needs some memory barriers to properly synchronise with
> > veth_newlink(). But can you not move initialisation of the peer
> > pointers before registration of the devices in veth_newlink(), so that
> > veth_get_stats64() cannot be called before they are initialised?
>
> The ->peer pointer cannot change once set. ( its never cleared )

We may still need an explicit barrier for data-dependency.

> So the problem would not be in veth_newlink(), but might be in
> veth_dellink()

A lot of things are done in between the unregister_netdevice_queue() and
the actual deletion which are probably sufficient to flush out any calls
to dev_get_stats(). But to make sure, I think we would need some small
amount of shared state that isn't freed until both devices are.

> It seems we would have a problem in veth_get_ethtool_stats() already...

That should be OK because both ethtool operations and the whole process
of interface deletion are serialised by the RTNL lock.

> More generally, what prevents a get_stats() being called while a
> dellink() (-> veth_dev_free() -> free_percpu()) is done ?

Anything calling dev_get_stats() must have a counted or RCU reference to
the device, and netdev_run_todo() waits for those to go away. For
mutually referencing devices we want a kind of weak reference and we
have no good way to implement those.

Ben.

> (Same thing is done for tunnel/dummy stats percpu data)

--
Ben Hutchings, Staff Engineer, Solarflare
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They asked us to note that Solarflare product names are trademarked.