Re: [RFC PATCH net-next 1/3] net: phy: don't bind genphy in phy_attach_direct if the specific driver defers probe

[Vladimir Oltean]

On Thu, Sep 02, 2021 at 11:24:39PM +0100, Russell King (Oracle) wrote:
> On Fri, Sep 03, 2021 at 12:39:49AM +0300, Vladimir Oltean wrote:
> > On Thu, Sep 02, 2021 at 10:33:03PM +0100, Russell King (Oracle) wrote:
> > > That's probably an unreliable indicator. DPAA2 has weirdness in the
> > > way it can dynamically create and destroy network interfaces, which
> > > does lead to problems with the rtnl lock. I've been carrying a patch
> > > from NXP for this for almost two years now, which NXP still haven't
> > > submitted:
> > >
> > > http://git.armlinux.org.uk/cgit/linux-arm.git/commit/?h=cex7&id=a600f2ee50223e9bcdcf86b65b4c427c0fd425a4
> > >
> > > ... and I've no idea why that patch never made mainline. I need it
> > > to avoid the stated deadlock on SolidRun Honeycomb platforms when
> > > creating additional network interfaces for the SFP cages in userspace.
> >
> > Ah, nice, I've copied that broken logic for the dpaa2-switch too:
> > https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/commit/?id=d52ef12f7d6c016f3b249db95af33f725e3dd065
> >
> > So why don't you send the patch? I can send it too if you want to, one
> > for the switch and one for the DPNI driver.
>
> Sorry, I mis-stated. NXP did submit that exact patch, but it's actually
> incorrect for the reason I stated when it was sent:
>
> https://patchwork.ozlabs.org/project/netdev/patch/1574363727-5437-2-git-send-email-ioana.ciornei@xxxxxxx/

So why are you carrying it then?

> I did miss the rtnl_lock() around phylink_disconnect_phy() in the
> description of the race, which goes someway towards hiding it, but
> there is still a race between phylink_destroy() and another thread
> calling dpaa2_eth_get_link_ksettings(), and priv->mac being freed:
>
> static int
> dpaa2_eth_get_link_ksettings(struct net_device *net_dev,
>                              struct ethtool_link_ksettings *link_settings)
> {
>         struct dpaa2_eth_priv *priv = netdev_priv(net_dev);
>
>         if (dpaa2_eth_is_type_phy(priv))
>                 return phylink_ethtool_ksettings_get(priv->mac->phylink,
>                                                      link_settings);
>
> which dereferences priv->mac and priv->mac->phylink, vs:
>
> static irqreturn_t dpni_irq0_handler_thread(int irq_num, void *arg)
> {
> ...
>         if (status & DPNI_IRQ_EVENT_ENDPOINT_CHANGED) {
>                 dpaa2_eth_set_mac_addr(netdev_priv(net_dev));
>                 dpaa2_eth_update_tx_fqids(priv);
>
>                 if (dpaa2_eth_has_mac(priv))
>                         dpaa2_eth_disconnect_mac(priv);
>                 else
>                         dpaa2_eth_connect_mac(priv);
>         }
>
> static void dpaa2_eth_disconnect_mac(struct dpaa2_eth_priv *priv)
> {
>         if (dpaa2_eth_is_type_phy(priv))
>                 dpaa2_mac_disconnect(priv->mac);
>
>         if (!dpaa2_eth_has_mac(priv))
>                 return;
>
>         dpaa2_mac_close(priv->mac);
>         kfree(priv->mac);		<== potential use after free bug by
>         priv->mac = NULL;		<== dpaa2_eth_get_link_ksettings()
> }

Okay, so this needs to stay under the rtnetlink mutex, to serialize with
dpaa2_eth_get_link_ksettings which is already under the rtnetlink mutex.
So the way in which rtnl_lock is taken right now is actually fine in a way.

>
> void dpaa2_mac_disconnect(struct dpaa2_mac *mac)
> {
>         if (!mac->phylink)
>                 return;
>
>         phylink_disconnect_phy(mac->phylink);
>         phylink_destroy(mac->phylink);	<== another use-after-free bug via
> 					    dpaa2_eth_get_link_ksettings()
>         dpaa2_pcs_destroy(mac);
> }
>
> Note that phylink_destroy() is documented as:
>
>  * Note: the rtnl lock must not be held when calling this function.
>
> because it calls sfp_bus_del_upstream(), which will take the rtnl lock
> itself. An alternative solution would be to remove the rtnl locking
> from sfp_bus_del_upstream(), but then force _everyone_ to take the
> rtnl lock before calling phylink_destroy() - meaning a larger block of
> code ends up executing under the lock than is really necessary.

So phylink_destroy has exactly 20 call sites, it is not that bad?

And as for "larger block than necessary" - doesn't the dpaa2 prolonged
usage count as necessary?

> However, as I stated in my review of the patch "As I've already stated,
> the phylink is not designed to be created and destroyed on a published
> network device." That still remains true today, and it seems that the
> issue has never been fixed in DPAA2 despite having been pointed out.

So what would you do, exactly, to "fix" the issue that a DPNI can
connect and disconnect at runtime from a DPMAC?

Also, "X is not designed to Y" doesn't really say much, given a bit of
will power. Linux was not designed to run on non-i386 either.

Any other issues besides needing to take rtnl_mutex top-level when
calling phylink_destroy? Since phylink_disconnect_phy needs it anyway,
and phylink_destroy ends up calling sfp_bus_del_upstream which takes the
same mutex again, and drivers that connect/disconnect at probe/remove
time end up calling both in a row, I don't think there is much of an
issue to speak of, or that the rework would be that difficult.