ppp: unlock all_ppp_mutex before registering device

ppp_dev_uninit(), which is the .ndo_uninit() handler of PPP devices,
needs to lock pn->all_ppp_mutex. Therefore we mustn't call
register_netdevice() with pn->all_ppp_mutex already locked, or we'd
deadlock in case register_netdevice() fails and calls .ndo_uninit().

Fortunately, we can unlock pn->all_ppp_mutex before calling
register_netdevice(). This lock protects pn->units_idr, which isn't
used in the device registration process.

However, keeping pn->all_ppp_mutex locked during device registration
did ensure that no device in transient state would be published in
pn->units_idr. In practice, unlocking it before calling
register_netdevice() doesn't change this property: ppp_unit_register()
is called with 'ppp_mutex' locked and all searches done in
pn->units_idr hold this lock too.

Fixes: 8cb775bc0a34 ("ppp: fix device unregistration upon netns deletion")
Reported-and-tested-by: syzbot+367889b9c9e279219175@syzkaller.appspotmail.com
Signed-off-by: Guillaume Nault <g.nault@alphalink.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ppp/ppp_generic.c

@@ -1006,17 +1006,18 @@ static int ppp_unit_register(struct ppp *ppp, int unit, bool ifname_is_set)
 	if (!ifname_is_set)
 		snprintf(ppp->dev->name, IFNAMSIZ, "ppp%i", ppp->file.index);
 
+	mutex_unlock(&pn->all_ppp_mutex);
+
 	ret = register_netdevice(ppp->dev);
 	if (ret < 0)
 		goto err_unit;
 
 	atomic_inc(&ppp_unit_count);
 
-	mutex_unlock(&pn->all_ppp_mutex);
-
 	return 0;
 
 err_unit:
+	mutex_lock(&pn->all_ppp_mutex);
 	unit_put(&pn->units_idr, ppp->file.index);
 err:
 	mutex_unlock(&pn->all_ppp_mutex);