3.16.60-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Guillaume Nault <redacted>

commit 0171c41835591e9aa2e384b703ef9a6ae367c610 upstream.

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 <redacted>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <redacted>
---
 drivers/net/ppp/ppp_generic.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c

@@ -2788,6 +2788,8 @@ static struct ppp *ppp_create_interface(
 	ppp->file.index = unit;
 	sprintf(dev->name, "ppp%d", unit);
 
+	mutex_unlock(&pn->all_ppp_mutex);
+
 	ret = register_netdevice(dev);
 	if (ret != 0) {
 		/* register_netdevice() may have called ppp_dev_init()

@@ -2796,6 +2798,7 @@ static struct ppp *ppp_create_interface(
 		 */
 		if (dev->qdisc_tx_busylock)
 			ppp_dev_priv_destructor(dev);
+		mutex_lock(&pn->all_ppp_mutex);
 		unit_put(&pn->units_idr, unit);
 		netdev_err(ppp->dev, "PPP: couldn't register device %s (%d)\n",
 			   dev->name, ret);

@@ -2803,7 +2806,6 @@ static struct ppp *ppp_create_interface(
 	}
 
 	atomic_inc(&ppp_unit_count);
-	mutex_unlock(&pn->all_ppp_mutex);
 	rtnl_unlock();
 
 	*retp = 0;