Patch "ppp: unlock all_ppp_mutex before registering device" has been added to the 4.9-stable tree

This is a note to let you know that I've just added the patch titled

ppp: unlock all_ppp_mutex before registering device

to the 4.9-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
ppp-unlock-all_ppp_mutex-before-registering-device.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Guillaume Nault <[email protected]>
Date: Wed, 10 Jan 2018 16:24:45 +0100
Subject: ppp: unlock all_ppp_mutex before registering device

From: Guillaume Nault <[email protected]>

[ Upstream commit 0171c41835591e9aa2e384b703ef9a6ae367c610 ]

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.

---
 drivers/net/ppp/ppp_generic.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

@@ -1002,17 +1002,18 @@ static int ppp_unit_register(struct ppp
 	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);

Patches currently in stable-queue which might be from [email protected] are

queue-4.9/pppoe-take-needed_headroom-of-lower-device-into-account-on-xmit.patch queue-4.9/ppp-unlock-all_ppp_mutex-before-registering-device.patch