[PATCH net] ppp: unlock all_ppp_mutex before registering device

[PATCH net] ppp: unlock all_ppp_mutex before registering device

[Guillaume Nault]

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

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index d8e5747ff4e3..264d4af0bf69 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/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);
-- 
2.15.1

Re: [PATCH net] ppp: unlock all_ppp_mutex before registering device

[David Miller]

From: Guillaume Nault <g.nault@alphalink.fr>
Date: Wed, 10 Jan 2018 16:24:45 +0100

> 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>

Applied and queued up for -stable, thank you.