netdev - [Patch net] ppp: defer netns reference release for ppp channel

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Date:	Tue,  5 Jul 2016 22:12:36 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	netdev@...r.kernel.org
Cc:	Matt.Bennett@...iedtelesis.co.nz,
	Cong Wang <xiyou.wangcong@...il.com>,
	Paul Mackerras <paulus@...ba.org>, linux-ppp@...r.kernel.org,
	Guillaume Nault <g.nault@...halink.fr>,
	Cyrill Gorcunov <gorcunov@...nvz.org>
Subject: [Patch net] ppp: defer netns reference release for ppp channel

Matt reported that we have a NULL pointer dereference
in ppp_pernet() from ppp_connect_channel(),
i.e. pch->chan_net is NULL.

This is due to that a parallel ppp_unregister_channel()
could happen while we are in ppp_connect_channel(), during
which pch->chan_net set to NULL. Since we need a reference
to net per channel, it makes sense to sync the refcnt
with the life time of the channel, therefore we should
release this reference when we destroy it.

Fixes: 1f461dcdd296 ("ppp: take reference on channels netns")
Reported-by: Matt Bennett <Matt.Bennett@...iedtelesis.co.nz>
Cc: Paul Mackerras <paulus@...ba.org>
Cc: linux-ppp@...r.kernel.org
Cc: Guillaume Nault <g.nault@...halink.fr>
Cc: Cyrill Gorcunov <gorcunov@...nvz.org>
Signed-off-by: Cong Wang <xiyou.wangcong@...il.com>
---
 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 8dedafa..a30ee42 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -2601,8 +2601,6 @@ ppp_unregister_channel(struct ppp_channel *chan)
 	spin_lock_bh(&pn->all_channels_lock);
 	list_del(&pch->list);
 	spin_unlock_bh(&pn->all_channels_lock);
-	put_net(pch->chan_net);
-	pch->chan_net = NULL;
 
 	pch->file.dead = 1;
 	wake_up_interruptible(&pch->file.rwait);
@@ -3136,6 +3134,9 @@ ppp_disconnect_channel(struct channel *pch)
  */
 static void ppp_destroy_channel(struct channel *pch)
 {
+	put_net(pch->chan_net);
+	pch->chan_net = NULL;
+
 	atomic_dec(&channel_count);
 
 	if (!pch->file.dead) {
-- 
1.8.4.5