From: WANG Cong <xiyou.wangc...@gmail.com>

3.12-stable review patch.  If anyone has any objections, please let me know.

===============

commit 205e1e255c479f3fd77446415706463b282f94e4 upstream.

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.benn...@alliedtelesis.co.nz>
Cc: Paul Mackerras <pau...@samba.org>
Cc: linux-ppp@vger.kernel.org
Cc: Guillaume Nault <g.na...@alphalink.fr>
Cc: Cyrill Gorcunov <gorcu...@openvz.org>
Signed-off-by: Cong Wang <xiyou.wangc...@gmail.com>
Reviewed-by: Cyrill Gorcunov <gorcu...@openvz.org>
Signed-off-by: David S. Miller <da...@davemloft.net>
Signed-off-by: Jiri Slaby <jsl...@suse.cz>
---
 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 14a8d2958698..ab79c0f13d0a 100644
--- a/drivers/net/ppp/ppp_generic.c
+++ b/drivers/net/ppp/ppp_generic.c
@@ -2317,8 +2317,6 @@ 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);
@@ -2925,6 +2923,9 @@ 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) {
-- 
2.10.1

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