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@alliedtelesis.co.nz>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linux-ppp@vger.kernel.org
Cc: Guillaume Nault <g.nault@alphalink.fr>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 2 deletions

diff --git a/drivers/net/ppp/ppp_generic.c b/drivers/net/ppp/ppp_generic.c
index 8dedafa1a95d..a30ee427efab 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) {