linux-kernel - [PATCH] tun: orphan an skb on tx

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Date:	Tue, 13 Apr 2010 17:59:44 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	unlisted-recipients:; (no To-header on input)
Cc:	"David S. Miller" <davem@...emloft.net>,
	Herbert Xu <herbert@...dor.hengli.com.au>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Paul Moore <paul.moore@...com>,
	David Woodhouse <David.Woodhouse@...el.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jan Kiszka <jan.kiszka@...mens.com>,
	qemu-devel <qemu-devel@...gnu.org>
Subject: [PATCH] tun: orphan an skb on tx

The following situation was observed in the field:
tap1 sends packets, tap2 does not consume them, as a result
tap1 can not be closed. This happens because
tun/tap devices can hang on to skbs undefinitely.

As noted by Herbert, possible solutions include a timeout followed by a
copy/change of ownership of the skb, or always copying/changing
ownership if we're going into a hostile device.

This patch implements the second approach.

Note: one issue still remaining is that since skbs
keep reference to tun socket and tun socket has a
reference to tun device, we won't flush backlog,
instead simply waiting for all skbs to get transmitted.
At least this is not user-triggerable, and
this was not reported in practice, my assumption is
other devices besides tap complete an skb
within finite time after it has been queued.

A possible solution for the second issue
would not to have socket reference the device,
instead, implement dev->destructor for tun, and
wait for all skbs to complete there, but this
needs some thought, probably too risky for 2.6.34.

Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
Tested-by: Yan Vugenfirer <yvugenfi@...hat.com>

---

Please review the below, and consider for 2.6.34,
and stable trees.

 drivers/net/tun.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/drivers/net/tun.c b/drivers/net/tun.c
index 96c39bd..4326520 100644
--- a/drivers/net/tun.c
+++ b/drivers/net/tun.c
@@ -387,6 +387,10 @@ static netdev_tx_t tun_net_xmit(struct sk_buff *skb, struct net_device *dev)
 		}
 	}

+	/* Orphan the skb - required as we might hang on to it
+	 * for indefinite time. */
+	skb_orphan(skb);
+
 	/* Enqueue packet */
 	skb_queue_tail(&tun->socket.sk->sk_receive_queue, skb);
 	dev->trans_start = jiffies;
-- 
1.7.0.2.280.gc6f05
--
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