Commit fa35864e authored Jan 22, 2014 by Dominic Curran Committed by David S. Miller Jan 22, 2014

tuntap: Fix for a race in accessing numqueues



A patch for fixing a race between queue selection and changing queues
was introduced in commit 92bb73ea("tuntap: fix a possible race between
queue selection and changing queues").

The fix was to prevent the driver from re-reading the tun->numqueues
more than once within tun_select_queue() using ACCESS_ONCE().

We have been experiancing 'Divide-by-zero' errors in tun_net_xmit()
since we moved from 3.6 to 3.10, and believe that they come from a
simular source where the value of tun->numqueues changes to zero
between the first and a subsequent read of tun->numqueues.

The fix is a simular use of ACCESS_ONCE(), as well as a multiply
instead of a divide in the if statement.

Signed-off-by: Dominic Curran <dominic.curran@citrix.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Maxim Krasnyansky <maxk@qti.qualcomm.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Acked-by: Max Krasnyansky <maxk@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent bdf4351b

drivers/net/tun.c

+6 −4

Original line number	Diff line number	Diff line
		@@ -738,15 +738,17 @@ static netdev_tx_t tun_net_xmit(struct sk_buff skb, struct net_device dev)
		struct tun_struct *tun = netdev_priv(dev);
		int txq = skb->queue_mapping;
		struct tun_file *tfile;
		u32 numqueues = 0;

		rcu_read_lock();
		tfile = rcu_dereference(tun->tfiles[txq]);
		numqueues = ACCESS_ONCE(tun->numqueues);

		/* Drop packet if interface is not attached */
		if (txq >= tun->numqueues)
		if (txq >= numqueues)
		goto drop;

		if (tun->numqueues == 1) {
		if (numqueues == 1) {
		/* Select queue was not called for the skbuff, so we extract the
		* RPS hash and save it into the flow_table here.
		*/
		@@ -779,8 +781,8 @@ static netdev_tx_t tun_net_xmit(struct sk_buff skb, struct net_device dev)
		/* Limit the number of packets queued by dividing txq length with the
		* number of queues.
		*/
		if (skb_queue_len(&tfile->socket.sk->sk_receive_queue)
		>= dev->tx_queue_len / tun->numqueues)
		if (skb_queue_len(&tfile->socket.sk->sk_receive_queue) * numqueues
		>= dev->tx_queue_len)
		goto drop;

		if (unlikely(skb_orphan_frags(skb, GFP_ATOMIC)))