[PATCH] virtio_net: fix use after free

From: Michael S. Tsirkin
Date: Wed Oct 15 2014 - 09:20:04 EST

commit 0b725a2ca61bedc33a2a63d0451d528b268cf975
net: Remove ndo_xmit_flush netdev operation, use signalling instead.

added code that looks at skb->xmit_more after the skb has
been put in TX VQ. Since some paths process the ring and free the skb
immediately, this can cause use after free.

Fix by storing xmit_more in a local variable.

Cc: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

David, am I using the API correctly?
Seems to work for me.
You used __netif_subqueue_stopped but that seems to use
a slightly more expensive test_bit internally.
The reason I added a variable for the txq here is because it's handy for
BQL patch later on.

drivers/net/virtio_net.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index 3d0ce44..13d0a8b 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -920,6 +920,8 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)
int qnum = skb_get_queue_mapping(skb);
struct send_queue *sq = &vi->sq[qnum];
int err;
+ struct netdev_queue *txq = netdev_get_tx_queue(dev, qnum);
+ bool kick = !skb->xmit_more;

/* Free up any pending old buffers before queueing new ones. */
@@ -956,7 +958,7 @@ static netdev_tx_t start_xmit(struct sk_buff *skb, struct net_device *dev)

- if (__netif_subqueue_stopped(dev, qnum) || !skb->xmit_more)
+ if (kick || netif_xmit_stopped(txq))

return NETDEV_TX_OK;
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