Subject: Re: [PATCH] virtio_net: fix use after free

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 15 Oct 2014 16:37:10 +0300

On Wed, Oct 15, 2014 at 01:24:57PM +0000, David Laight wrote:
> From: Michael S. Tsirkin
> > 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 <[email-masked]>
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > ---
> > 
> > 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);
> 
> Do you need to cache 'txq' on stack for the entire call?
> Looks like it is only needed when 'kick' is true.
> I've not looked to see if saves both 'dev' and 'qnum' being kept.
> 
> In any case it isn't mentioned in the commit message.
> 
> 	David

Code seems slightly neater this way, I haven't bothered to
micro-optimize it to this level yet.
Want to benchmark and send a patch on top?

> > +	bool kick = !skb->xmit_more;
> > 
> >  	/* Free up any pending old buffers before queueing new ones. */
> >  	free_old_xmit_skbs(sq);
> > @@ -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))
> >  		virtqueue_kick(sq->vq);
> > 
> >  	return NETDEV_TX_OK;
> > --
> > MST
> > --
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