Subject: Re: [PATCH] hv_netvsc: Make sure out channel is fully opened on send

From: Mohammed Gamal <[email-masked]>
Date: Wed, 14 Mar 2018 10:22:01 +0100

On Tue, 2018-03-13 at 12:35 -0700, Stephen Hemminger wrote:
> On Tue, 13 Mar 2018 20:06:50 +0100
> Mohammed Gamal <[email-masked]> wrote:
> 
> > Dring high network traffic changes to network interface parameters
> > such as number of channels or MTU can cause a kernel panic with a
> > NULL
> > pointer dereference. This is due to netvsc_device_remove() being
> > called and deallocating the channel ring buffers, which can then be
> > accessed by netvsc_send_pkt() before they're allocated on calling
> > netvsc_device_add()
> > 
> > The patch fixes this problem by checking the channel state and
> > returning
> > ENODEV if not yet opened. We also move the call to
> > hv_ringbuf_avail_percent()
> > which may access the uninitialized ring buffer.
> > 
> > Signed-off-by: Mohammed Gamal <[email-masked]>
> > ---
> >  drivers/net/hyperv/netvsc.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/net/hyperv/netvsc.c
> > b/drivers/net/hyperv/netvsc.c
> > index 0265d70..44a8358 100644
> > --- a/drivers/net/hyperv/netvsc.c
> > +++ b/drivers/net/hyperv/netvsc.c
> > @@ -757,7 +757,7 @@ static inline int netvsc_send_pkt(
> >  	struct netdev_queue *txq = netdev_get_tx_queue(ndev,
> > packet->q_idx);
> >  	u64 req_id;
> >  	int ret;
> > -	u32 ring_avail = hv_ringbuf_avail_percent(&out_channel-
> > >outbound);
> > +	u32 ring_avail;
> >  
> >  	nvmsg.hdr.msg_type = NVSP_MSG1_TYPE_SEND_RNDIS_PKT;
> >  	if (skb)
> > @@ -773,7 +773,7 @@ static inline int netvsc_send_pkt(
> >  
> >  	req_id = (ulong)skb;
> >  
> > -	if (out_channel->rescind)
> > +	if (out_channel->rescind || out_channel->state !=
> > CHANNEL_OPENED_STATE)
> >  		return -ENODEV;
> >  
> >  	if (packet->page_buf_cnt) {
> > @@ -791,6 +791,7 @@ static inline int netvsc_send_pkt(
> >  				       VMBUS_DATA_PACKET_FLAG_COMP
> > LETION_REQUESTED);
> >  	}
> >  
> > +	ring_avail = hv_ringbuf_avail_percent(&out_channel-
> > >outbound);
> >  	if (ret == 0) {
> >  		atomic_inc_return(&nvchan->queue_sends);
> >  
> 
> Thanks for your patch. Yes there are races with the current update
> logic. The root cause goes higher up in the flow; the send queues
> should
> be stopped before netvsc_device_remove is called. Solving it where
> you tried
> to is racy and not going to work reliably.
> 
> Network patches should go to [email-masked]
> 
> You can't move the ring_avail check until after the vmbus_sendpacket
> because
> that will break the flow control logic.
> 
Why? I don't see ring_avail being used before that point.

> Instead, you should just move the avail_read check until just after
> the existing rescind
> check.
> 
> Also, you shouldn't need to check for OPENED_STATE, just rescind is
> enough.

That rarely mitigated the race. channel->rescind flag is set on vmbus
exit - called on module unload - and when a rescind offer is received
from the host, which AFAICT doesn't happen on every call to
netvsc_device_remove, so it's quite possible that the ringbuffer is
accessed before it's allocated again on channel open and hence the
check for OPENED_STAT - which is only set after all vmbus data is
initialized.

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