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

From: Mohammed Gamal <[email-masked]>
Date: Thu, 27 Sep 2018 10:57:05 +0200

On Wed, 2018-09-26 at 17:13 +0000, Haiyang Zhang wrote:
> > -----Original Message-----
> > From: Mohammed Gamal <[email-masked]>
> > Sent: Wednesday, September 26, 2018 12:34 PM
> > To: Stephen Hemminger <[email-masked]>; [email-masked].
> > org
> > Cc: KY Srinivasan <[email-masked]>; Haiyang Zhang
> > <[email-masked]>; vkuznets <[email-masked]>;
> > [email-masked]; cavery <[email-masked]>; linux-
> > [email-masked]; [email-masked]; Mohammed
> > Gamal
> > <[email-masked]>
> > Subject: [PATCH] hv_netvsc: Make sure out channel is fully opened
> > on send
> > 
> > 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 | 7 ++++++-
> >  1 file changed, 6 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/net/hyperv/netvsc.c
> > b/drivers/net/hyperv/netvsc.c index
> > fe01e14..75f1b31 100644
> > --- a/drivers/net/hyperv/netvsc.c
> > +++ b/drivers/net/hyperv/netvsc.c
> > @@ -825,7 +825,12 @@ 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_get_avail_to_write_percent(&out_channel-
> > > outbound);
> > 
> > +	u32 ring_avail;
> > +
> > +	if (out_channel->state != CHANNEL_OPENED_STATE)
> > +		return -ENODEV;
> > +
> > +	ring_avail = hv_get_avail_to_write_percent(&out_channel-
> > >outbound);
> 
> When you reproducing the NULL ptr panic, does your kernel include the
> following patch?
> hv_netvsc: common detach logic
> https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/c
> ommit/?id=7b2ee50c0cd513a176a26a71f2989facdd75bfea
> 
Yes it is included. And the commit did reduce the occurrence of this
race condition, but it still nevertheless occurs albeit rarely.

> We call netif_tx_disable(ndev) and netif_device_detach(ndev) before
> doing the changes 
> on MTU or #channels. So there should be no call to start_xmit() when
> channel is not ready.
> 
> If you see the check for CHANNEL_OPENED_STATE is still necessary on
> upstream kernel (including 
> the patch " common detach logic "), we should debug further on the
> code and find out the 
> root cause.
> 
> Thanks,
> - Haiyang
> 
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