Re: [PATCH net-next] hv_netvsc: fix a race between netvsc_send() and netvsc_init_buf()

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Stephen Hemminger <sthemmin@microsoft.com> writes:

> Do we need ACCESS_ONCE() here to avoid check/use issues?
>

I think we don't: this is the only place in the function where we read
the variable so we'll get normal read. We're not trying to syncronize
with netvsc_init_buf() as that would require locking, if we read stale
NULL value after it was already updated on a different CPU we're fine,
we'll just return -EAGAIN.

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@redhat.com] 
> Sent: Wednesday, October 19, 2016 2:53 PM
> To: netdev@vger.kernel.org
> Cc: Stephen Hemminger <sthemmin@microsoft.com>; devel@linuxdriverproject.org; linux-kernel@vger.kernel.org; KY Srinivasan <kys@microsoft.com>; Haiyang Zhang <haiyangz@microsoft.com>
> Subject: [PATCH net-next] hv_netvsc: fix a race between netvsc_send() and netvsc_init_buf()
>
> Fix in commit 880988348270 ("hv_netvsc: set nvdev link after populating
> chn_table") turns out to be incomplete. A crash in
> netvsc_get_next_send_section() is observed on mtu change when the device is under load. The race I identified is: if we get to netvsc_send() after we set net_device_ctx->nvdev link in netvsc_device_add() but before we finish netvsc_connect_vsp()->netvsc_init_buf() send_section_map is not allocated and we crash. Unfortunately we can't set net_device_ctx->nvdev link after the netvsc_init_buf() call as during the negotiation we need to receive packets and on the receive path we check for it. It would probably be possible to split nvdev into a pair of nvdev_in and nvdev_out links and check them accordingly in get_outbound_net_device()/
> get_inbound_net_device() but this looks like an overkill.
>
> Check that send_section_map is allocated in netvsc_send().
>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  drivers/net/hyperv/netvsc.c | 7 +++++++
>  1 file changed, 7 insertions(+)
>
> diff --git a/drivers/net/hyperv/netvsc.c b/drivers/net/hyperv/netvsc.c index 720b5fa..e2bfaac 100644
> --- a/drivers/net/hyperv/netvsc.c
> +++ b/drivers/net/hyperv/netvsc.c
> @@ -888,6 +888,13 @@ int netvsc_send(struct hv_device *device,
>  	if (!net_device)
>  		return -ENODEV;
>
> +	/* We may race with netvsc_connect_vsp()/netvsc_init_buf() and get
> +	 * here before the negotiation with the host is finished and
> +	 * send_section_map may not be allocated yet.
> +	 */
> +	if (!net_device->send_section_map)
> +		return -EAGAIN;
> +
>  	out_channel = net_device->chn_table[q_idx];
>
>  	packet->send_buf_index = NETVSC_INVALID_INDEX;
> --
> 2.7.4

-- 
  Vitaly