[PATCH] netvsc: fix use-after-free in netvsc_change_mtu()

[Dexuan Cui]

'nvdev' is freed in rndis_filter_device_remove -> netvsc_device_remove ->
free_netvsc_device, so we mustn't access it, before it's re-created in
rndis_filter_device_add -> netvsc_device_add.

Signed-off-by: Dexuan Cui <de...@microsoft.com>
Cc: "K. Y. Srinivasan" <k...@microsoft.com>
Cc: Haiyang Zhang <haiya...@microsoft.com>
Cc: Stephen Hemminger <sthem...@microsoft.com>
---
 drivers/net/hyperv/netvsc_drv.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/drivers/net/hyperv/netvsc_drv.c b/drivers/net/hyperv/netvsc_drv.c
index 2d3cdb0..bc05c89 100644
--- a/drivers/net/hyperv/netvsc_drv.c
+++ b/drivers/net/hyperv/netvsc_drv.c
@@ -859,15 +859,22 @@ static int netvsc_change_mtu(struct net_device *ndev, int 
mtu)
        if (ret)
                goto out;
 
+       memset(&device_info, 0, sizeof(device_info));
+       device_info.ring_size = ring_size;
+       device_info.num_chn = nvdev->num_chn;
+       device_info.max_num_vrss_chns = nvdev->num_chn;
+
        ndevctx->start_remove = true;
        rndis_filter_device_remove(hdev, nvdev);
 
+       /* 'nvdev' has been freed in rndis_filter_device_remove() ->
+        * netvsc_device_remove () -> free_netvsc_device().
+        * We mustn't access it before it's re-created in
+        * rndis_filter_device_add() -> netvsc_device_add().
+        */
+
        ndev->mtu = mtu;
 
-       memset(&device_info, 0, sizeof(device_info));
-       device_info.ring_size = ring_size;
-       device_info.num_chn = nvdev->num_chn;
-       device_info.max_num_vrss_chns = nvdev->num_chn;
        rndis_filter_device_add(hdev, &device_info);
 
 out:
-- 
2.7.4