hv_netvsc: Reset the RSC count if NVSP_STAT_FAIL in netvsc_receive()

Commit 4414418 ("hv_netvsc: Add validation for untrusted Hyper-V
values") added validation to rndis_filter_receive_data() (and
rndis_filter_receive()) which introduced NVSP_STAT_FAIL-scenarios where
the count is not updated/reset.  Fix this omission, and prevent similar
scenarios from occurring in the future.

Reported-by: Juan Vazquez <juvazq@microsoft.com>
Signed-off-by: Andrea Parri (Microsoft) <parri.andrea@gmail.com>
Fixes: 4414418 ("hv_netvsc: Add validation for untrusted Hyper-V values")
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Link: https://lore.kernel.org/r/20210203113602.558916-1-parri.andrea@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/hyperv/netvsc.c

@@ -1262,8 +1262,11 @@ static int netvsc_receive(struct net_device *ndev,
 		ret = rndis_filter_receive(ndev, net_device,
 					   nvchan, data, buflen);
 
-		if (unlikely(ret != NVSP_STAT_SUCCESS))
+		if (unlikely(ret != NVSP_STAT_SUCCESS)) {
+			/* Drop incomplete packet */
+			nvchan->rsc.cnt = 0;
 			status = NVSP_STAT_FAIL;
+		}
 	}
 
 	enq_receive_complete(ndev, net_device, q_idx,

drivers/net/hyperv/rndis_filter.c

@@ -509,8 +509,6 @@ static int rndis_filter_receive_data(struct net_device *ndev,
 	return ret;
 
 drop:
-	/* Drop incomplete packet */
-	nvchan->rsc.cnt = 0;
 	return NVSP_STAT_FAIL;
 }