[PATCH NET] vrf: fix null pointer dereference in vrf_finish_output()

After 14ee70ca89e6 ("vrf: use skb_expand_head in vrf_finish_output")
skb->dev  is accessed after skb free.
Let's replace skb->dev by dev = skb_dst(skb)->dev:
vrf_finish_output() is only called from vrf_output(),
it set skb->dev to skb_dst(skb)->dev and calls POSTROUTING netfilter
hooks, where output device should not be changed.

Fixes: 14ee70ca89e6 ("vrf: use skb_expand_head in vrf_finish_output")
Reported-by: Julian Wiedmann <redacted>
Signed-off-by: Vasily Averin <redacted>
---
 drivers/net/vrf.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/net/vrf.c b/drivers/net/vrf.c
index 726adf0..168d4ef 100644
--- a/drivers/net/vrf.c
+++ b/drivers/net/vrf.c

@@ -864,7 +864,7 @@ static int vrf_finish_output(struct net *net, struct sock *sk, struct sk_buff *s
 	if (unlikely(skb_headroom(skb) < hh_len && dev->header_ops)) {
 		skb = skb_expand_head(skb, hh_len);
 		if (!skb) {
-			skb->dev->stats.tx_errors++;
+			dev->stats.tx_errors++;
 			return -ENOMEM;
 		}
 	}

@@ -883,7 +883,7 @@ static int vrf_finish_output(struct net *net, struct sock *sk, struct sk_buff *s
 	}
 
 	rcu_read_unlock_bh();
-	vrf_tx_error(skb->dev, skb);
+	vrf_tx_error(dev, skb);
 	return -EINVAL;
 }
 

-- 
1.8.3.1