gre: fix a possible skb leak

commit 68c33163 ("v4 GRE: Add TCP segmentation offload for GRE")
added a possible skb leak, because it frees only the head of segment
list, in case a skb_linearize() call fails.

This patch adds a kfree_skb_list() helper to fix the bug.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Pravin B Shelar <pshelar@nicira.com>
Cc: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -627,6 +627,7 @@ static inline struct rtable *skb_rtable(const struct sk_buff *skb)
 }
 
 extern void kfree_skb(struct sk_buff *skb);
+extern void kfree_skb_list(struct sk_buff *segs);
 extern void skb_tx_error(struct sk_buff *skb);
 extern void consume_skb(struct sk_buff *skb);
 extern void	       __kfree_skb(struct sk_buff *skb);

net/core/skbuff.c

@@ -483,15 +483,8 @@ EXPORT_SYMBOL(skb_add_rx_frag);
 
 static void skb_drop_list(struct sk_buff **listp)
 {
-	struct sk_buff *list = *listp;
-
+	kfree_skb_list(*listp);
 	*listp = NULL;
-
-	do {
-		struct sk_buff *this = list;
-		list = list->next;
-		kfree_skb(this);
-	} while (list);
 }
 
 static inline void skb_drop_fraglist(struct sk_buff *skb)
@@ -651,6 +644,17 @@ void kfree_skb(struct sk_buff *skb)
 }
 EXPORT_SYMBOL(kfree_skb);
 
+void kfree_skb_list(struct sk_buff *segs)
+{
+	while (segs) {
+		struct sk_buff *next = segs->next;
+
+		kfree_skb(segs);
+		segs = next;
+	}
+}
+EXPORT_SYMBOL(kfree_skb_list);
+
 /**
  *	skb_tx_error - report an sk_buff xmit error
  *	@skb: buffer that triggered an error

net/ipv4/gre.c

@@ -178,7 +178,7 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 
 				err = __skb_linearize(skb);
 				if (err) {
-					kfree_skb(segs);
+					kfree_skb_list(segs);
 					segs = ERR_PTR(err);
 					goto out;
 				}