linux-kernel - [PATCH] connector: bump skb->users before callback invocation

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Date:	Thu, 31 Dec 2015 14:26:33 +0100
From:	Florian Westphal <fw@...len.de>
To:	Dmitry Vyukov <dvyukov@...gle.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Thomas Graf <tgraf@...g.ch>,
	Daniel Borkmann <daniel@...earbox.net>,
	Ken-ichirou MATSUZAWA <chamaken@...il.com>,
	Nicolas Dichtel <nicolas.dichtel@...nd.com>,
	Florian Westphal <fw@...len.de>,
	netdev <netdev@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	syzkaller <syzkaller@...glegroups.com>,
	Kostya Serebryany <kcc@...gle.com>,
	Alexander Potapenko <glider@...gle.com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Eric Dumazet <edumazet@...gle.com>, zbr@...emap.net
Subject: [PATCH] connector: bump skb->users before callback invocation

Dmitry reports memleak with syskaller program.
Problem is that connector bumps skb usecount but might not invoke callback.

So move skb_get to where we invoke the callback.

Reported-by: Dmitry Vyukov <dvyukov@...gle.com>
Signed-off-by: Florian Westphal <fw@...len.de>
---
 I wonder wth userspace can cram skb->len < NLMSG_HDRLEN
 down the kernel, it seems to beg for trouble...

diff --git a/drivers/connector/connector.c b/drivers/connector/connector.c
index d7373ca..25693b0 100644
--- a/drivers/connector/connector.c
+++ b/drivers/connector/connector.c
@@ -179,26 +179,21 @@ static int cn_call_callback(struct sk_buff *skb)
  *
  * It checks skb, netlink header and msg sizes, and calls callback helper.
  */
-static void cn_rx_skb(struct sk_buff *__skb)
+static void cn_rx_skb(struct sk_buff *skb)
 {
 	struct nlmsghdr *nlh;
-	struct sk_buff *skb;
 	int len, err;
 
-	skb = skb_get(__skb);
-
 	if (skb->len >= NLMSG_HDRLEN) {
 		nlh = nlmsg_hdr(skb);
 		len = nlmsg_len(nlh);
 
 		if (len < (int)sizeof(struct cn_msg) ||
 		    skb->len < nlh->nlmsg_len ||
-		    len > CONNECTOR_MAX_MSG_SIZE) {
-			kfree_skb(skb);
+		    len > CONNECTOR_MAX_MSG_SIZE)
 			return;
-		}
 
-		err = cn_call_callback(skb);
+		err = cn_call_callback(skb_get(skb));
 		if (err < 0)
 			kfree_skb(skb);
 	}
--
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