netdev - [PATCH] ipv4: fix a potential use after free in gre

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Message-Id: <1413624364-12944-1-git-send-email-roy.qing.li@gmail.com>
Date:	Sat, 18 Oct 2014 17:26:04 +0800
From:	roy.qing.li@...il.com
To:	netdev@...r.kernel.org
Cc:	weichunc@...mgrid.com
Subject: [PATCH] ipv4: fix a potential use after free in gre_offload.c

From: Li RongQing <roy.qing.li@...il.com>

pskb_may_pull() may change skb->data and make greh pointer oboslete;
so need to reassign greh;
but since first calling pskb_may_pull already ensured that skb->data
has enough space for greh, so move the reference of greh before second
calling pskb_may_pull(), to avoid reassign greh.

Fixes: 7a7ffbabf9("ipv4: fix tunneled VM traffic over hw VXLAN/GRE GSO NIC")
Cc: Wei-Chun Chao <weichunc@...mgrid.com>
Signed-off-by: Li RongQing <roy.qing.li@...il.com>
---
 net/ipv4/gre_offload.c |    6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/gre_offload.c b/net/ipv4/gre_offload.c
index a777295..ccda096 100644
--- a/net/ipv4/gre_offload.c
+++ b/net/ipv4/gre_offload.c
@@ -55,13 +55,13 @@ static struct sk_buff *gre_gso_segment(struct sk_buff *skb,
 	if (csum)
 		skb->encap_hdr_csum = 1;
 
-	if (unlikely(!pskb_may_pull(skb, ghl)))
-		goto out;
-
 	/* setup inner skb. */
 	skb->protocol = greh->protocol;
 	skb->encapsulation = 0;
 
+	if (unlikely(!pskb_may_pull(skb, ghl)))
+		goto out;
+
 	__skb_pull(skb, ghl);
 	skb_reset_mac_header(skb);
 	skb_set_network_header(skb, skb_inner_network_offset(skb));
-- 
1.7.10.4

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