netdev - [PATCH] ipv4: ip_check_defrag must not modify skb before unsharing

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Message-ID: <1355132466.9857.6.camel@jlt4.sipsolutions.net>
Date:	Mon, 10 Dec 2012 10:41:06 +0100
From:	Johannes Berg <johannes@...solutions.net>
To:	David Miller <davem@...emloft.net>
Cc:	eric@...it.org, netdev@...r.kernel.org,
	linux-wireless@...r.kernel.org, linville@...driver.com,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: [PATCH] ipv4: ip_check_defrag must not modify skb before unsharing

From: Johannes Berg <johannes.berg@...el.com>

ip_check_defrag() might be called from af_packet within the
RX path where shared SKBs are used, so it must not modify
the input SKB before it has unshared it for defragmentation.
Use skb_copy_bits() to get the IP header and only pull in
everything later.

The same is true for the other caller in macvlan as it is
called from dev->rx_handler which can also get a shared SKB.

Reported-by: Eric Leblond <eric@...it.org>
Cc: stable@...r.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@...el.com>
---
For some versions of the kernel, this code goes into af_packet.c

 net/ipv4/ip_fragment.c | 19 +++++++++----------
 1 file changed, 9 insertions(+), 10 deletions(-)

diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index 448e685..8d5cc75 100644
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -707,28 +707,27 @@ EXPORT_SYMBOL(ip_defrag);
 
 struct sk_buff *ip_check_defrag(struct sk_buff *skb, u32 user)
 {
-	const struct iphdr *iph;
+	struct iphdr iph;
 	u32 len;
 
 	if (skb->protocol != htons(ETH_P_IP))
 		return skb;
 
-	if (!pskb_may_pull(skb, sizeof(struct iphdr)))
+	if (!skb_copy_bits(skb, 0, &iph, sizeof(iph)))
 		return skb;
 
-	iph = ip_hdr(skb);
-	if (iph->ihl < 5 || iph->version != 4)
+	if (iph.ihl < 5 || iph.version != 4)
 		return skb;
-	if (!pskb_may_pull(skb, iph->ihl*4))
-		return skb;
-	iph = ip_hdr(skb);
-	len = ntohs(iph->tot_len);
-	if (skb->len < len || len < (iph->ihl * 4))
+
+	len = ntohs(iph.tot_len);
+	if (skb->len < len || len < (iph.ihl * 4))
 		return skb;
 
-	if (ip_is_fragment(ip_hdr(skb))) {
+	if (ip_is_fragment(&iph)) {
 		skb = skb_share_check(skb, GFP_ATOMIC);
 		if (skb) {
+			if (!pskb_may_pull(skb, iph.ihl*4))
+				return skb;
 			if (pskb_trim_rcsum(skb, len))
 				return skb;
 			memset(IPCB(skb), 0, sizeof(struct inet_skb_parm));
-- 
1.8.0



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