ipv4: ip_check_defrag must not modify skb before unsharing

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@regit.org>
Cc: stable@vger.kernel.org
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 9 insertions, 10 deletions

diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index 448e68546827..8d5cc75dac88 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));