ipv4: ip_check_defrag must not modify skb before unsharing

[ Upstream commit 1bf3751ec90cc3174e01f0d701e8449ce163d113 ]

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>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/net/ipv4/ip_fragment.c b/net/ipv4/ip_fragment.c
index fdaabf2f2b68c13a1f66f31e92ac961a69fa7061..a4e71314bf2c06c032aa7eba05d18db76c4cd291 100644 (file)
--- a/net/ipv4/ip_fragment.c
+++ b/net/ipv4/ip_fragment.c
@@ -684,28 +684,27 @@ 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));