netlink: netlink_recvmsg() fix

commit 1dacc76d0014
(net/compat/wext: send different messages to compat tasks)
introduced a race condition on netlink, in case MSG_PEEK is used.

An skb given by skb_recv_datagram() might be shared, we must copy it
before any modification, or risk fatal corruption.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netlink/af_netlink.c

@@ -1406,7 +1406,7 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
 	struct netlink_sock *nlk = nlk_sk(sk);
 	int noblock = flags&MSG_DONTWAIT;
 	size_t copied;
-	struct sk_buff *skb, *frag __maybe_unused = NULL;
+	struct sk_buff *skb;
 	int err;
 
 	if (flags&MSG_OOB)
@@ -1441,7 +1441,21 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
 			kfree_skb(skb);
 			skb = compskb;
 		} else {
-			frag = skb_shinfo(skb)->frag_list;
+			/*
+			 * Before setting frag_list to NULL, we must get a
+			 * private copy of skb if shared (because of MSG_PEEK)
+			 */
+			if (skb_shared(skb)) {
+				struct sk_buff *nskb;
+
+				nskb = pskb_copy(skb, GFP_KERNEL);
+				kfree_skb(skb);
+				skb = nskb;
+				err = -ENOMEM;
+				if (!skb)
+					goto out;
+			}
+			kfree_skb(skb_shinfo(skb)->frag_list);
 			skb_shinfo(skb)->frag_list = NULL;
 		}
 	}
@@ -1478,10 +1492,6 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
 	if (flags & MSG_TRUNC)
 		copied = skb->len;
 
-#ifdef CONFIG_COMPAT_NETLINK_MESSAGES
-	skb_shinfo(skb)->frag_list = frag;
-#endif
-
 	skb_free_datagram(sk, skb);
 
 	if (nlk->cb && atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf / 2)