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>

1 files changed, 16 insertions, 6 deletions

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 8648a9922aab..2cbf380377d5 100644
--- a/net/netlink/af_netlink.c
+++ b/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)