netlink: fix compat recvmsg

Since
commit 1dacc76d0014a034b8aca14237c127d7c19d7726
Author: Johannes Berg <johannes@sipsolutions.net>
Date:   Wed Jul 1 11:26:02 2009 +0000

    net/compat/wext: send different messages to compat tasks

we had a race condition when setting and then
restoring frag_list. Eric attempted to fix it,
but the fix created even worse problems.

However, the original motivation I had when I
added the code that turned out to be racy is
no longer clear to me, since we only copy up
to skb->len to userspace, which doesn't include
the frag_list length. As a result, not doing
any frag_list clearing and restoring avoids
the race condition, while not introducing any
other problems.

Additionally, while preparing this patch I found
that since none of the remaining netlink code is
really aware of the frag_list, we need to use the
original skb's information for packet information
and credentials. This fixes, for example, the
group information received by compat tasks.

Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: stable@kernel.org [2.6.31+, for 2.6.35 revert 1235f504aa]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 8648a99..980fe4a 100644 (file)
--- 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, *data_skb;
        int err;
 
        if (flags&MSG_OOB)
@@ -1418,45 +1418,35 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
        if (skb == NULL)
                goto out;
 
+       data_skb = skb;
+
 #ifdef CONFIG_COMPAT_NETLINK_MESSAGES
        if (unlikely(skb_shinfo(skb)->frag_list)) {
-               bool need_compat = !!(flags & MSG_CMSG_COMPAT);
-
                /*
-                * If this skb has a frag_list, then here that means that
-                * we will have to use the frag_list skb for compat tasks
-                * and the regular skb for non-compat tasks.
+                * If this skb has a frag_list, then here that means that we
+                * will have to use the frag_list skb's data for compat tasks
+                * and the regular skb's data for normal (non-compat) tasks.
                 *
-                * The skb might (and likely will) be cloned, so we can't
-                * just reset frag_list and go on with things -- we need to
-                * keep that. For the compat case that's easy -- simply get
-                * a reference to the compat skb and free the regular one
-                * including the frag. For the non-compat case, we need to
-                * avoid sending the frag to the user -- so assign NULL but
-                * restore it below before freeing the skb.
+                * If we need to send the compat skb, assign it to the
+                * 'data_skb' variable so that it will be used below for data
+                * copying. We keep 'skb' for everything else, including
+                * freeing both later.
                 */
-               if (need_compat) {
-                       struct sk_buff *compskb = skb_shinfo(skb)->frag_list;
-                       skb_get(compskb);
-                       kfree_skb(skb);
-                       skb = compskb;
-               } else {
-                       frag = skb_shinfo(skb)->frag_list;
-                       skb_shinfo(skb)->frag_list = NULL;
-               }
+               if (flags & MSG_CMSG_COMPAT)
+                       data_skb = skb_shinfo(skb)->frag_list;
        }
 #endif
 
        msg->msg_namelen = 0;
 
-       copied = skb->len;
+       copied = data_skb->len;
        if (len < copied) {
                msg->msg_flags |= MSG_TRUNC;
                copied = len;
        }
 
-       skb_reset_transport_header(skb);
-       err = skb_copy_datagram_iovec(skb, 0, msg->msg_iov, copied);
+       skb_reset_transport_header(data_skb);
+       err = skb_copy_datagram_iovec(data_skb, 0, msg->msg_iov, copied);
 
        if (msg->msg_name) {
                struct sockaddr_nl *addr = (struct sockaddr_nl *)msg->msg_name;
@@ -1476,11 +1466,7 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
        }
        siocb->scm->creds = *NETLINK_CREDS(skb);
        if (flags & MSG_TRUNC)
-               copied = skb->len;
-
-#ifdef CONFIG_COMPAT_NETLINK_MESSAGES
-       skb_shinfo(skb)->frag_list = frag;
-#endif
+               copied = data_skb->len;
 
        skb_free_datagram(sk, skb);