[090/114] netlink: fix compat recvmsg

From: Greg KH
Date: Tue Aug 24 2010 - 19:08:09 EST


2.6.35-stable review patch. If anyone has any objections, please let us know.

------------------

From: Johannes Berg <johannes.berg@xxxxxxxxx>

commit 68d6ac6d2740b6a55f3ae92a4e0be6d881904b32 upstream.

Since
commit 1dacc76d0014a034b8aca14237c127d7c19d7726
Author: Johannes Berg <johannes@xxxxxxxxxxxxxxxx>
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@xxxxxxxxx>
Signed-off-by: Johannes Berg <johannes.berg@xxxxxxxxx>
Acked-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxx>

---
net/netlink/af_netlink.c | 46 ++++++++++++++++------------------------------
1 file changed, 16 insertions(+), 30 deletions(-)

--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -1400,7 +1400,7 @@ static int netlink_recvmsg(struct kiocb
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)
@@ -1412,45 +1412,35 @@ static int netlink_recvmsg(struct kiocb
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;
@@ -1470,11 +1460,7 @@ static int netlink_recvmsg(struct kiocb
}
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);



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