Commit 68d6ac6d authored by Johannes Berg's avatar Johannes Berg Committed by David S. Miller
Browse files

netlink: fix compat recvmsg 



Since
commit 1dacc76d
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 1235f504]
Signed-off-by: default avatarJohannes Berg <johannes.berg@intel.com>
Acked-by: default avatarEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent e243f5b6

net/netlink/af_netlink.c

+16 −30
Original line number Diff line number Diff line
@@ -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);

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