Subject: [PATCH] netlink: enable skb header refcounting before sending first broadcast

From: Konstantin Khlebnikov <[email-masked]>
Date: Fri, 10 Jul 2015 14:51:41 +0300

This fixes race between non-atomic updates of adjacent bit-fields:
skb->cloned could be lost because netlink broadcast clones skb after
sending it to the first listener who sets skb->peeked at the same skb.
As a result atomic refcounting of skb header stays disabled and
skb_release_data() frees it twice. Race leads to double-free in kmalloc-xxx.

Signed-off-by: Konstantin Khlebnikov <[email-masked]>
Fixes: b19372273164 ("net: reorganize sk_buff for faster __copy_skb_header()")
---
 net/netlink/af_netlink.c |    6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index dea925388a5b..921e0d8dfe3a 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2028,6 +2028,12 @@ int netlink_broadcast_filtered(struct sock *ssk, struct sk_buff *skb, u32 portid
 	info.tx_filter = filter;
 	info.tx_data = filter_data;
 
+	/* Enable atomic refcounting in skb_release_data() before first send:
+	 * non-atomic set of that bit-field in __skb_clone() could race with
+	 * __skb_recv_datagram() which touches the same set of bit-fields.
+	 */
+	skb->cloned = 1;
+
 	/* While we sleep in clone, do not allow to change socket list */
 
 	netlink_lock_table();