netlink: fix splat in skb_clone with large messages

Since (c05cdb1 netlink: allow large data transfers from user-space),
netlink splats if it invokes skb_clone on large netlink skbs since:

* skb_shared_info was not correctly initialized.
* skb->destructor is not set in the cloned skb.

This was spotted by trinity:

[  894.990671] BUG: unable to handle kernel paging request at ffffc9000047b001
[  894.991034] IP: [<ffffffff81a212c4>] skb_clone+0x24/0xc0
[...]
[  894.991034] Call Trace:
[  894.991034]  [<ffffffff81ad299a>] nl_fib_input+0x6a/0x240
[  894.991034]  [<ffffffff81c3b7e6>] ? _raw_read_unlock+0x26/0x40
[  894.991034]  [<ffffffff81a5f189>] netlink_unicast+0x169/0x1e0
[  894.991034]  [<ffffffff81a601e1>] netlink_sendmsg+0x251/0x3d0

Fix it by:

1) introducing a new netlink_skb_clone function that is used in nl_fib_input,
   that sets our special skb->destructor in the cloned skb. Moreover, handle
   the release of the large cloned skb head area in the destructor path.

2) not allowing large skbuffs in the netlink broadcast path. I cannot find
   any reasonable use of the large data transfer using netlink in that path,
   moreover this helps to skip extra skb_clone handling.

I found two more netlink clients that are cloning the skbs, but they are
not in the sendmsg path. Therefore, the sole client cloning that I found
seems to be the fib frontend.

Thanks to Eric Dumazet for helping to address this issue.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 16 insertions, 0 deletions

diff --git a/include/linux/netlink.h b/include/linux/netlink.h
index 86fde81a..7a6c396 100644
--- a/include/linux/netlink.h
+++ b/include/linux/netlink.h
@@ -85,6 +85,22 @@ int netlink_attachskb(struct sock *sk, struct sk_buff *skb,
 void netlink_detachskb(struct sock *sk, struct sk_buff *skb);
 int netlink_sendskb(struct sock *sk, struct sk_buff *skb);
 
+static inline struct sk_buff *
+netlink_skb_clone(struct sk_buff *skb, gfp_t gfp_mask)
+{
+	struct sk_buff *nskb;
+
+	nskb = skb_clone(skb, gfp_mask);
+	if (!nskb)
+		return NULL;
+
+	/* This is a large skb, set destructor callback to release head */
+	if (is_vmalloc_addr(skb->head))
+		nskb->destructor = skb->destructor;
+
+	return nskb;
+}
+
 /*
  *	skb should fit one page. This choice is good for headerless malloc.
  *	But we should limit to 8K so that userspace does not have to