[PATCH net] net: ovs: prevent underflow in queue_userspace_packet()

[Dan Carpenter <dan.carpenter@linaro.org>]

If "hlen" less than "cutlen" then when we call upcall_msg_size()
the "hlen - cutlen" parameter will be a very high positive
number.

Later in the function we use "skb->len - cutlen" but this change
addresses that potential underflow since skb->len is always going to
be greater than or equal to hlen.

Fixes: f2a4d086ed4c ("openvswitch: Add packet truncation support.")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
---
From code review not testing.

 net/openvswitch/datapath.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/net/openvswitch/datapath.c b/net/openvswitch/datapath.c
index 225f6048867f..bb25a2bbe8a0 100644
--- a/net/openvswitch/datapath.c
+++ b/net/openvswitch/datapath.c
@@ -477,6 +477,11 @@ static int queue_userspace_packet(struct datapath *dp, struct sk_buff *skb,
 	else
 		hlen = skb->len;
 
+	if (hlen < cutlen) {
+		err = -EINVAL;
+		goto out;
+	}
+
 	len = upcall_msg_size(upcall_info, hlen - cutlen,
 			      OVS_CB(skb)->acts_origlen);
 	user_skb = genlmsg_new(len, GFP_ATOMIC);
-- 
2.45.2