net: neighbour: prohibit negative value for unres_qlen_bytes parameter

unres_qlen_bytes and unres_qlen are int type.
But multiple relation(unres_qlen_bytes = unres_qlen * SKB_TRUESIZE(ETH_FRAME_LEN))
will cause type overflow when seting unres_qlen. e.g.

$ echo 1027506 > /proc/sys/net/ipv4/neigh/eth1/unres_qlen
$ cat /proc/sys/net/ipv4/neigh/eth1/unres_qlen
1182657265
$ cat /proc/sys/net/ipv4/neigh/eth1/unres_qlen_bytes
-2147479756

The gutted value is not that we setting。
But user/administrator don't know this is caused by int type overflow.

what's more, it is meaningless and even dangerous that unres_qlen_bytes is set
with negative number. Because, for unresolved neighbour address, kernel will cache packets
without limit in __neigh_event_send()(e.g. (u32)-1 = 2GB).

Signed-off-by: Shan Wei <davidshan@tencent.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/neighbour.c

@@ -62,6 +62,9 @@ static void __neigh_notify(struct neighbour *n, int type, int flags);
 static void neigh_update_notify(struct neighbour *neigh);
 static int pneigh_ifdown(struct neigh_table *tbl, struct net_device *dev);
 
+static int zero;
+static int unres_qlen_max = INT_MAX / SKB_TRUESIZE(ETH_FRAME_LEN);
+
 static struct neigh_table *neigh_tables;
 #ifdef CONFIG_PROC_FS
 static const struct file_operations neigh_stat_seq_fops;
@@ -1787,8 +1790,7 @@ static int neightbl_fill_parms(struct sk_buff *skb, struct neigh_parms *parms)
 	    nla_put_u32(skb, NDTPA_QUEUE_LENBYTES, parms->queue_len_bytes) ||
 	    /* approximative value for deprecated QUEUE_LEN (in packets) */
 	    nla_put_u32(skb, NDTPA_QUEUE_LEN,
-			DIV_ROUND_UP(parms->queue_len_bytes,
-				     SKB_TRUESIZE(ETH_FRAME_LEN))) ||
+			parms->queue_len_bytes / SKB_TRUESIZE(ETH_FRAME_LEN)) ||
 	    nla_put_u32(skb, NDTPA_PROXY_QLEN, parms->proxy_qlen) ||
 	    nla_put_u32(skb, NDTPA_APP_PROBES, parms->app_probes) ||
 	    nla_put_u32(skb, NDTPA_UCAST_PROBES, parms->ucast_probes) ||
@@ -2777,9 +2779,13 @@ static int proc_unres_qlen(ctl_table *ctl, int write, void __user *buffer,
 	int size, ret;
 	ctl_table tmp = *ctl;
 
+	tmp.extra1 = &zero;
+	tmp.extra2 = &unres_qlen_max;
 	tmp.data = &size;
-	size = DIV_ROUND_UP(*(int *)ctl->data, SKB_TRUESIZE(ETH_FRAME_LEN));
-	ret = proc_dointvec(&tmp, write, buffer, lenp, ppos);
+
+	size = *(int *)ctl->data / SKB_TRUESIZE(ETH_FRAME_LEN);
+	ret = proc_dointvec_minmax(&tmp, write, buffer, lenp, ppos);
+
 	if (write && !ret)
 		*(int *)ctl->data = size * SKB_TRUESIZE(ETH_FRAME_LEN);
 	return ret;
@@ -2865,7 +2871,8 @@ static struct neigh_sysctl_table {
 			.procname	= "unres_qlen_bytes",
 			.maxlen		= sizeof(int),
 			.mode		= 0644,
-			.proc_handler	= proc_dointvec,
+			.extra1		= &zero,
+			.proc_handler   = proc_dointvec_minmax,
 		},
 		[NEIGH_VAR_PROXY_QLEN] = {
 			.procname	= "proxy_qlen",