psd: add basic validation of userspace matchinfo data

psd multiplies weight_thresh by HZ, so it could overflow.

Userspace libxt_psd refuses values exceeding PSD_MAX_RATE, so check
that on kernel side, too.

Also, setting 0 weight for both privileged and highports will cause
psd to never match at all.

Reject 0 weight threshold, too because it makes no sense (triggers
match for every initial packet).

doc/changelog.txt

@@ -3,6 +3,8 @@ HEAD
 ====
 Fixes:
 - xt_psd: avoid crash due to curr->next corruption
+Changes:
+- xt_psd: reject invalid match options
 
 
 v1.42 (2012-04-05)

extensions/xt_psd.c

@@ -278,10 +278,32 @@ out_match:
 	return true;
 }
 
+static int psd_mt_check(const struct xt_mtchk_param *par)
+{
+	const struct xt_psd_info *info = par->matchinfo;
+
+	if (info->weight_threshold == 0)
+		/* 0 would match on every 1st packet */
+		return -EINVAL;
+
+	if ((info->lo_ports_weight | info->hi_ports_weight) == 0)
+		/* would never match */
+		return -EINVAL;
+
+	if (info->delay_threshold > PSD_MAX_RATE ||
+	    info->weight_threshold > PSD_MAX_RATE ||
+	    info->lo_ports_weight > PSD_MAX_RATE ||
+	    info->hi_ports_weight > PSD_MAX_RATE)
+		return -EINVAL;
+
+	return 0;
+}
+
 static struct xt_match xt_psd_reg __read_mostly = {
 	.name       = "psd",
 	.family     = NFPROTO_IPV4,
 	.revision   = 1,
+	.checkentry = psd_mt_check,
 	.match      = xt_psd_match,
 	.matchsize  = sizeof(struct xt_psd_info),
 	.me         = THIS_MODULE,