netdev - [PATCH net-next 2/3] l2tp: configure l2specific_len according to l2specific

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Date:   Wed, 10 Jan 2018 18:20:41 +0100
From:   Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
To:     davem@...emloft.net
Cc:     netdev@...r.kernel.org, jchapman@...alix.com, g.nault@...halink.fr
Subject: [PATCH net-next 2/3] l2tp: configure l2specific_len according to l2specific_type

Set l2specific_len according to the L2-Specific Sublayer type specified
by the userspace and not rely on a user supplied value for sublayer len
since invalid usage of L2TP_ATTR_L2SPEC_LEN allows leaking memory on the
network and sending corrupted frames.
Moreover add a sanity check on l2specific_type provided by userspace

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@...hat.com>
---
 net/l2tp/l2tp_netlink.c | 21 ++++++++++++++++-----
 1 file changed, 16 insertions(+), 5 deletions(-)

diff --git a/net/l2tp/l2tp_netlink.c b/net/l2tp/l2tp_netlink.c
index 48b5bf30ec50..404e5482c4e7 100644
--- a/net/l2tp/l2tp_netlink.c
+++ b/net/l2tp/l2tp_netlink.c
@@ -550,13 +550,24 @@ static int l2tp_nl_cmd_session_create(struct sk_buff *skb, struct genl_info *inf
 		if (info->attrs[L2TP_ATTR_DATA_SEQ])
 			cfg.data_seq = nla_get_u8(info->attrs[L2TP_ATTR_DATA_SEQ]);
 
-		cfg.l2specific_type = L2TP_L2SPECTYPE_DEFAULT;
-		if (info->attrs[L2TP_ATTR_L2SPEC_TYPE])
+		if (info->attrs[L2TP_ATTR_L2SPEC_TYPE]) {
 			cfg.l2specific_type = nla_get_u8(info->attrs[L2TP_ATTR_L2SPEC_TYPE]);
 
-		cfg.l2specific_len = 4;
-		if (info->attrs[L2TP_ATTR_L2SPEC_LEN])
-			cfg.l2specific_len = nla_get_u8(info->attrs[L2TP_ATTR_L2SPEC_LEN]);
+			switch (cfg.l2specific_type) {
+			case L2TP_L2SPECTYPE_DEFAULT:
+				cfg.l2specific_len = 4;
+				break;
+			case L2TP_L2SPECTYPE_NONE:
+				cfg.l2specific_len = 0;
+				break;
+			default:
+				ret = -EINVAL;
+				goto out_tunnel;
+			}
+		} else {
+			cfg.l2specific_type = L2TP_L2SPECTYPE_DEFAULT;
+			cfg.l2specific_len = 4;
+		}
 
 		if (info->attrs[L2TP_ATTR_COOKIE]) {
 			u16 len = nla_len(info->attrs[L2TP_ATTR_COOKIE]);
-- 
2.13.6