[PATCH AUTOSEL 5.4 21/23] ipv6: Fix signed integer overflow in l2tp_ip6_sendmsg

[Sasha Levin <sashal@kernel.org>]

From: Wang Yufen <wangyufen@huawei.com>

[ Upstream commit f638a84afef3dfe10554c51820c16e39a278c915 ]

When len >= INT_MAX - transhdrlen, ulen = len + transhdrlen will be
overflow. To fix, we can follow what udpv6 does and subtract the
transhdrlen from the max.

Signed-off-by: Wang Yufen <wangyufen@huawei.com>
Link: https://lore.kernel.org/r/20220607120028.845916-2-wangyufen@huawei.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 net/l2tp/l2tp_ip6.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/net/l2tp/l2tp_ip6.c b/net/l2tp/l2tp_ip6.c
index 0fa694bd3f6a..23e9ce985ed6 100644
--- a/net/l2tp/l2tp_ip6.c
+++ b/net/l2tp/l2tp_ip6.c
@@ -515,14 +515,15 @@ static int l2tp_ip6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	struct ipcm6_cookie ipc6;
 	int addr_len = msg->msg_namelen;
 	int transhdrlen = 4; /* zero session-id */
-	int ulen = len + transhdrlen;
+	int ulen;
 	int err;
 
 	/* Rough check on arithmetic overflow,
 	   better check is made in ip6_append_data().
 	 */
-	if (len > INT_MAX)
+	if (len > INT_MAX - transhdrlen)
 		return -EMSGSIZE;
+	ulen = len + transhdrlen;
 
 	/* Mirror BSD error message compatibility */
 	if (msg->msg_flags & MSG_OOB)
-- 
2.35.1