netdev - [PATCH net-next v5 2/2] ipv6: Fix signed integer overflow in l2tp_ip6

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Date:   Tue, 7 Jun 2022 20:00:28 +0800
From:   Wang Yufen <wangyufen@...wei.com>
To:     <davem@...emloft.net>, <yoshfuji@...ux-ipv6.org>,
        <dsahern@...nel.org>, <edumazet@...gle.com>, <kuba@...nel.org>,
        <pabeni@...hat.com>, <ast@...nel.org>, <daniel@...earbox.net>,
        <andrii@...nel.org>, <kafai@...com>, <songliubraving@...com>,
        <yhs@...com>, <john.fastabend@...il.com>, <kpsingh@...nel.org>
CC:     <netdev@...r.kernel.org>, <bpf@...r.kernel.org>
Subject: [PATCH net-next v5 2/2] ipv6: Fix signed integer overflow in l2tp_ip6_sendmsg

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: Jakub Kicinski <kuba@...nel.org>
Signed-off-by: Wang Yufen <wangyufen@...wei.com>
---
 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 c6ff8bf9b55f..9dbd801ddb98 100644
--- a/net/l2tp/l2tp_ip6.c
+++ b/net/l2tp/l2tp_ip6.c
@@ -504,14 +504,15 @@ 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)
-- 
2.25.1