netdev - [PATCH net] l2tp: pass correct message length to ip6_append

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Message-Id: <20240220122156.43131-1-tparkin@katalix.com>
Date: Tue, 20 Feb 2024 12:21:56 +0000
From: Tom Parkin <tparkin@...alix.com>
To: netdev@...r.kernel.org
Cc: Tom Parkin <tparkin@...alix.com>,
	David Howells <dhowells@...hat.com>,
	stable@...r.kernel.org
Subject: [PATCH net] l2tp: pass correct message length to ip6_append_data

l2tp_ip6_sendmsg needs to avoid accounting for the transport header
twice when splicing more data into an already partially-occupied skbuff.

To manage this, we check whether the skbuff contains data using
skb_queue_empty when deciding how much data to append using
ip6_append_data.

However, the code which performed the calculation was incorrect:

     ulen = len + skb_queue_empty(&sk->sk_write_queue) ? transhdrlen : 0;

...due to C operator precedence, this ends up setting ulen to
transhdrlen for messages with a non-zero length, which results in
corrupted packets on the wire.

Add parentheses to correct the calculation in line with the original
intent.

Fixes: 9d4c75800f61 ("ipv4, ipv6: Fix handling of transhdrlen in __ip{,6}_append_data()")
Cc: David Howells <dhowells@...hat.com>
Cc: stable@...r.kernel.org
Signed-off-by: Tom Parkin <tparkin@...alix.com>
---
This issue was uncovered by Debian build-testing for the
golang-github-katalix-go-l2tp package[1].

It seems 9d4c75800f61 has been backported to the linux-6.1.y stable
kernel (and possibly others), so I think this fix will also need
backporting.

The bug is currently seen on at least Debian Bookworm, Ubuntu Jammy, and 
Debian testing/unstable.

Unfortunately tests using "ip l2tp" and which focus on dataplane
transport will not uncover this bug: it's necessary to send a packet
using an L2TPIP6 socket opened by userspace, and to verify the packet on
the wire.  The l2tp-ktest[2] test suite has been extended to cover this.

[1]. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1063746
[2]. https://github.com/katalix/l2tp-ktest

---
 net/l2tp/l2tp_ip6.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/l2tp/l2tp_ip6.c b/net/l2tp/l2tp_ip6.c
index dd3153966173..7bf14cf9ffaa 100644
--- a/net/l2tp/l2tp_ip6.c
+++ b/net/l2tp/l2tp_ip6.c
@@ -627,7 +627,7 @@ static int l2tp_ip6_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)

 back_from_confirm:
 	lock_sock(sk);
-	ulen = len + skb_queue_empty(&sk->sk_write_queue) ? transhdrlen : 0;
+	ulen = len + (skb_queue_empty(&sk->sk_write_queue) ? transhdrlen : 0);
 	err = ip6_append_data(sk, ip_generic_getfrag, msg,
 			      ulen, transhdrlen, &ipc6,
 			      &fl6, (struct rt6_info *)dst,
-- 
2.34.1