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@redhat.com Cc: stable@vger.kernel.org Signed-off-by: Tom Parkin tparkin@katalix.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,