On Tue, Feb 20, 2024 at 12:21:56PM +0000, Tom Parkin wrote:
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
Reviewed-by: Simon Horman horms@kernel.org
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.
In that case perhaps this is appropriate - citing the patch that 9d4c75800f61 tried to fix?
Fixes: a32e0eec7042 ("l2tp: introduce L2TPv3 IP encapsulation support for IPv6")
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
...