l2tp: pass correct message length to ip6_append_data

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: 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.

References

• https://git.kernel.org/stable/c/4c3ce64bc9d36ca9164dd6c77ff144c121011aae
• https://git.kernel.org/stable/c/c1d3a84a67db910ce28a871273c992c3d7f9efb5
• https://git.kernel.org/stable/c/dcb4d14268595065c85dc5528056713928e17243
• https://git.kernel.org/stable/c/0da15a70395182ee8cb75716baf00dddc0bea38d
• https://git.kernel.org/stable/c/13cd1daeea848614e585b2c6ecc11ca9c8ab2500
• https://git.kernel.org/stable/c/804bd8650a3a2bf3432375f8c97d5049d845ce56
• https://git.kernel.org/stable/c/83340c66b498e49353530e41542500fc8a4782d6
• https://git.kernel.org/stable/c/359e54a93ab43d32ee1bff3c2f9f10cb9f6b6e79