CVE-2026-80847

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

tcp: clamp route advmss to TCP_MIN_MSS

tcp_select_initial_window() assumes that callers never pass an MSS

smaller than 1, but route-derived advmss values can violate that

assumption.

A too-small explicit RTAX_ADVMSS is one way to get there, but it is not

the only one. The same divide-by-zero can also be reached through the

"default advmss" path when RTAX_ADVMSS is left at 0 and the effective

advmss is later driven down by route MTU and min_adv_mss.

Introduce a tcp_dst_advmss() helper that clamps route advmss to

TCP_MIN_MSS before TCP consumes it, and use it in the TCP paths that

derive advmss from dst metrics. This keeps the effective MSS from

dropping to zero before tcp_select_initial_window() rounds the receive

window.