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.