CVE-2026-80847tcp: clamp route advmss to TCP_MIN_MSS

Official description

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.

Affected packages & fixed versions

Affected products (as reported)

References

• https://git.kernel.org/stable/c/1aedef9ca04aad427a077c7c427ffd89768038bc
• https://git.kernel.org/stable/c/788fbbf7d61f9742298417c207c91b6db159fbbc
• https://git.kernel.org/stable/c/eaf4af9bd6975c2eff8d48820937c83032d968dc
• https://git.kernel.org/stable/c/cfb44c6028e6f979720315e0772bbd0d4d33ed61
• https://git.kernel.org/stable/c/6b8c20bf61924dfc38fefb145c9f7406d73fae53
• https://git.kernel.org/stable/c/31cf2349361902769dc323e4dbf4b449795ec288
• https://git.kernel.org/stable/c/870a9e42ecc6fe1b8c25d87af043cb0d9c178fe1