UBUNTU-CVE-2026-80847

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UBUNTU-CVE-2026-80847

Summary:

Details: 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.

References: https://ubuntu.com/security/CVE-2026-80847, https://www.cve.org/CVERecord?id=CVE-2026-80847, https://git.kernel.org/linus/870a9e42ecc6fe1b8c25d87af043cb0d9c178fe1