Re: [PATCH net 1/1] ipv4: reject RTAX_ADVMSS values below TCP_MIN_MSS

On Tue, 28 Jul 2026 13:08:17 +0800 Ren Wei wrote:
> ip_metrics_convert() only caps RTAX_ADVMSS at the upper bound and
> still accepts undersized non-zero values from userspace.
> 
> A route installed with "advmss 12" can later reach the passive TCP
> open path. When SYN timestamps are enabled, tcp_openreq_init_rwin()
> subtracts TCPOLEN_TSTAMP_ALIGNED from the route advmss before calling
> tcp_select_initial_window(). This can reduce the effective MSS to
> zero and trigger a divide-by-zero in the rounddown(space, mss) path.
> 
> Reject non-zero RTAX_ADVMSS values smaller than TCP_MIN_MSS while
> keeping the existing "0 means use default advmss" behavior intact.
> 
> This matches the existing TCP_MIN_MSS based validation used for
> TCP_MAXSEG and fixes the bug at the route metric input point rather
> than adding a redundant guard deeper in the TCP stack.

Could you repost this? maybe we'll get more review attention that way.