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

[Ido Schimmel <idosch@nvidia.com>]

On Fri, Aug 14, 2026 at 01:05:34AM +0800, Ren Wei wrote:
> From: Yong Wang <edragain@163.com>
> 
> 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.

Eric / Neal,

Both sashiko instances [1][2] claim that this patch doesn't completely
fix the divide-by-zero issue: it is still reachable by lowering
net.ipv4.route.min_adv_mss to 0 and configuring a route with an MTU
metric of 52.

Given the above and the "We assume here that mss >= 1. This MUST be
enforced by all callers" comment above tcp_select_initial_window(), do
you prefer to fix this in TCP by enforcing a minimum MSS value?
Something like [3].

Thanks

[1] https://sashiko.dev/#/patchset/2c3901162c65a1d85cc1756a83a458db834d70c1.1786610865.git.edragain%40163.com
[2] https://netdev-ai.bots.linux.dev/sashiko/#/patchset/2c3901162c65a1d85cc1756a83a458db834d70c1.1786610865.git.edragain%40163.com
[3]
diff --git a/include/net/tcp.h b/include/net/tcp.h
index 2c5b889530b5..670c20876f26 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1782,6 +1782,11 @@ static inline int tcp_full_space(const struct sock *sk)
 	return tcp_win_from_space(sk, READ_ONCE(sk->sk_rcvbuf));
 }
 
+static inline u32 tcp_dst_advmss(const struct dst_entry *dst)
+{
+	return max_t(u32, dst_metric_advmss(dst), TCP_MIN_MSS);
+}
+
 static inline void __tcp_adjust_rcv_ssthresh(struct sock *sk, u32 new_ssthresh)
 {
 	int unused_mem = sk_unused_reserved_mem(sk);
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 7f413f509d7d..497b1a0370cd 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1736,7 +1736,7 @@ struct sock *tcp_v4_syn_recv_sock(const struct sock *sk, struct sk_buff *skb,
 	tcp_ca_openreq_child(newsk, dst);
 
 	tcp_sync_mss(newsk, dst4_mtu(dst));
-	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), dst_metric_advmss(dst));
+	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), tcp_dst_advmss(dst));
 
 	tcp_initialize_rcv_mss(newsk);
 
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index 6ab3e3a0b431..fb901b368ecc 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -440,7 +440,7 @@ void tcp_openreq_init_rwin(struct request_sock *req,
 	u32 rcv_wnd;
 	int mss;
 
-	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
+	mss = tcp_mss_clamp(tp, tcp_dst_advmss(dst));
 	window_clamp = READ_ONCE(tp->window_clamp);
 	/* Set this up on the first call only */
 	req->rsk_window_clamp = window_clamp ? : dst_metric(dst, RTAX_WINDOW);
diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
index d7c1444b5e30..7b761edf86ee 100644
--- a/net/ipv4/tcp_output.c
+++ b/net/ipv4/tcp_output.c
@@ -143,7 +143,7 @@ static __u16 tcp_advertise_mss(struct sock *sk)
 	int mss = tp->advmss;
 
 	if (dst) {
-		unsigned int metric = dst_metric_advmss(dst);
+		unsigned int metric = tcp_dst_advmss(dst);
 
 		if (metric < mss) {
 			mss = metric;
@@ -3972,7 +3972,7 @@ struct sk_buff *tcp_make_synack(const struct sock *sk, struct dst_entry *dst,
 	}
 	skb_dst_set(skb, dst);
 
-	mss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
+	mss = tcp_mss_clamp(tp, tcp_dst_advmss(dst));
 
 	memset(&opts, 0, sizeof(opts));
 	now = tcp_clock_ns();
@@ -4127,7 +4127,7 @@ static void tcp_connect_init(struct sock *sk)
 
 	if (!tp->window_clamp)
 		WRITE_ONCE(tp->window_clamp, dst_metric(dst, RTAX_WINDOW));
-	tp->advmss = tcp_mss_clamp(tp, dst_metric_advmss(dst));
+	tp->advmss = tcp_mss_clamp(tp, tcp_dst_advmss(dst));
 
 	tcp_initialize_rcv_mss(sk);
 
diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
index 9e9155b1b3aa..df9c29eb5c1f 100644
--- a/net/ipv6/tcp_ipv6.c
+++ b/net/ipv6/tcp_ipv6.c
@@ -1487,7 +1487,7 @@ static struct sock *tcp_v6_syn_recv_sock(const struct sock *sk, struct sk_buff *
 	tcp_ca_openreq_child(newsk, dst);
 
 	tcp_sync_mss(newsk, dst6_mtu(dst));
-	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), dst_metric_advmss(dst));
+	newtp->advmss = tcp_mss_clamp(tcp_sk(sk), tcp_dst_advmss(dst));
 
 	tcp_initialize_rcv_mss(newsk);