[PATCH net v2] tcp: reject zero CWND route metrics

A process with CAP_NET_ADMIN in a user-created network namespace can send
an RTM_NEWROUTE request with RTAX_CWND set to zero. It can also set the
RTAX_CWND lock bit while omitting RTAX_CWND, leaving the zero-initialized
metric value locked. tcpm_suck_dst() copies the zero into the TCP metrics
cache, and tcp_init_metrics() later installs it as snd_cwnd_clamp. This
makes the initial cwnd zero and can reach a divide by zero in
tcp_cong_avoid_ai().

Reject zero RTAX_CWND and RTAX_INITCWND metrics while parsing route
metrics. Also reject a locked CWND without a value, so a zero CWND cannot
enter the TCP metrics cache.

Fixes: 51c5d0c4b169 ("tcp: Maintain dynamic metrics in local cache.")
Reported-by: co+71b6509cb791e531@bugs.sh
Suggested-by: Eric Dumazet <edumazet@kernel.org>
Assisted-by: LLM
Cc: stable@vger.kernel.org
Signed-off-by: Weiming Shi <redacted>
---
 net/ipv4/metrics.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/net/ipv4/metrics.c b/net/ipv4/metrics.c
index ad40762a8b38..c26f95126ae7 100644
--- a/net/ipv4/metrics.c
+++ b/net/ipv4/metrics.c

@@ -54,9 +54,19 @@ static int ip_metrics_convert(struct nlattr *fc_mx,
 			NL_SET_ERR_MSG(extack, "Unknown flag set in feature mask in metrics attribute");
 			return -EINVAL;
 		}
+		if ((type == RTAX_CWND || type == RTAX_INITCWND) && !val) {
+			NL_SET_ERR_MSG(extack, "CWND metric must be greater than zero");
+			return -EINVAL;
+		}
 		metrics[type - 1] = val;
 	}
 
+	if ((metrics[RTAX_LOCK - 1] & (1U << RTAX_CWND)) &&
+	    !metrics[RTAX_CWND - 1]) {
+		NL_SET_ERR_MSG(extack, "Locked CWND metric requires a value");
+		return -EINVAL;
+	}
+
 	if (ecn_ca)
 		metrics[RTAX_FEATURES - 1] |= DST_FEATURE_ECN_CA;
 

-- 
2.55.0