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

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

[Weiming Shi]

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 <bestswngs@gmail.com>
---
 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

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

[netdev-bot+sinfo]

Hi!

This is an automated message. This series looks like a fix, but its
commit messages seem to be missing some information:

 - Whether the issue was actually triggered, or is only theoretical
   (e.g. found by code inspection). If it was triggered please include
   the symptoms, like the stack trace or error messages.

Please do not repost the series just to address the above. Instead,
reply to this email with the missing information, so that reviewers
can take it into account. If the series needs another revision for
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then.

The evaluation is done by an LLM so it may be wrong, if you think
that is the case please reply and explain.