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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman Cc: netdev@vger.kernel.org, linux-kernel@vger.kernel.org, co+71b6509cb791e531@bugs.sh, Xiang Mei , Weiming Shi , Eric Dumazet , stable@vger.kernel.org Subject: [PATCH net v2] tcp: reject zero CWND route metrics Date: Mon, 5 Oct 2026 00:49:44 +0800 Message-ID: <20261004164944.2006741-1-bestswngs@gmail.com> X-Mailer: git-send-email 2.55.0 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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 Assisted-by: LLM Cc: stable@vger.kernel.org Signed-off-by: Weiming Shi --- 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