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Sat, 03 Oct 2026 08:32:45 -0700 (PDT) Received: from 192.168.50.3 ([198.176.50.208]) by smtp.gmail.com with ESMTPSA id a92af1059eb24-151fc39a7d1sm6576061c88.3.2026.10.03.08.32.39 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 03 Oct 2026 08:32:45 -0700 (PDT) From: bestswngs@gmail.com To: edumazet@google.com, ncardwell@google.com, netdev@vger.kernel.org Cc: kuniyu@google.com, davem@davemloft.net, horms@kernel.org, xmei5@asu.edu, co+71b6509cb791e531@bugs.sh, Weiming Shi Subject: [PATCH net] tcp: ignore locked zero CWND route metric Date: Sat, 3 Oct 2026 23:32:25 +0800 Message-ID: <20261003153224.2683219-2-bestswngs@gmail.com> X-Mailer: git-send-email 2.55.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Weiming Shi A locked RTAX_CWND route metric of zero is copied into the TCP metrics cache and then into snd_cwnd_clamp. This sets the initial cwnd to zero. With a cached SSTHRESH metric, a later Reno ACK can reach a divide by zero in tcp_cong_avoid_ai(). A user with CAP_NET_ADMIN in a user-created network namespace can trigger the oops. Ignore a zero cached CWND metric when applying the locked clamp, leaving the socket's initialized nonzero clamp in place. Oops: divide error: 0000 [#1] SMP KASAN NOPTI RIP: 0010:tcp_cong_avoid_ai (net/ipv4/tcp_cong.c:482) Call Trace: tcp_ack (net/ipv4/tcp_input.c:4440) tcp_rcv_established (net/ipv4/tcp_input.c:6678) tcp_v4_do_rcv (net/ipv4/tcp_ipv4.c:1854) tcp_v4_rcv (net/ipv4/tcp_ipv4.c:2248) Kernel panic - not syncing: Fatal exception in interrupt Fixes: 51c5d0c4b169 ("tcp: Maintain dynamic metrics in local cache.") Reported-by: Assisted-by: LLM Signed-off-by: Weiming Shi --- net/ipv4/tcp_metrics.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/net/ipv4/tcp_metrics.c b/net/ipv4/tcp_metrics.c index dc0c081fc1f3..ff41804ad086 100644 --- a/net/ipv4/tcp_metrics.c +++ b/net/ipv4/tcp_metrics.c @@ -484,8 +484,11 @@ void tcp_init_metrics(struct sock *sk) goto reset; } - if (tcp_metric_locked(tm, TCP_METRIC_CWND)) - tp->snd_cwnd_clamp = tcp_metric_get(tm, TCP_METRIC_CWND); + if (tcp_metric_locked(tm, TCP_METRIC_CWND)) { + val = tcp_metric_get(tm, TCP_METRIC_CWND); + if (val) + tp->snd_cwnd_clamp = val; + } val = READ_ONCE(net->ipv4.sysctl_tcp_no_ssthresh_metrics_save) ? 0 : tcp_metric_get(tm, TCP_METRIC_SSTHRESH); -- 2.55.0