From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-yx1-f41.google.com (mail-yx1-f41.google.com [74.125.224.41]) (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 33F6931E845 for ; Thu, 6 Aug 2026 03:26:22 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=74.125.224.41 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785986783; cv=none; b=PdxHisba8HUHZUwoSHvDRORm2U6EoZ8Q81CFHemJgyTCyegPK14vPZqS+MS2huw0pEn1BA9JzZdfBMdJtuTMgproyYPQC9gVCcPYSU7iuuvGFMvLsBV/UqdrlhQbvuJK/usYcEADqO2J8ot3GFBGFIZiNAQ5B3m0d1Dx1veRPvE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785986783; c=relaxed/simple; bh=Go5avKT1QvaWrORePMMZlk0/5Uri0rRR5CIKExBX8XQ=; h=Date:From:To:Cc:Message-ID:In-Reply-To:References:Subject: Mime-Version:Content-Type; b=U4fmvFPuAPxtnR468JvYY3p0XdJoZ9t5Sgkxmdqzhg/z2Jn2W73L0V3C89VPmfSTaN2dZhk0ds2lcdk2SgRup9LpV212UG+ADOz0SIbNSmrFjDX0YuY5ej6t6SvZ4/1oClCmg/v6GEPbqP1xPeP1IiqGnouQRzwgH/+VVkpoq+E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com; spf=pass smtp.mailfrom=gmail.com; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b=UT1jzFJj; arc=none smtp.client-ip=74.125.224.41 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=gmail.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=gmail.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="UT1jzFJj" Received: by mail-yx1-f41.google.com with SMTP id 956f58d0204a3-669944f60b3so2115921d50.1 for ; Wed, 05 Aug 2026 20:26:22 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20251104; t=1785986781; x=1786591581; darn=vger.kernel.org; h=content-transfer-encoding:content-type:mime-version:subject :references:in-reply-to:message-id:cc:to:from:date:from:to:cc :subject:date:message-id:reply-to:content-type; bh=28Nn44OGd3bbYGO0uu6bNbKhsRmKcVMK2M9qsrDgIYQ=; b=UT1jzFJj0OzFttjRnEhuIYq1r5DTO2mi6JqEHI+RPTdEgOgIt5aXy2mLsLiRCu4W6d k7rrBcloBzOFB/hCnKt0/7p4/zpOGwFF3hVryzjFHzHFqjBvzAnzSl0+s6MstDDHtNns v7KpLhBOw/iiUeO7C2fliqRupcGVDhfKuRftTql4Vq1sPsiNZ9C6EolkmGbcefSS45nR 3C7bGHjZTPS89zmkX/GJ6kauYSlQVQTp21I1y9MkcUXDeWKWPQnkBX7MdMNfmYgKZR7a yW+eeIE2zyqiLSnaK4kkDpKxDdpLht8egD0UMvUJn1wyegLwRGNVYU0oFOZ7JKXV/XCV H6Sg== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1785986781; x=1786591581; h=content-transfer-encoding:content-type:mime-version:subject :references:in-reply-to:message-id:cc:to:from:date:x-gm-gg :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to :content-type; bh=28Nn44OGd3bbYGO0uu6bNbKhsRmKcVMK2M9qsrDgIYQ=; b=rpQSrVZHBmYqSJJlA+LdKRmPE9h0BtpKQHaQhGI0FDgjpEnL8K7LXkOaSALjInPhV8 jVYPQlRegnzcgBK/Kt/vx7O5lV5FLPdeH7edo30IH0MKssOOMfka1c9j+f0Et9P0THIy tLQomq7p3QLF6COsY2xIRxu9J6QjigmXgdmRltpn7qnzUXie0hIhLpaKyKYprFzi+z0b M28zrPldBeo7+HvV/VEzGTGCmyZHomwINMK1/NkAWXp0VPV2rUAAj3iQ8ZEhIWwwMKeh bQCR4Ec9W5KIAUGDGmeb4aVM7WO8Xpmj6Pp2LfQicSH1IxF2D4FP3EVVX19rrI+mQo5K qCsQ== X-Forwarded-Encrypted: i=1; AHgh+Ro6q6mMNN9Oy7w23Wp7btpx9uylFKtKQzBhcSx985giEz2nkTWmrfaRlZI99S5r69P/ExWA0/I=@vger.kernel.org X-Gm-Message-State: AOJu0YylZHEQkA7mfgeObckVVXs9evOasxZIC0OX8yJxfykEGqBu7XNp FPdOu+Si86/gIKL89n3YJ1i9X27ZKUH2R6DRk7goAUou0cVV1pBim+gH X-Gm-Gg: AR+sD12KGknygr3mqzOMuoXv0RWipzBZqTsX2HHXPHbjXTpbD8AzXxqovz9H7lrwFxR dLHwtzKbCIfaRyOIgCADmeo1tZfMDs93Mpi0x3PWrju4WTiVbsRL8OTZyDZOGGd4VqQ+doeKPwy DvW3rZhzLCdcDgHcrrnmmURq2iDpFilJ7Khsfk62OXn5CNVZ+5VtfDBEM3zW8jIhPA7JMIvDbU9 zkwmIJxIaBk5juzP9YJvyvvYnC7dvlsyVpBNgt7seI/2HME8/K91b4J7/X2OG8sDJdgyDOlEwLs jZ4cmA807ZGyRGizXk+tNQ+B18xh2sQq9P5OJoDqMtUHHbDZaSHdHIOmNWTwbKE46DwEia27Q8p UGNaP0hsNKdWz1kGxFntzXmtkM1YQ32+gu3wok9DEMp/IGEvqTUWrAin2fMeZXbCDAg7G+NgvGN AWUjelBGdPWN3lrki9SIDsW+FudAm1oSwjIEIgGAuT5lclD+rIW13hi/0jqbAEOwy/Z08du+zDL WQ+KMXMXb6f80gJLFp+0Pd1G4CZUdgUYx6p X-Received: by 2002:a05:690e:ed2:b0:668:15aa:4f79 with SMTP id 956f58d0204a3-6699a919b25mr5215994d50.2.1785986780972; Wed, 05 Aug 2026 20:26:20 -0700 (PDT) Received: from gmail.com (250.4.48.34.bc.googleusercontent.com. [34.48.4.250]) by smtp.gmail.com with ESMTPSA id 956f58d0204a3-6699166dc17sm4222731d50.19.2026.08.05.20.26.19 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 05 Aug 2026 20:26:19 -0700 (PDT) Date: Wed, 05 Aug 2026 23:26:19 -0400 From: Willem de Bruijn To: Alice Mikityanska , Willem de Bruijn , David Ahern , Ido Schimmel , Jakub Kicinski , Paolo Abeni Cc: "David S. Miller" , Eric Dumazet , Simon Horman , Shuah Khan , netdev@vger.kernel.org, linux-kselftest@vger.kernel.org, Alice Mikityanska , syzbot+ce13c07d96d04716eaa2@syzkaller.appspotmail.com, vadim.fedorenko@linux.dev Message-ID: In-Reply-To: <20260805205957.1652619-1-alice.kernel@fastmail.im> References: <20260805205957.1652619-1-alice.kernel@fastmail.im> Subject: Re: [PATCH net-next 4/4] net: Fix UDP length overflow with PMTU discover and big MTU Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Alice Mikityanska wrote: > From: Alice Mikityanska > > This commit bounds cork->base.fragsize to IP(6)_MAX_MTU to avoid a > possible overflow of UDP length that triggers a WARN in > udp_set_len_short when setsockopt IP(V6)_MTU_DISCOVER is set to > IPV6_PMTUDISC_DO or IP(V6)_PMTUDISC_PROBE, and a large packet is sent > over a netdev with an unusually large MTU. > > Steps to reproduce (included in the new selftest): > > 1. Set device MTU bigger than IP6_MAX_MTU (or IP_MAX_MTU + 20). > cork->base.fragsize will be set to that MTU in ip(6)_setup_cork. > 2. Set IP(V6)_MTU_DISCOVER to IP(V6)_PMTUDISC_PROBE or IPV6_PMTUDISC_DO. > It lets maxnonfragsize be set to device MTU (cork->fragsize) in > __ip(6)_append_data, rather than to IP(6)_MAX_MTU. > 3. Send 65528 bytes of payload (+8 bytes of UDP header, +20/40 bytes of > IPv4/IPv6 header). Device MTU allows it (it's only one byte bigger > than IP6_MAX_MTU or IP_MAX_MTU + IPv4 header, and the device MTU is > bigger than that). > 4. The UDP length in the built packet is 65536, which overflows the > 16-bit length field and triggers the WARN in udp_set_len_short. This is discovered thanks to udp_set_len_short, but is this a preexisting bug and the fix go to net with a Fixes tag? > Note: IP_PMTUDISC_DO with IPv4 is safe, because ip_dst_mtu_maybe_forward > always clamps at IP_MAX_MTU, unlike ip6_dst_mtu_maybe_forward. That was introduced in commit 14972cbd34ff ("net: lwtunnel: Handle fragmentation"), the message of which includes "This includes .. some mtu fixes" without elaborating on those. That introduced the same clamp in ip6_mtu. Which was removed in commit 427faee167bc ("net: ipv6: introduce ip6_dst_mtu_maybe_forward") Tangential to this fix, but maybe that should be reinstated. I don't immediately see why the two would diverge on this point. > Reported-by: syzbot+ce13c07d96d04716eaa2@syzkaller.appspotmail.com > Closes: https://lore.kernel.org/netdev/6a6a966c.86abc875.e5c3d.0054.GAE@google.com/ > Signed-off-by: Alice Mikityanska > Assisted-by: Claude:claude-sonnet-4.6 > Cc: Willem de Bruijn I only see this patch 4/4. Is there more that did not make it to the list? > --- > net/ipv4/ip_output.c | 2 + > net/ipv6/ip6_output.c | 2 +- > tools/testing/selftests/net/Makefile | 1 + > tools/testing/selftests/net/cork_fragsize.py | 77 ++++++++++++++++++++ > 4 files changed, 81 insertions(+), 1 deletion(-) > create mode 100755 tools/testing/selftests/net/cork_fragsize.py > > diff --git a/net/ipv4/ip_output.c b/net/ipv4/ip_output.c > index e6dd1e5b8c32..2bc997c4fa7b 100644 > --- a/net/ipv4/ip_output.c > +++ b/net/ipv4/ip_output.c > @@ -1299,6 +1299,8 @@ static int ip_setup_cork(struct sock *sk, struct inet_cork *cork, > > cork->fragsize = ip_sk_use_pmtu(sk) ? > dst4_mtu(&rt->dst) : READ_ONCE(rt->dst.dev->mtu); > + if (cork->fragsize > IP_MAX_MTU) > + cork->fragsize = IP_MAX_MTU; > > if (!inetdev_valid_mtu(cork->fragsize)) > return -ENETUNREACH; > diff --git a/net/ipv6/ip6_output.c b/net/ipv6/ip6_output.c > index 368e4fa3b43c..91c92cb8e529 100644 > --- a/net/ipv6/ip6_output.c > +++ b/net/ipv6/ip6_output.c > @@ -1430,7 +1430,7 @@ static int ip6_setup_cork(struct sock *sk, struct inet_cork_full *cork, > if (frag_size && frag_size < mtu) > mtu = frag_size; > > - cork->base.fragsize = mtu; > + cork->base.fragsize = min(mtu, IP6_MAX_MTU); > cork->base.gso_size = ipc6->gso_size; > cork->base.tx_flags = 0; > cork->base.mark = ipc6->sockc.mark; > diff --git a/tools/testing/selftests/net/Makefile b/tools/testing/selftests/net/Makefile > index b43ddf192ecc..628203fda424 100644 > --- a/tools/testing/selftests/net/Makefile > +++ b/tools/testing/selftests/net/Makefile > @@ -25,6 +25,7 @@ TEST_PROGS := \ > cmsg_so_mark.sh \ > cmsg_so_priority.sh \ > cmsg_time.sh \ > + cork_fragsize.py \ > double_udp_encap.sh \ > drop_monitor_tests.sh \ > ecmp_rehash.sh \ > diff --git a/tools/testing/selftests/net/cork_fragsize.py b/tools/testing/selftests/net/cork_fragsize.py > new file mode 100755 > index 000000000000..7a1bb069227a > --- /dev/null > +++ b/tools/testing/selftests/net/cork_fragsize.py > @@ -0,0 +1,77 @@ > +#!/usr/bin/env python3 > +# SPDX-License-Identifier: GPL-2.0 > + > +# Test possible UDP length overflow in udp_send_skb/udp_v6_send_skb. > + > +from lib.py import ksft_run, ksft_exit, ksft_true > +from lib.py import ip, NetNS, NetNSEnter > +import errno > +import socket > +import subprocess > + > + > +IP_MTU_DISCOVER = 10 > +IP_PMTUDISC_PROBE = 3 > +IPV6_MTU_DISCOVER = 23 > +IPV6_PMTUDISC_DO = 2 > + > + > +def check_dmesg_clean(func) -> bool: > + dmesg = subprocess.Popen(['dmesg'], stdout=subprocess.PIPE) > + result = subprocess.run(['grep', '-q', f'WARNING:.*{func}'], stdin=dmesg.stdout) > + dmesg.wait() > + return result.returncode != 0 and dmesg.returncode == 0 > + > + > +def test_ipv6() -> None: > + with NetNS() as ns: > + ip('link add dummy type dummy', ns=ns) > + ip('link set dummy mtu 65576', ns=ns) > + ip('link set dummy up', ns=ns) > + ip('-6 addr add fd00::1/64 dev dummy nodad', ns=ns) > + ip('-6 neigh add fd00::2 lladdr 02:00:00:00:00:02 dev dummy nud permanent', ns=ns) > + > + with NetNSEnter(ns): > + with socket.socket(socket.AF_INET6, socket.SOCK_DGRAM) as fd: > + fd.setsockopt(socket.IPPROTO_IPV6, IPV6_MTU_DISCOVER, IPV6_PMTUDISC_DO) > + try: > + fd.sendto(b' ' * 65528, ('fd00::2', 1234)) > + except OSError as e: > + # Ignore EMSGSIZE: it happens on kernels with the fix. > + if e.errno != errno.EMSGSIZE: > + raise > + > + ip('link del dummy', ns=ns) > + > + ksft_true(check_dmesg_clean('udp_v6_send_skb'), 'WARNING detected in dmesg') > + > + > +def test_ipv4() -> None: > + with NetNS() as ns: > + ip('link add dummy type dummy', ns=ns) > + ip('link set dummy mtu 65556', ns=ns) > + ip('link set dummy up', ns=ns) > + ip('addr add 10.0.0.1/24 dev dummy', ns=ns) > + ip('neigh add 10.0.0.2 lladdr 02:00:00:00:00:02 dev dummy nud permanent', ns=ns) > + > + with NetNSEnter(ns): > + with socket.socket(socket.AF_INET, socket.SOCK_DGRAM) as fd: > + fd.setsockopt(socket.IPPROTO_IP, IP_MTU_DISCOVER, IP_PMTUDISC_PROBE) > + try: > + fd.sendto(b' ' * 65528, ('10.0.0.2', 1234)) > + except OSError as e: > + # Ignore EMSGSIZE: the check happens after the WARN is printed. > + if e.errno != errno.EMSGSIZE: > + raise > + > + ip('link del dummy', ns=ns) > + > + ksft_true(check_dmesg_clean('udp_send_skb'), 'WARNING detected in dmesg') > + > + > +if __name__ == "__main__": > + ksft_run([ > + test_ipv6, > + test_ipv4, > + ]) > + ksft_exit() > -- > 2.55.0 >