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[34.85.207.234]) by smtp.gmail.com with ESMTPSA id 00721157ae682-84caabb82ecsm13256917b3.30.2026.08.22.11.39.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 22 Aug 2026 11:39:43 -0700 (PDT) Date: Sat, 22 Aug 2026 14:39:42 -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 , Hannes Frederic Sowa , Vadim Fedorenko , netdev@vger.kernel.org, Alice Mikityanska , syzbot+ce13c07d96d04716eaa2@syzkaller.appspotmail.com Message-ID: In-Reply-To: <20260822115717.1161782-3-alice.kernel@fastmail.im> References: <20260822115717.1161782-1-alice.kernel@fastmail.im> <20260822115717.1161782-3-alice.kernel@fastmail.im> Subject: Re: [PATCH net v3 2/4] net: ipv6: 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 IP6_MAX_MTU to avoid a in v3 it only does so for UDP due to IPv6 jumbograms. > possible overflow of UDP length that triggers a WARN in > udp_set_len_short when setsockopt IPV6_MTU_DISCOVER is set to > IPV6_PMTUDISC_DO or IPV6_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. cork->base.fragsize will be > set to that MTU in ip6_setup_cork. > 2. Set IPV6_MTU_DISCOVER to IPV6_PMTUDISC_PROBE or IPV6_PMTUDISC_DO. It > lets maxnonfragsize be set to device MTU (cork->fragsize) in > __ip6_append_data, rather than to IP6_MAX_MTU. > 3. Send 65528 bytes of payload (+8 bytes of UDP header, +40 bytes of > IPv6 header). Device MTU allows it (it's only one byte bigger than > IP6_MAX_MTU, 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. > > The original overflow bug with IPv6 and IPV6_PMTUDISC_DO seems to > predate git history (verified reproduction on 2.6.21), was fixed later, > and then reappeared in commit 427faee167bc ("net: ipv6: introduce > ip6_dst_mtu_maybe_forward"), which is chosen as the Fixes tag here. The > overflow with IPV6_PMTUDISC_PROBE reproduces since its introduction in > commit 628a5c561890 ("[INET]: Add IP(V6)_PMTUDISC_RPOBE"). > > Fixes: 427faee167bc ("net: ipv6: introduce ip6_dst_mtu_maybe_forward") > 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: Codex:gpt-5.6-sol > Cc: Willem de Bruijn Reviewed-by: Willem de Bruijn