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[34.85.207.234]) by smtp.gmail.com with ESMTPSA id 00721157ae682-84caacaf1e9sm13253657b3.23.2026.08.22.11.35.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sat, 22 Aug 2026 11:35:59 -0700 (PDT) Date: Sat, 22 Aug 2026 14:35:59 -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: <20260822115717.1161782-2-alice.kernel@fastmail.im> References: <20260822115717.1161782-1-alice.kernel@fastmail.im> <20260822115717.1161782-2-alice.kernel@fastmail.im> Subject: Re: [PATCH net v3 1/4] net: ipv4: 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_MAX_MTU to avoid a > possible overflow of UDP length that triggers a WARN in > udp_set_len_short when setsockopt IP_MTU_DISCOVER is set to > IP_PMTUDISC_PROBE, and a large packet is sent over a netdev with an > unusually large MTU. > > Steps to reproduce: > > 1. Set device MTU bigger than IP_MAX_MTU + 20. cork->base.fragsize will > be set to that MTU in ip_setup_cork. > 2. Set IP_MTU_DISCOVER to IP_PMTUDISC_PROBE. It lets maxnonfragsize be > set to device MTU (cork->fragsize) in __ip_append_data, rather than > to IP_MAX_MTU. > 3. Send 65528 bytes of payload (+8 bytes of UDP header, +20 bytes of > IPv4 header). Device MTU allows it (it's only one byte bigger than > 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. > > 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. > > The Fixes tag points at the first commit where I could reproduce the > overflow with IPv4 and IP_PMTUDISC_PROBE. > > Fixes: daba287b299e ("ipv4: fix DO and PROBE pmtu mode regarding local fragmentation with UFO/CORK") > 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 Reviewed-by: Willem de Bruijn