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[34.48.4.250]) by smtp.gmail.com with ESMTPSA id 956f58d0204a3-6692c6e9bc7sm1337186d50.1.2026.07.30.07.10.50 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Jul 2026 07:10:50 -0700 (PDT) Date: Thu, 30 Jul 2026 10:10:48 -0400 From: Willem de Bruijn To: Willem de Bruijn , syzbot , davem@davemloft.net, edumazet@google.com, horms@kernel.org, kuba@kernel.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, pabeni@redhat.com, syzkaller-bugs@googlegroups.com, willemdebruijn.kernel@gmail.com, alice.kernel@fastmail.im Message-ID: In-Reply-To: References: <6a6a966c.86abc875.e5c3d.0054.GAE@google.com> Subject: Re: [syzbot] [net?] WARNING in udp_v6_send_skb Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Willem de Bruijn wrote: > syzbot wrote: > > Hello, > > > > syzbot found the following issue on: > > > > HEAD commit: 04026c998c24 net/rds: use krealloc_array() for iovector gr.. > > git tree: net-next > > console output: https://syzkaller.appspot.com/x/log.txt?x=163a6499580000 > > kernel config: https://syzkaller.appspot.com/x/.config?x=f9f592a4e0cc86da > > dashboard link: https://syzkaller.appspot.com/bug?extid=ce13c07d96d04716eaa2 > > compiler: Debian clang version 22.1.8 (++20260613092233+e80beda6e255-1~exp1~20260613092250.77), Debian LLD 22.1.8 > > > > Unfortunately, I don't have any reproducer for this issue yet. > > > > Downloadable assets: > > disk image: https://storage.googleapis.com/syzbot-assets/4f82b6352d02/disk-04026c99.raw.xz > > vmlinux: https://storage.googleapis.com/syzbot-assets/9b29e52c71b2/vmlinux-04026c99.xz > > kernel image: https://storage.googleapis.com/syzbot-assets/0b1e84e828d6/bzImage-04026c99.xz > > > > IMPORTANT: if you fix the issue, please add the following tag to the commit: > > Reported-by: syzbot+ce13c07d96d04716eaa2@syzkaller.appspotmail.com > > > > ------------[ cut here ]------------ > > len >= 65536u > > WARNING: ./include/linux/udp.h:49 at udp_set_len_short include/linux/udp.h:49 [inline], CPU#0: syz.4.4430/24120 > > WARNING: ./include/linux/udp.h:49 at udp_v6_send_skb+0xd3b/0x13a0 net/ipv6/udp.c:1375, CPU#0: syz.4.4430/24120 > > Modules linked in: > > CPU: 0 UID: 0 PID: 24120 Comm: syz.4.4430 Not tainted syzkaller #0 PREEMPT(full) > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/16/2026 > > RIP: 0010:udp_set_len_short include/linux/udp.h:49 [inline] > > RIP: 0010:udp_v6_send_skb+0xd3b/0x13a0 net/ipv6/udp.c:1375 > > Code: 42 0f b6 04 38 84 c0 0f 85 37 06 00 00 41 0f b7 1e e9 60 f3 ff ff e8 04 c3 43 f7 90 0f 0b 90 e9 02 f4 ff ff e8 f6 c2 43 f7 90 <0f> 0b 90 e9 e9 f4 ff ff e8 e8 c2 43 f7 90 0f 0b 90 e9 94 f5 ff ff > > RSP: 0018:ffffc9000e0a74c8 EFLAGS: 00010287 > > RAX: ffffffff8a833e8a RBX: ffff8880355e4a00 RCX: 0000000000080000 > > RDX: ffffc900100ca000 RSI: 00000000000049e3 RDI: 00000000000049e4 > > RBP: 000000000000244c R08: ffff888049ed5d00 R09: 0000000000000003 > > R10: 000000000000000b R11: 0000000000000002 R12: dffffc0000000000 > > R13: ffff888027cb0c80 R14: 0000000000010000 R15: ffff8880355e4a02 > > FS: 00007f2d4a07b6c0(0000) GS:ffff888124f54000(0000) knlGS:0000000000000000 > > CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 > > CR2: 0000200000010000 CR3: 0000000029d28000 CR4: 00000000003526f0 > > Call Trace: > > > > udp_v6_push_pending_frames+0xa2/0x140 net/ipv6/udp.c:1451 > > udpv6_sendmsg+0xfa0/0x2610 net/ipv6/udp.c:1740 > > sock_sendmsg_nosec+0x90/0x180 net/socket.c:775 > > __sock_sendmsg net/socket.c:790 [inline] > > ____sys_sendmsg+0x54e/0x850 net/socket.c:2684 > > ___sys_sendmsg+0x2a5/0x360 net/socket.c:2738 > > __sys_sendmmsg+0x273/0x4d0 net/socket.c:2827 > > __do_sys_sendmmsg net/socket.c:2854 [inline] > > __se_sys_sendmmsg net/socket.c:2851 [inline] > > __x64_sys_sendmmsg+0xa0/0xc0 net/socket.c:2851 > > do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] > > do_syscall_64+0x174/0x580 arch/x86/entry/syscall_64.c:94 > > entry_SYSCALL_64_after_hwframe+0x77/0x7f > > RIP: 0033:0x7f2d4919de99 > > Interesting trigger of the recently added DEBUG_NET_WARN_ON_ONCE in > udp_set_len_short called in udp_v6_send_skb. > > No reproducer yet, so probably not very actionable at this point. > A mostly straightforward looking ipv6 udp sendmsg path. Actually, this may just be a valid UDP_SEGMENT skb with max payload: IP6_MAX_MTU (0xFFFF + sizeof(struct ipv6hdr)) This is what __ip6_append_data checks on egress mtu = cork->gso_size ? IP6_MAX_MTU : cork->fragsize; Not sure whether UDP_SEGMENT can legitimately build even larger BIG-TCP variants of UDP_SEGMENT skbs at this point too today.