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Date:   Mon, 18 Sep 2023 03:33:32 -0700
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Subject: Re: [syzbot] [net?] WARNING in __ip6_append_data
From:   syzbot <syzbot+62cbf263225ae13ff153@syzkaller.appspotmail.com>
To:     bpf@vger.kernel.org, davem@davemloft.net, dhowells@redhat.com,
        dsahern@kernel.org, edumazet@google.com, kuba@kernel.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        pabeni@redhat.com, syzkaller-bugs@googlegroups.com
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Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
WARNING in __ip6_append_data

l2tp_ip6_sendmsg()
MAXPLEN
check 0 4100 65575 40, 4 65536
l2tp_ip6_sendmsg()
MAXPLEN
check 4100 4100 65575 40, 0 65536
------------[ cut here ]------------
WARNING: CPU: 0 PID: 5455 at net/ipv6/ip6_output.c:1812 __ip6_append_data.isra.0+0x1c6d/0x4900 net/ipv6/ip6_output.c:1812
Modules linked in:
CPU: 0 PID: 5455 Comm: syz-executor.0 Not tainted 6.5.0-syzkaller-11938-g65d6e954e378-dirty #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/04/2023
RIP: 0010:__ip6_append_data.isra.0+0x1c6d/0x4900 net/ipv6/ip6_output.c:1812
Code: c4 f6 ff ff e8 84 d4 97 f8 49 8d 44 24 ff 48 89 44 24 68 49 8d 6c 24 07 e9 ab f6 ff ff 4c 8b b4 24 90 01 00 00 e8 63 d4 97 f8 <0f> 0b 48 8b 44 24 10 45 89 f4 48 8d 98 74 02 00 00 e8 4d d4 97 f8
RSP: 0018:ffffc90004f373b8 EFLAGS: 00010293
RAX: 0000000000000000 RBX: 0000000000001004 RCX: 0000000000000000
RDX: ffff888019e8bb80 RSI: ffffffff88efcf9d RDI: 0000000000000006
RBP: 0000000000001000 R08: 0000000000000006 R09: 0000000000001004
R10: 0000000000001000 R11: 0000000000000001 R12: 0000000000000001
R13: dffffc0000000000 R14: 0000000000001004 R15: ffff888027b1d640
FS:  00007feae40ff6c0(0000) GS:ffff8880b9800000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f0f01e4e378 CR3: 000000007d467000 CR4: 00000000003506f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 <TASK>
 ip6_append_data+0x1e6/0x510 net/ipv6/ip6_output.c:1909
 l2tp_ip6_sendmsg+0xe0c/0x1ce0 net/l2tp/l2tp_ip6.c:633
 inet_sendmsg+0x9d/0xe0 net/ipv4/af_inet.c:840
 sock_sendmsg_nosec net/socket.c:730 [inline]
 sock_sendmsg+0xd9/0x180 net/socket.c:753
 splice_to_socket+0xade/0x1010 fs/splice.c:881
 do_splice_from fs/splice.c:933 [inline]
 direct_splice_actor+0x118/0x180 fs/splice.c:1142
 splice_direct_to_actor+0x347/0xa30 fs/splice.c:1088
 do_splice_direct+0x1af/0x280 fs/splice.c:1194
 do_sendfile+0xb88/0x1390 fs/read_write.c:1254
 __do_sys_sendfile64 fs/read_write.c:1322 [inline]
 __se_sys_sendfile64 fs/read_write.c:1308 [inline]
 __x64_sys_sendfile64+0x1d6/0x220 fs/read_write.c:1308
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x38/0xb0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd
RIP: 0033:0x7feae347cae9
Code: 28 00 00 00 75 05 48 83 c4 28 c3 e8 e1 20 00 00 90 48 89 f8 48 89 f7 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 b0 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007feae40ff0c8 EFLAGS: 00000246 ORIG_RAX: 0000000000000028
RAX: ffffffffffffffda RBX: 00007feae359bf80 RCX: 00007feae347cae9
RDX: 0000000000000000 RSI: 0000000000000004 RDI: 0000000000000005
RBP: 00007feae34c847a R08: 0000000000000000 R09: 0000000000000000
R10: 000000010000a006 R11: 0000000000000246 R12: 0000000000000000
R13: 000000000000000b R14: 00007feae359bf80 R15: 00007ffc444d03c8
 </TASK>


Tested on:

commit:         65d6e954 Merge tag 'gfs2-v6.5-rc5-fixes' of git://git...
git tree:       git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
console output: https://syzkaller.appspot.com/x/log.txt?x=12133ac4680000
kernel config:  https://syzkaller.appspot.com/x/.config?x=b273cdfbc13e9a4b
dashboard link: https://syzkaller.appspot.com/bug?extid=62cbf263225ae13ff153
compiler:       gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
patch:          https://syzkaller.appspot.com/x/patch.diff?x=139cae54680000