WARNING in sctp_assoc_update_frag_point

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    6f0d349d922b Merge git://git.kernel.org/pub/scm/linux/kern..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12a423c0400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=a63be0c83e84d370
dashboard link: https://syzkaller.appspot.com/bug?extid=f0d9d7cba052f9344b03
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+f0d9d7cba052f9344b03@xxxxxxxxxxxxxxxxxxxxxxxxx

WARNING: CPU: 0 PID: 22543 at include/net/sctp/sctp.h:598 sctp_mtu_payload  
include/net/sctp/sctp.h:598 [inline]
WARNING: CPU: 0 PID: 22543 at include/net/sctp/sctp.h:598  
sctp_assoc_update_frag_point+0x252/0x2c0 net/sctp/associola.c:1401
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 22543 Comm: syz-executor2 Not tainted 4.18.0-rc2+ #117
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x1c9/0x2b4 lib/dump_stack.c:113
 panic+0x238/0x4e7 kernel/panic.c:184
 __warn.cold.8+0x163/0x1ba kernel/panic.c:536
 report_bug+0x252/0x2d0 lib/bug.c:186
 fixup_bug arch/x86/kernel/traps.c:178 [inline]
 do_error_trap+0x1fc/0x4d0 arch/x86/kernel/traps.c:296
 do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:316
 invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:992
RIP: 0010:sctp_mtu_payload include/net/sctp/sctp.h:598 [inline]
RIP: 0010:sctp_assoc_update_frag_point+0x252/0x2c0 net/sctp/associola.c:1401
Code: 76 fa 45 39 e5 76 1e e8 0c 69 76 fa 45 29 e5 45 89 ec e9 34 ff ff ff  
e8 fc 68 76 fa 45 8d 66 34 e9 09 ff ff ff e8 ee 68 76 fa <0f> 0b 45 31 e4  
e9 17 ff ff ff e8 7f 3c b4 fa e9 31 fe ff ff 4c 89
RSP: 0018:ffff8801d7def378 EFLAGS: 00010216
RAX: 0000000000040000 RBX: ffff8801d8580ac0 RCX: ffffc900133ca000
RDX: 00000000000001b9 RSI: ffffffff8705a382 RDI: 0000000000000004
RBP: ffff8801d7def3a0 R08: ffff8801cfaa6000 R09: ffffed002e0421af
R10: ffffed002e0421af R11: ffff880170210d7f R12: 0000000000000044
R13: 0000000000000044 R14: 0000000000000010 R15: ffff8801d8580ac0
 sctp_assoc_set_pmtu net/sctp/associola.c:1417 [inline]
 sctp_assoc_sync_pmtu+0x251/0x2e0 net/sctp/associola.c:1445
 sctp_sendmsg_to_asoc+0x1851/0x21a0 net/sctp/socket.c:1921
 sctp_sendmsg+0x13c2/0x1d90 net/sctp/socket.c:2124
 inet_sendmsg+0x1a1/0x690 net/ipv4/af_inet.c:798
 sock_sendmsg_nosec net/socket.c:645 [inline]
 sock_sendmsg+0xd5/0x120 net/socket.c:655
 ___sys_sendmsg+0x51d/0x930 net/socket.c:2161
 __sys_sendmmsg+0x240/0x6f0 net/socket.c:2256
 __do_sys_sendmmsg net/socket.c:2285 [inline]
 __se_sys_sendmmsg net/socket.c:2282 [inline]
 __x64_sys_sendmmsg+0x9d/0x100 net/socket.c:2282
 do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x455a99
Code: 1d ba fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 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 0f 83 eb b9 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f75b488ec68 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
RAX: ffffffffffffffda RBX: 00007f75b488f6d4 RCX: 0000000000455a99
RDX: 0000000000000001 RSI: 0000000020005900 RDI: 0000000000000014
RBP: 000000000072bff0 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 00000000004c0b1b R14: 00000000004d0938 R15: 0000000000000002
Dumping ftrace buffer:
   (ftrace buffer empty)
Kernel Offset: disabled
Rebooting in 86400 seconds..


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