general protection fault in n_tty_flush_buffer

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    e6506eb24187 Merge tag 'trace-v4.17-rc4-2' of git://git.ke..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1295d637800000
kernel config:  https://syzkaller.appspot.com/x/.config?x=f3b4e30da84ec1ed
dashboard link: https://syzkaller.appspot.com/bug?extid=083228748980909eb453
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=1340a7c7800000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=15b25e37800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+083228748980909eb453@xxxxxxxxxxxxxxxxxxxxxxxxx

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] SMP KASAN
Dumping ftrace buffer:
   (ftrace buffer empty)
Modules linked in:
CPU: 1 PID: 4530 Comm: syz-executor147 Not tainted 4.17.0-rc5+ #54
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:reset_buffer_flags drivers/tty/n_tty.c:318 [inline]
RIP: 0010:n_tty_flush_buffer+0x64/0x320 drivers/tty/n_tty.c:358
RSP: 0018:ffff8801c60f77a0 EFLAGS: 00010202
RAX: dffffc0000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 000000000000044c RSI: ffff8801ae012ef8 RDI: 0000000000002260
RBP: ffff8801c60f77b8 R08: 0000000000007ad5 R09: 0000000000000002
R10: ffff8801ae012f48 R11: ffff8801ae0126c0 R12: ffff8801d96321c0
R13: ffff8801d96323f8 R14: 0000000000000000 R15: ffff8801c60f7808
FS:  00007f53b10c8700(0000) GS:ffff8801daf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f53b10a6e78 CR3: 00000001ac8cf000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 set_termios+0x390/0x7d0 drivers/tty/tty_ioctl.c:404
 tty_mode_ioctl+0x265/0xb50 drivers/tty/tty_ioctl.c:745
 n_tty_ioctl_helper+0x54/0x3b0 drivers/tty/tty_ioctl.c:940
 n_tty_ioctl+0x54/0x320 drivers/tty/n_tty.c:2441
 tty_ioctl+0x5e1/0x1870 drivers/tty/tty_io.c:2655
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:500 [inline]
 do_vfs_ioctl+0x1cf/0x16a0 fs/ioctl.c:684
 ksys_ioctl+0xa9/0xd0 fs/ioctl.c:701
 __do_sys_ioctl fs/ioctl.c:708 [inline]
 __se_sys_ioctl fs/ioctl.c:706 [inline]
 __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:706
 do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x445c19
RSP: 002b:00007f53b10c7da8 EFLAGS: 00000297 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00000000006dac24 RCX: 0000000000445c19
RDX: 0000000020000000 RSI: 0000000000005404 RDI: 0000000000000005
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000297 R12: 00000000006dac20
R13: 6d74702f7665642f R14: 00007f53b10c89c0 R15: 0000000000000007
Code: 03 80 3c 02 00 0f 85 b1 02 00 00 49 8b 9c 24 70 05 00 00 48 b8 00 00  
00 00 00 fc ff df 48 8d bb 60 22 00 00 48 89 fa 48 c1 ea 03 <80> 3c 02 00  
0f 85 9b 02 00 00 48 b8 00 00 00 00 00 fc ff df 48
RIP: reset_buffer_flags drivers/tty/n_tty.c:318 [inline] RSP:  
ffff8801c60f77a0
RIP: n_tty_flush_buffer+0x64/0x320 drivers/tty/n_tty.c:358 RSP:  
ffff8801c60f77a0
---[ end trace 75cd8a017cb978de ]---


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