general protection fault in n_tty_set_termios

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    8188fc8bef8c Merge  
git://git.kernel.org/pub/scm/linux/kerne...
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?id=5093449355231232
kernel config:   
https://syzkaller.appspot.com/x/.config?id=6493557782959164711
dashboard link: https://syzkaller.appspot.com/bug?extid=ed02be0ad5f26ef4e31b
compiler:       gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller  
repro:https://syzkaller.appspot.com/x/repro.syz?id=6543533393575936
C reproducer:   https://syzkaller.appspot.com/x/repro.c?id=5754063643738112

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+ed02be0ad5f26ef4e31b@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: 4509 Comm: syz-executor654 Not tainted 4.17.0-rc3+ #26
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
RIP: 0010:n_tty_set_termios+0x2d9/0xe80 drivers/tty/n_tty.c:1782
RSP: 0018:ffff8801b42df698 EFLAGS: 00010203
RAX: 0000000000000001 RBX: 0000000000000000 RCX: 000000000000000b
RDX: dffffc0000000000 RSI: 0000000000000000 RDI: 0000000000000005
RBP: ffff8801b42df6d0 R08: ffff8801d97aa000 R09: 0000000000000002
R10: ffff8801d97aa888 R11: ffff8801d97aa000 R12: ffff8801d9bea500
R13: ffff8801d9bea8b4 R14: 000000000000005d R15: ffff8801b42df730
FS:  00007f082d3d7700(0000) GS:ffff8801daf00000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f082d3b5e78 CR3: 00000001ac8aa000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
 tty_set_termios+0x7a0/0xac0 drivers/tty/tty_ioctl.c:341
 set_termios+0x41e/0x7d0 drivers/tty/tty_ioctl.c:414
 tty_mode_ioctl+0x855/0xb50 drivers/tty/tty_ioctl.c:749
 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: 0033:0x445d19
RSP: 002b:00007f082d3d6da8 EFLAGS: 00000297 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00000000006dac3c RCX: 0000000000445d19
RDX: 0000000020000040 RSI: 0000000000005402 RDI: 0000000000000033
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000297 R12: 00000000006dac38
R13: 6d74702f7665642f R14: 00007f082d3d79c0 R15: 0000000000000007
Code: 8b 45 d0 31 ff 83 e0 02 89 c6 89 45 d0 e8 50 4a e1 fd 8b 45 d0 4c 89  
f1 48 ba 00 00 00 00 00 fc ff df 85 c0 0f 95 c0 48 c1 e9 03 <0f> b6 14 11  
4c 89 f1 83 e1 07 38 ca 7f 08 84 d2 0f 85 96 09 00
RIP: n_tty_set_termios+0x2d9/0xe80 drivers/tty/n_tty.c:1782 RSP:  
ffff8801b42df698
---[ end trace b89be7398398fc5c ]---


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