general protection fault in fbcon_cursor

[syzbot <syzbot+6acf28c23c81badd89a7@syzkaller.appspotmail.com>]

Hello,

syzbot found the following crash on:

HEAD commit:    ae4b064e Merge tag 'afs-fixes-20191211' of git://git.kerne..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x\x1218c1dee00000
kernel config:  https://syzkaller.appspot.com/x/.config?xyf79de2a27d3e3d
dashboard link: https://syzkaller.appspot.com/bug?extidjcf28c23c81badd89a7
compiler:       gcc (GCC) 9.0.0 20181231 (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+6acf28c23c81badd89a7@syzkaller.appspotmail.com

kasan: CONFIG_KASAN_INLINE enabled
kasan: GPF could be caused by NULL-ptr deref or user memory access
general protection fault: 0000 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 17 Comm: kworker/1:0 Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Workqueue: events console_callback
RIP: 0010:fbcon_cursor+0x114/0x660 drivers/video/fbdev/core/fbcon.c:1380
Code: 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 e6 04 00 00 4d 8b b4 24 a0 03  
00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 f2 48 c1 ea 03 <0f> b6 14 02 4c  
89 f0 83 e0 07 83 c0 01 38 d0 7c 08 84 d2 0f 85 ba
RSP: 0018:ffffc90000d8fb00 EFLAGS: 00010206
RAX: dffffc0000000000 RBX: ffff8880a4309400 RCX: ffffffff83e01590
RDX: 000000001fffffe7 RSI: ffffffff83b2804c RDI: ffff8880a282b3a0
RBP: ffffc90000d8fb40 R08: ffff8880a9a4a480 R09: ffffed10147a3e1c
R10: ffffed10147a3e1b R11: ffff8880a3d1f0df R12: ffff8880a282b000
R13: ffff888218c76000 R14: 00000000ffffff3a R15: ffff888218c76468
FS:  0000000000000000(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000004bf9b0 CR3: 000000008e75d000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
  set_cursor drivers/tty/vt/vt.c:908 [inline]
  set_cursor+0x1fb/0x280 drivers/tty/vt/vt.c:899
  redraw_screen+0x4e1/0x7d0 drivers/tty/vt/vt.c:1013
  complete_change_console+0x105/0x3a0 drivers/tty/vt/vt_ioctl.c:1264
  change_console+0x19b/0x2c0 drivers/tty/vt/vt_ioctl.c:1389
  console_callback+0x3a1/0x400 drivers/tty/vt/vt.c:2824
  process_one_work+0x9af/0x1740 kernel/workqueue.c:2264
  worker_thread+0x98/0xe40 kernel/workqueue.c:2410
  kthread+0x361/0x430 kernel/kthread.c:255
  ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
Modules linked in:
---[ end trace a825343a1e8757e1 ]---
RIP: 0010:fbcon_cursor+0x114/0x660 drivers/video/fbdev/core/fbcon.c:1380
Code: 48 89 fa 48 c1 ea 03 80 3c 02 00 0f 85 e6 04 00 00 4d 8b b4 24 a0 03  
00 00 48 b8 00 00 00 00 00 fc ff df 4c 89 f2 48 c1 ea 03 <0f> b6 14 02 4c  
89 f0 83 e0 07 83 c0 01 38 d0 7c 08 84 d2 0f 85 ba
RSP: 0018:ffffc90000d8fb00 EFLAGS: 00010206
RAX: dffffc0000000000 RBX: ffff8880a4309400 RCX: ffffffff83e01590
RDX: 000000001fffffe7 RSI: ffffffff83b2804c RDI: ffff8880a282b3a0
RBP: ffffc90000d8fb40 R08: ffff8880a9a4a480 R09: ffffed10147a3e1c
R10: ffffed10147a3e1b R11: ffff8880a3d1f0df R12: ffff8880a282b000
R13: ffff888218c76000 R14: 00000000ffffff3a R15: ffff888218c76468
FS:  0000000000000000(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00000000004bf9b0 CR3: 000000008e75d000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


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