==================================================================
BUG: KASAN: use-after-free in soft_cursor+0x96a/0xa50 drivers/video/fbdev/core/softcursor.c:61
Read of size 1 at addr ffff8880b383caf0 by task kworker/1:4/9225

CPU: 1 PID: 9225 Comm: kworker/1:4 Not tainted 4.14.231-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events_power_efficient fb_flashcursor
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0x1b2/0x281 lib/dump_stack.c:58
 print_address_description.cold+0x54/0x1d3 mm/kasan/report.c:252
 kasan_report_error.cold+0x8a/0x191 mm/kasan/report.c:351
 kasan_report mm/kasan/report.c:409 [inline]
 __asan_report_load1_noabort+0x68/0x70 mm/kasan/report.c:427
 soft_cursor+0x96a/0xa50 drivers/video/fbdev/core/softcursor.c:61
 bit_cursor+0xf7a/0x1580 drivers/video/fbdev/core/bitblit.c:377
 fb_flashcursor+0x356/0x3f0 drivers/video/fbdev/core/fbcon.c:373
 process_one_work+0x793/0x14a0 kernel/workqueue.c:2116
 worker_thread+0x5cc/0xff0 kernel/workqueue.c:2250
 kthread+0x30d/0x420 kernel/kthread.c:232
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:404

Allocated by task 26010:
 save_stack mm/kasan/kasan.c:447 [inline]
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_kmalloc+0xeb/0x160 mm/kasan/kasan.c:551
 kmem_cache_alloc_trace+0x131/0x3d0 mm/slab.c:3618
 kmalloc include/linux/slab.h:488 [inline]
 kzalloc include/linux/slab.h:661 [inline]
 cdev_alloc+0x3d/0xd0 fs/char_dev.c:641
 tty_cdev_add+0x4a/0x280 drivers/tty/tty_io.c:2897
 tty_register_device_attr+0x400/0x710 drivers/tty/tty_io.c:3011
 gsmld_attach_gsm drivers/tty/n_gsm.c:2282 [inline]
 gsmld_open+0x548/0x7b0 drivers/tty/n_gsm.c:2408
 tty_ldisc_open+0x6c/0xb0 drivers/tty/tty_ldisc.c:481
 tty_set_ldisc+0x287/0x5d0 drivers/tty/tty_ldisc.c:606
 tiocsetd drivers/tty/tty_io.c:2351 [inline]
 tty_ioctl+0xa2a/0x13c0 drivers/tty/tty_io.c:2595
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:500 [inline]
 do_vfs_ioctl+0x75a/0xff0 fs/ioctl.c:684
 SYSC_ioctl fs/ioctl.c:701 [inline]
 SyS_ioctl+0x7f/0xb0 fs/ioctl.c:692
 do_syscall_64+0x1d5/0x640 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x46/0xbb

Freed by task 26007:
 save_stack mm/kasan/kasan.c:447 [inline]
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_slab_free+0xc3/0x1a0 mm/kasan/kasan.c:524
 __cache_free mm/slab.c:3496 [inline]
 kfree+0xc9/0x250 mm/slab.c:3815
 cdev_dynamic_release+0x3a/0x50 fs/char_dev.c:622
 kobject_cleanup lib/kobject.c:646 [inline]
 kobject_release lib/kobject.c:675 [inline]
 kref_put include/linux/kref.h:70 [inline]
 kobject_put+0x251/0x550 lib/kobject.c:692
 tty_unregister_device drivers/tty/tty_io.c:3046 [inline]
 tty_unregister_device+0x101/0x1a0 drivers/tty/tty_io.c:3041
 gsmld_detach_gsm drivers/tty/n_gsm.c:2303 [inline]
 gsmld_close+0xaa/0x1f0 drivers/tty/n_gsm.c:2372
 tty_ldisc_close+0x8c/0xc0 drivers/tty/tty_ldisc.c:505
 tty_ldisc_kill drivers/tty/tty_ldisc.c:651 [inline]
 tty_ldisc_release+0xe8/0x400 drivers/tty/tty_ldisc.c:818
 tty_release_struct+0x20/0xe0 drivers/tty/tty_io.c:1603
 tty_release+0xb3f/0x10d0 drivers/tty/tty_io.c:1776
 __fput+0x25f/0x7a0 fs/file_table.c:210
 task_work_run+0x11f/0x190 kernel/task_work.c:113
 tracehook_notify_resume include/linux/tracehook.h:191 [inline]
 exit_to_usermode_loop+0x1ad/0x200 arch/x86/entry/common.c:164
 prepare_exit_to_usermode arch/x86/entry/common.c:199 [inline]
 syscall_return_slowpath arch/x86/entry/common.c:270 [inline]
 do_syscall_64+0x4a3/0x640 arch/x86/entry/common.c:297
 entry_SYSCALL_64_after_hwframe+0x46/0xbb

The buggy address belongs to the object at ffff8880b383ca80
 which belongs to the cache kmalloc-128 of size 128
The buggy address is located 112 bytes inside of
 128-byte region [ffff8880b383ca80, ffff8880b383cb00)
The buggy address belongs to the page:
page:ffffea0002ce0f00 count:1 mapcount:0 mapping:ffff8880b383c000 index:0xffff8880b383ce40
flags: 0xfff00000000100(slab)
raw: 00fff00000000100 ffff8880b383c000 ffff8880b383ce40 0000000100000008
raw: ffffea0002875c20 ffffea00026b1060 ffff88813fe80640 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8880b383c980: fc fc fc fc fc fc fc fc fb fb fb fb fb fb fb fb
 ffff8880b383ca00: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
>ffff8880b383ca80: fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb fb
                                                             ^
 ffff8880b383cb00: fc fc fc fc fc fc fc fc 00 00 00 00 00 00 00 00
 ffff8880b383cb80: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
==================================================================