bisecting fixing commit since a844dc4c544291470aa69edbe2434b040794e269
building syzkaller on eef6e5808d6507716d331b9eff67fdd991be891a
testing commit a844dc4c544291470aa69edbe2434b040794e269 with gcc (GCC) 8.1.0
kernel signature: 25f379f0b1dd639642edfc4e8fbd1d6901f0c377e50ba7295fe1092437a0d65b
all runs: crashed: KASAN: use-after-free Read in fbcon_cursor
testing current HEAD d71f695ce745df9544a85d8a762f16d72e72df00
testing commit d71f695ce745df9544a85d8a762f16d72e72df00 with gcc (GCC) 8.1.0
kernel signature: f416d5b5a67ca8bcf8c78fec92189db6709e85d8af05b8df21a1e66db0b5a544
all runs: crashed: KASAN: use-after-free Read in fbcon_cursor
revisions tested: 2, total time: 27m45.239016439s (build: 19m22.658322914s, test: 6m55.580128667s)
the crash still happens on HEAD
commit msg: Linux 4.14.179
crash: KASAN: use-after-free Read in fbcon_cursor
IPv6: ADDRCONF(NETDEV_CHANGE): vxcan0: link becomes ready
8021q: adding VLAN 0 to HW filter on device batadv0
==================================================================
BUG: KASAN: use-after-free in fbcon_cursor+0x583/0x6c0 drivers/video/fbdev/core/fbcon.c:1328
Read of size 2 at addr ffff88808ddb548c by task syz-executor.1/10583

CPU: 1 PID: 10583 Comm: syz-executor.1 Not tainted 4.14.179-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:17 [inline]
 dump_stack+0xf7/0x13b lib/dump_stack.c:58
 print_address_description.cold.7+0x9/0x1c9 mm/kasan/report.c:252
 kasan_report_error mm/kasan/report.c:351 [inline]
 kasan_report.cold.8+0x11a/0x2d3 mm/kasan/report.c:409
 __asan_report_load2_noabort+0x14/0x20 mm/kasan/report.c:428
 fbcon_cursor+0x583/0x6c0 drivers/video/fbdev/core/fbcon.c:1328
 fbcon_scrolldelta+0x521/0xff0 drivers/video/fbdev/core/fbcon.c:2835
 fbcon_set_origin+0x16/0x20 drivers/video/fbdev/core/fbcon.c:2886
 set_origin+0x2b7/0x390 drivers/tty/vt/vt.c:614
 vc_do_resize+0x652/0xfa0 drivers/tty/vt/vt.c:947
 vc_resize+0x3d/0x60 drivers/tty/vt/vt.c:987
 vt_ioctl+0x32d/0x21f0 drivers/tty/vt/vt_ioctl.c:850
 tty_ioctl+0x434/0x1260 drivers/tty/tty_io.c:2661
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:500 [inline]
 do_vfs_ioctl+0x180/0xfb0 fs/ioctl.c:684
 SYSC_ioctl fs/ioctl.c:701 [inline]
 SyS_ioctl+0x74/0x80 fs/ioctl.c:692
 do_syscall_64+0x1c7/0x5b0 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x45a909
RSP: 002b:00007fad5b2bbc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 000000000045a909
RDX: 0000000020000000 RSI: 0000000000005609 RDI: 0000000000000003
RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fad5b2bc6d4
R13: 00000000004c7009 R14: 00000000004dd670 R15: 00000000ffffffff

Allocated by task 10439:
 save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
 save_stack+0x43/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_kmalloc+0xc7/0xe0 mm/kasan/kasan.c:551
 __do_kmalloc mm/slab.c:3720 [inline]
 __kmalloc+0x15b/0x7b0 mm/slab.c:3729
 kmalloc include/linux/slab.h:493 [inline]
 kzalloc include/linux/slab.h:661 [inline]
 vc_do_resize+0x1b6/0xfa0 drivers/tty/vt/vt.c:885
 vc_resize+0x3d/0x60 drivers/tty/vt/vt.c:987
 vt_ioctl+0x32d/0x21f0 drivers/tty/vt/vt_ioctl.c:850
 tty_ioctl+0x434/0x1260 drivers/tty/tty_io.c:2661
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:500 [inline]
 do_vfs_ioctl+0x180/0xfb0 fs/ioctl.c:684
 SYSC_ioctl fs/ioctl.c:701 [inline]
 SyS_ioctl+0x74/0x80 fs/ioctl.c:692
 do_syscall_64+0x1c7/0x5b0 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x42/0xb7

Freed by task 10583:
 save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
 save_stack+0x43/0xd0 mm/kasan/kasan.c:447
 set_track mm/kasan/kasan.c:459 [inline]
 kasan_slab_free+0x71/0xc0 mm/kasan/kasan.c:524
 __cache_free mm/slab.c:3496 [inline]
 kfree+0xcc/0x270 mm/slab.c:3815
 vc_do_resize+0x5ef/0xfa0 drivers/tty/vt/vt.c:944
 vc_resize+0x3d/0x60 drivers/tty/vt/vt.c:987
 vt_ioctl+0x32d/0x21f0 drivers/tty/vt/vt_ioctl.c:850
 tty_ioctl+0x434/0x1260 drivers/tty/tty_io.c:2661
 vfs_ioctl fs/ioctl.c:46 [inline]
 file_ioctl fs/ioctl.c:500 [inline]
 do_vfs_ioctl+0x180/0xfb0 fs/ioctl.c:684
 SYSC_ioctl fs/ioctl.c:701 [inline]
 SyS_ioctl+0x74/0x80 fs/ioctl.c:692
 do_syscall_64+0x1c7/0x5b0 arch/x86/entry/common.c:292
 entry_SYSCALL_64_after_hwframe+0x42/0xb7

The buggy address belongs to the object at ffff88808ddb5480
 which belongs to the cache kmalloc-32 of size 32
The buggy address is located 12 bytes inside of
 32-byte region [ffff88808ddb5480, ffff88808ddb54a0)
The buggy address belongs to the page:
page:ffffea0002376d40 count:1 mapcount:0 mapping:ffff88808ddb5000 index:0xffff88808ddb5fc1
flags: 0xfffe0000000100(slab)
raw: 00fffe0000000100 ffff88808ddb5000 ffff88808ddb5fc1 000000010000003f
raw: ffffea000241d7a0 ffffea00023ece20 ffff8880aa8001c0 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88808ddb5380: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
 ffff88808ddb5400: 00 00 fc fc fc fc fc fc 00 06 fc fc fc fc fc fc
>ffff88808ddb5480: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
                      ^
 ffff88808ddb5500: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
 ffff88808ddb5580: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
==================================================================