bisecting fixing commit since a844dc4c544291470aa69edbe2434b040794e269
building syzkaller on eef6e5808d6507716d331b9eff67fdd991be891a
testing commit a844dc4c544291470aa69edbe2434b040794e269 with gcc (GCC) 8.1.0
kernel signature: 6a87e6dbf62a99c7fd703a4baf4cc4d453afb79494f26f9ab06466dc6fe75865
all runs: crashed: KASAN: use-after-free Read in fbcon_cursor
testing current HEAD e0f8b8a65a473a8baa439cf865a694bbeb83fe90
testing commit e0f8b8a65a473a8baa439cf865a694bbeb83fe90 with gcc (GCC) 8.1.0
kernel signature: ab1df693fe97e0c7cf1fbea25517ee708cbae7f815d21f24c27b4e14156ddae3
all runs: crashed: KASAN: use-after-free Read in fbcon_cursor
revisions tested: 2, total time: 24m29.366036731s (build: 16m33.413786313s, test: 7m22.789900353s)
the crash still happens on HEAD
commit msg: Linux 4.14.170
crash: KASAN: use-after-free Read in fbcon_cursor
==================================================================
BUG: KASAN: use-after-free in fbcon_cursor+0x583/0x6c0 drivers/video/fbdev/core/fbcon.c:1325
Read of size 2 at addr ffff8880a8a315cc by task syz-executor.0/13841

CPU: 0 PID: 13841 Comm: syz-executor.0 Not tainted 4.14.170-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:1325
 fbcon_scrolldelta+0x521/0xff0 drivers/video/fbdev/core/fbcon.c:2832
 fbcon_set_origin+0x16/0x20 drivers/video/fbdev/core/fbcon.c:2883
 set_origin+0x2b7/0x390 drivers/tty/vt/vt.c:613
 vc_do_resize+0x68f/0x1010 drivers/tty/vt/vt.c:934
 vc_resize+0x3d/0x60 drivers/tty/vt/vt.c:974
 vt_ioctl+0x34c/0x21d0 drivers/tty/vt/vt_ioctl.c:840
 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:00007f7c12a91c78 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: 00007f7c12a926d4
R13: 00000000004c7009 R14: 00000000004dd670 R15: 00000000ffffffff

Allocated by task 13821:
 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+0x1bb/0x1010 drivers/tty/vt/vt.c:872
 vc_resize+0x3d/0x60 drivers/tty/vt/vt.c:974
 vt_ioctl+0x34c/0x21d0 drivers/tty/vt/vt_ioctl.c:840
 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 13841:
 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+0x62a/0x1010 drivers/tty/vt/vt.c:931
 vc_resize+0x3d/0x60 drivers/tty/vt/vt.c:974
 vt_ioctl+0x34c/0x21d0 drivers/tty/vt/vt_ioctl.c:840
 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 ffff8880a8a315c0
 which belongs to the cache kmalloc-32 of size 32
The buggy address is located 12 bytes inside of
 32-byte region [ffff8880a8a315c0, ffff8880a8a315e0)
The buggy address belongs to the page:
page:ffffea0002a28c40 count:1 mapcount:0 mapping:ffff8880a8a31000 index:0xffff8880a8a31fc1
flags: 0xfffe0000000100(slab)
raw: 00fffe0000000100 ffff8880a8a31000 ffff8880a8a31fc1 000000010000003f
raw: ffffea0002637720 ffffea0002a16c20 ffff8880aa8001c0 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8880a8a31480: fb fb fb fb fc fc fc fc 00 00 02 fc fc fc fc fc
 ffff8880a8a31500: 00 00 03 fc fc fc fc fc fb fb fb fb fc fc fc fc
>ffff8880a8a31580: 00 06 fc fc fc fc fc fc fb fb fb fb fc fc fc fc
                                              ^
 ffff8880a8a31600: fb fb fb fb fc fc fc fc 07 fc fc fc fc fc fc fc
 ffff8880a8a31680: 00 00 01 fc fc fc fc fc 00 05 fc fc fc fc fc fc
==================================================================