KASAN: global-out-of-bounds Read in bit_putcs

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit:    b9c5ef25 Add linux-next specific files for 20191218
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=17d57b46e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2eb13492323f151f
dashboard link: https://syzkaller.appspot.com/bug?extid=38a3699c7eaf165b97a6
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13ce1f2ee00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=125727dee00000

Bisection is inconclusive: the bug happens on the oldest tested release.

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=12caa5b6e00000
final crash:    https://syzkaller.appspot.com/x/report.txt?x=11caa5b6e00000
console output: https://syzkaller.appspot.com/x/log.txt?x=16caa5b6e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+38a3699c7eaf165b97a6 at syzkaller.appspotmail.com

==================================================================
BUG: KASAN: global-out-of-bounds in __fb_pad_aligned_buffer  
include/linux/fb.h:654 [inline]
BUG: KASAN: global-out-of-bounds in bit_putcs_aligned  
drivers/video/fbdev/core/bitblit.c:96 [inline]
BUG: KASAN: global-out-of-bounds in bit_putcs+0xd5d/0xf10  
drivers/video/fbdev/core/bitblit.c:185
Read of size 1 at addr ffffffff8872bb44 by task syz-executor093/14101

CPU: 1 PID: 14101 Comm: syz-executor093 Not tainted  
5.5.0-rc2-next-20191218-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS  
Google 01/01/2011
Call Trace:
  __dump_stack lib/dump_stack.c:77 [inline]
  dump_stack+0x197/0x210 lib/dump_stack.c:118
  print_address_description.constprop.0.cold+0x5/0x30b mm/kasan/report.c:374
  __kasan_report.cold+0x1b/0x41 mm/kasan/report.c:506
  kasan_report+0x12/0x20 mm/kasan/common.c:639
  __asan_report_load1_noabort+0x14/0x20 mm/kasan/generic_report.c:132
  __fb_pad_aligned_buffer include/linux/fb.h:654 [inline]
  bit_putcs_aligned drivers/video/fbdev/core/bitblit.c:96 [inline]
  bit_putcs+0xd5d/0xf10 drivers/video/fbdev/core/bitblit.c:185
  fbcon_putcs+0x33c/0x3e0 drivers/video/fbdev/core/fbcon.c:1353
  do_update_region+0x42b/0x6f0 drivers/tty/vt/vt.c:677
  redraw_screen+0x676/0x7d0 drivers/tty/vt/vt.c:1011
  fbcon_do_set_font+0x829/0x960 drivers/video/fbdev/core/fbcon.c:2605
  fbcon_copy_font+0x12c/0x190 drivers/video/fbdev/core/fbcon.c:2620
  con_font_copy drivers/tty/vt/vt.c:4594 [inline]
  con_font_op+0x6b2/0x1270 drivers/tty/vt/vt.c:4609
  vt_ioctl+0x181a/0x26d0 drivers/tty/vt/vt_ioctl.c:965
  tty_ioctl+0xa37/0x14f0 drivers/tty/tty_io.c:2660
  vfs_ioctl fs/ioctl.c:47 [inline]
  file_ioctl fs/ioctl.c:545 [inline]
  do_vfs_ioctl+0x977/0x14e0 fs/ioctl.c:732
  ksys_ioctl+0xab/0xd0 fs/ioctl.c:749
  __do_sys_ioctl fs/ioctl.c:756 [inline]
  __se_sys_ioctl fs/ioctl.c:754 [inline]
  __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:754
  do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
  entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x449c49
Code: e8 7c e6 ff ff 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7  
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff  
ff 0f 83 1b 05 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fa99f42ace8 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00000000006e6a38 RCX: 0000000000449c49
RDX: 0000000020000000 RSI: 0000000000004b72 RDI: 0000000000000004
RBP: 00000000006e6a30 R08: 00007fa99f42b700 R09: 0000000000000000
R10: 00007fa99f42b700 R11: 0000000000000246 R12: 00000000006e6a3c
R13: 00007ffe46ffe5df R14: 00007fa99f42b9c0 R15: 20c49ba5e353f7cf

The buggy address belongs to the variable:
  __func__.44397+0x104/0x1c0

Memory state around the buggy address:
  ffffffff8872ba00: 00 00 00 00 fa fa fa fa 00 03 fa fa fa fa fa fa
  ffffffff8872ba80: 00 01 fa fa fa fa fa fa 04 fa fa fa fa fa fa fa
> ffffffff8872bb00: 04 fa fa fa fa fa fa fa 04 fa fa fa fa fa fa fa
                                            ^
  ffffffff8872bb80: 04 fa fa fa fa fa fa fa 07 fa fa fa fa fa fa fa
  ffffffff8872bc00: 04 fa fa fa fa fa fa fa 00 00 01 fa fa fa fa fa
==================================================================