Re: KASAN: global-out-of-bounds Read in fb_pad_aligned_buffer

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit:    9455d25f Merge tag 'ntb-5.5' of git://github.com/jonmason/..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=171e09dae00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=7a3b8f5088d4043a
dashboard link: https://syzkaller.appspot.com/bug?extid=0568d05e486eee0a1ba2
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=15ddeca6e00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=16df9e41e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+0568d05e486eee0a1ba2@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: global-out-of-bounds in __fb_pad_aligned_buffer  
include/linux/fb.h:655 [inline]
BUG: KASAN: global-out-of-bounds in fb_pad_aligned_buffer+0x138/0x160  
drivers/video/fbdev/core/fbmem.c:115
Read of size 1 at addr ffffffff8872a77c by task syz-executor208/9953

CPU: 1 PID: 9953 Comm: syz-executor208 Not tainted 5.4.0-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:655 [inline]
 fb_pad_aligned_buffer+0x138/0x160 drivers/video/fbdev/core/fbmem.c:115
 bit_putcs_aligned drivers/video/fbdev/core/bitblit.c:99 [inline]
 bit_putcs+0xd14/0xf10 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: 0033:0x440269
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 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 5b 14 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007fff65832458 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 0000000000440269
RDX: 0000000020000000 RSI: 0000000000004b72 RDI: 0000000000000004
RBP: 00000000006cb018 R08: 000000000000000d R09: 00000000004002c8
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000401b50
R13: 0000000000401be0 R14: 0000000000000000 R15: 0000000000000000

The buggy address belongs to the variable:
 oid_index+0x93c/0xb80

Memory state around the buggy address:
 ffffffff8872a600: 04 fa fa fa fa fa fa fa 00 01 fa fa fa fa fa fa
 ffffffff8872a680: 04 fa fa fa fa fa fa fa 04 fa fa fa fa fa fa fa
> ffffffff8872a700: 07 fa fa fa fa fa fa fa 04 fa fa fa fa fa fa fa
                                                                ^
 ffffffff8872a780: 05 fa fa fa fa fa fa fa 01 fa fa fa fa fa fa fa
 ffffffff8872a800: 00 00 02 fa fa fa fa fa 00 00 00 05 fa fa fa fa
==================================================================