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SubjectRe: KASAN: slab-out-of-bounds Read in bit_putcs
From
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=16b1d1bce00000
kernel config: https://syzkaller.appspot.com/x/.config?x=7a3b8f5088d4043a
dashboard link: https://syzkaller.appspot.com/bug?extid=998dec6452146bd7a90c
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=12fa5c2ee00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=12e327f2e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+998dec6452146bd7a90c@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: slab-out-of-bounds in __fb_pad_aligned_buffer
include/linux/fb.h:655 [inline]
BUG: KASAN: slab-out-of-bounds in bit_putcs_aligned
drivers/video/fbdev/core/bitblit.c:96 [inline]
BUG: KASAN: slab-out-of-bounds in bit_putcs+0xd5d/0xf10
drivers/video/fbdev/core/bitblit.c:185
Read of size 1 at addr ffff8880a66ec808 by task syz-executor523/8819

CPU: 1 PID: 8819 Comm: syz-executor523 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+0xd4/0x30b 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]
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: 0033:0x446a79
Code: e8 ec e7 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 3b 07 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f451fcd2d08 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00000000006dbc58 RCX: 0000000000446a79
RDX: 0000000020000180 RSI: 0000000000004b72 RDI: 0000000000000003
RBP: 00000000006dbc50 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006dbc5c
R13: 0000000000000000 R14: 00000000f72a8fce R15: 0000000000000000

Allocated by task 8802:
save_stack+0x23/0x90 mm/kasan/common.c:72
set_track mm/kasan/common.c:80 [inline]
__kasan_kmalloc mm/kasan/common.c:513 [inline]
__kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:486
kasan_kmalloc+0x9/0x10 mm/kasan/common.c:527
__do_kmalloc mm/slab.c:3656 [inline]
__kmalloc+0x163/0x770 mm/slab.c:3665
kmalloc include/linux/slab.h:561 [inline]
fbcon_set_font+0x32d/0x860 drivers/video/fbdev/core/fbcon.c:2663
con_font_set drivers/tty/vt/vt.c:4538 [inline]
con_font_op+0xe30/0x1270 drivers/tty/vt/vt.c:4603
vt_ioctl+0xd2e/0x26d0 drivers/tty/vt/vt_ioctl.c:913
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

Freed by task 8528:
save_stack+0x23/0x90 mm/kasan/common.c:72
set_track mm/kasan/common.c:80 [inline]
kasan_set_free_info mm/kasan/common.c:335 [inline]
__kasan_slab_free+0x102/0x150 mm/kasan/common.c:474
kasan_slab_free+0xe/0x10 mm/kasan/common.c:483
__cache_free mm/slab.c:3426 [inline]
kfree+0x10a/0x2c0 mm/slab.c:3757
kvfree+0x61/0x70 mm/util.c:603
__free_fdtable+0x34/0x80 fs/file.c:31
put_files_struct fs/file.c:420 [inline]
put_files_struct+0x253/0x2f0 fs/file.c:413
exit_files+0x83/0xb0 fs/file.c:445
do_exit+0x8b5/0x2ef0 kernel/exit.c:792
do_group_exit+0x135/0x360 kernel/exit.c:895
__do_sys_exit_group kernel/exit.c:906 [inline]
__se_sys_exit_group kernel/exit.c:904 [inline]
__x64_sys_exit_group+0x44/0x50 kernel/exit.c:904
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff8880a66ec000
which belongs to the cache kmalloc-2k of size 2048
The buggy address is located 8 bytes to the right of
2048-byte region [ffff8880a66ec000, ffff8880a66ec800)
The buggy address belongs to the page:
page:ffffea000299bb00 refcount:1 mapcount:0 mapping:ffff8880aa400e00
index:0x0
raw: 00fffe0000000200 ffffea000299bac8 ffffea000299bb48 ffff8880aa400e00
raw: 0000000000000000 ffff8880a66ec000 0000000100000001 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8880a66ec700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8880a66ec780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
> ffff8880a66ec800: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
^
ffff8880a66ec880: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8880a66ec900: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================

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