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SubjectKASAN: slab-out-of-bounds Read in vcs_scr_readw
From
Hello,

syzbot found the following crash on:

HEAD commit: 596cf45c Merge branch 'akpm' (patches from Andrew)
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14c1d196e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=8eb54eee6e6ca4a7
dashboard link: https://syzkaller.appspot.com/bug?extid=7d027845265d531ba506
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=11c6090ee00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=13821b7ae00000

The bug was bisected to:

commit d21b0be246bf3bbf569e6e239f56abb529c7154e
Author: Nicolas Pitre <nicolas.pitre@linaro.org>
Date: Wed Jun 27 03:56:41 2018 +0000

vt: introduce unicode mode for /dev/vcs

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=1292fbf2e00000
final crash: https://syzkaller.appspot.com/x/report.txt?x=1192fbf2e00000
console output: https://syzkaller.appspot.com/x/log.txt?x=1692fbf2e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+7d027845265d531ba506@syzkaller.appspotmail.com
Fixes: d21b0be246bf ("vt: introduce unicode mode for /dev/vcs")

==================================================================
BUG: KASAN: slab-out-of-bounds in vcs_scr_readw+0xc2/0xd0
drivers/tty/vt/vt.c:4665
Read of size 2 at addr ffff8882192c52c0 by task syz-executor391/9679

CPU: 0 PID: 9679 Comm: syz-executor391 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:638
__asan_report_load2_noabort+0x14/0x20 mm/kasan/generic_report.c:133
vcs_scr_readw+0xc2/0xd0 drivers/tty/vt/vt.c:4665
vcs_write+0x646/0xcf0 drivers/tty/vt/vc_screen.c:545
__vfs_write+0x8a/0x110 fs/read_write.c:494
vfs_write+0x268/0x5d0 fs/read_write.c:558
ksys_write+0x14f/0x290 fs/read_write.c:611
__do_sys_write fs/read_write.c:623 [inline]
__se_sys_write fs/read_write.c:620 [inline]
__x64_sys_write+0x73/0xb0 fs/read_write.c:620
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x444399
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 7b d8 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffe50ca6938 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007ffe50ca6940 RCX: 0000000000444399
RDX: 00000000fffffecb RSI: 0000000020000300 RDI: 0000000000000003
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000400c60
R10: 00007ffe50ca6480 R11: 0000000000000246 R12: 00000000004020a0
R13: 0000000000402130 R14: 0000000000000000 R15: 0000000000000000

Allocated by task 1:
save_stack+0x23/0x90 mm/kasan/common.c:71
set_track mm/kasan/common.c:79 [inline]
__kasan_kmalloc mm/kasan/common.c:512 [inline]
__kasan_kmalloc.constprop.0+0xcf/0xe0 mm/kasan/common.c:485
kasan_kmalloc+0x9/0x10 mm/kasan/common.c:526
__do_kmalloc mm/slab.c:3656 [inline]
__kmalloc+0x163/0x770 mm/slab.c:3665
kmalloc include/linux/slab.h:561 [inline]
kzalloc include/linux/slab.h:670 [inline]
vc_do_resize+0x262/0x1460 drivers/tty/vt/vt.c:1187
vc_resize+0x4d/0x60 drivers/tty/vt/vt.c:1304
fbcon_init+0x122d/0x1a90 drivers/video/fbdev/core/fbcon.c:1212
visual_init+0x30a/0x5e0 drivers/tty/vt/vt.c:1051
do_bind_con_driver+0x54c/0x8b0 drivers/tty/vt/vt.c:3532
do_take_over_console+0x449/0x5a0 drivers/tty/vt/vt.c:4113
do_fbcon_takeover+0x116/0x220 drivers/video/fbdev/core/fbcon.c:581
fbcon_fb_registered+0x275/0x340 drivers/video/fbdev/core/fbcon.c:3252
do_register_framebuffer drivers/video/fbdev/core/fbmem.c:1652 [inline]
register_framebuffer+0x5c3/0xa10 drivers/video/fbdev/core/fbmem.c:1821
vga16fb_probe+0x711/0x825 drivers/video/fbdev/vga16fb.c:1373
platform_drv_probe+0x8d/0x140 drivers/base/platform.c:725
really_probe+0x291/0x710 drivers/base/dd.c:548
driver_probe_device+0x110/0x220 drivers/base/dd.c:721
__device_attach_driver+0x1c9/0x230 drivers/base/dd.c:828
bus_for_each_drv+0x172/0x1f0 drivers/base/bus.c:430
__device_attach+0x237/0x390 drivers/base/dd.c:894
device_initial_probe+0x1b/0x20 drivers/base/dd.c:941
bus_probe_device+0x1f1/0x2a0 drivers/base/bus.c:490
device_add+0x14fe/0x1d00 drivers/base/core.c:2487
platform_device_add+0x34d/0x6c0 drivers/base/platform.c:562
vga16fb_init+0x15f/0x1d6 drivers/video/fbdev/vga16fb.c:1431
do_one_initcall+0x120/0x81a init/main.c:938
do_initcall_level init/main.c:1006 [inline]
do_initcalls init/main.c:1014 [inline]
do_basic_setup init/main.c:1031 [inline]
kernel_init_freeable+0x4ca/0x5b9 init/main.c:1191
kernel_init+0x12/0x1bf init/main.c:1109
ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352

Freed by task 0:
(stack is not available)

The buggy address belongs to the object at ffff8882192c4000
which belongs to the cache kmalloc-8k of size 8192
The buggy address is located 4800 bytes inside of
8192-byte region [ffff8882192c4000, ffff8882192c6000)
The buggy address belongs to the page:
page:ffffea000864b100 refcount:1 mapcount:0 mapping:ffff8880aa4021c0
index:0x0 compound_mapcount: 0
raw: 057ffe0000010200 ffffea000864ae08 ffffea000863fe08 ffff8880aa4021c0
raw: 0000000000000000 ffff8882192c4000 0000000100000001 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff8882192c5180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffff8882192c5200: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
> ffff8882192c5280: 00 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc
^
ffff8882192c5300: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff8882192c5380: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#status for how to communicate with syzbot.
For information about bisection process see: https://goo.gl/tpsmEJ#bisection
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches

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