KASAN: use-after-free Read in fbcon_cursor

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 312017a4 Linux 4.19.89
git tree: linux-4.19.y
console output: https://syzkaller.appspot.com/x/log.txt?x=11bf1b89e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=67c148d572dbab48
dashboard link: https://syzkaller.appspot.com/bug?extid=164d60ee54cdb30597a9
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=108b0056e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+164d60...@syzkaller.appspotmail.com

audit: type=1400 audit(1576381391.863:38): avc: denied { associate } for
pid=7812 comm="syz-executor.0" name="syz0"
scontext=unconfined_u:object_r:unlabeled_t:s0
tcontext=system_u:object_r:unlabeled_t:s0 tclass=filesystem permissive=1
==================================================================
BUG: KASAN: use-after-free in fbcon_cursor+0x686/0x7b0
drivers/video/fbdev/core/fbcon.c:1347
Read of size 2 at addr ffff88809fedd44c by task syz-executor.0/8323

CPU: 0 PID: 8323 Comm: syz-executor.0 Not tainted 4.19.89-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.cold+0x7c/0x20d mm/kasan/report.c:256
kasan_report_error mm/kasan/report.c:354 [inline]
kasan_report mm/kasan/report.c:412 [inline]
kasan_report.cold+0x8c/0x2ba mm/kasan/report.c:396
__asan_report_load2_noabort+0x14/0x20 mm/kasan/report.c:431
fbcon_cursor+0x686/0x7b0 drivers/video/fbdev/core/fbcon.c:1347
fbcon_scrolldelta+0x675/0x1330 drivers/video/fbdev/core/fbcon.c:2855
fbcon_set_origin+0x43/0x50 drivers/video/fbdev/core/fbcon.c:2906
set_origin+0x341/0x440 drivers/tty/vt/vt.c:919
vc_do_resize+0xacc/0x14a0 drivers/tty/vt/vt.c:1267
vc_resize+0x4d/0x60 drivers/tty/vt/vt.c:1307
vt_ioctl+0x146c/0x2530 drivers/tty/vt/vt_ioctl.c:840
tty_ioctl+0x7f3/0x1510 drivers/tty/tty_io.c:2669
vfs_ioctl fs/ioctl.c:46 [inline]
file_ioctl fs/ioctl.c:501 [inline]
do_vfs_ioctl+0xd5f/0x1380 fs/ioctl.c:688
ksys_ioctl+0xab/0xd0 fs/ioctl.c:705
__do_sys_ioctl fs/ioctl.c:712 [inline]
__se_sys_ioctl fs/ioctl.c:710 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:710
do_syscall_64+0xfd/0x620 arch/x86/entry/common.c:293
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x45a909
Code: ad b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 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 b6 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f77cb9d3c78 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: 00007f77cb9d46d4
R13: 00000000004c7009 R14: 00000000004dd670 R15: 00000000ffffffff

Allocated by task 7820:
save_stack+0x45/0xd0 mm/kasan/kasan.c:448
set_track mm/kasan/kasan.c:460 [inline]
kasan_kmalloc mm/kasan/kasan.c:553 [inline]
kasan_kmalloc+0xce/0xf0 mm/kasan/kasan.c:531
__do_kmalloc mm/slab.c:3727 [inline]
__kmalloc+0x15d/0x750 mm/slab.c:3736
kmalloc include/linux/slab.h:520 [inline]
kzalloc include/linux/slab.h:709 [inline]
vc_do_resize+0x262/0x14a0 drivers/tty/vt/vt.c:1190
vc_resize+0x4d/0x60 drivers/tty/vt/vt.c:1307
vt_ioctl+0x146c/0x2530 drivers/tty/vt/vt_ioctl.c:840
tty_ioctl+0x7f3/0x1510 drivers/tty/tty_io.c:2669
vfs_ioctl fs/ioctl.c:46 [inline]
file_ioctl fs/ioctl.c:501 [inline]
do_vfs_ioctl+0xd5f/0x1380 fs/ioctl.c:688
ksys_ioctl+0xab/0xd0 fs/ioctl.c:705
__do_sys_ioctl fs/ioctl.c:712 [inline]
__se_sys_ioctl fs/ioctl.c:710 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:710
do_syscall_64+0xfd/0x620 arch/x86/entry/common.c:293
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 8323:
save_stack+0x45/0xd0 mm/kasan/kasan.c:448
set_track mm/kasan/kasan.c:460 [inline]
__kasan_slab_free+0x102/0x150 mm/kasan/kasan.c:521
kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528
__cache_free mm/slab.c:3503 [inline]
kfree+0xcf/0x220 mm/slab.c:3822
vc_do_resize+0xa69/0x14a0 drivers/tty/vt/vt.c:1264
vc_resize+0x4d/0x60 drivers/tty/vt/vt.c:1307
vt_ioctl+0x146c/0x2530 drivers/tty/vt/vt_ioctl.c:840
tty_ioctl+0x7f3/0x1510 drivers/tty/tty_io.c:2669
vfs_ioctl fs/ioctl.c:46 [inline]
file_ioctl fs/ioctl.c:501 [inline]
do_vfs_ioctl+0xd5f/0x1380 fs/ioctl.c:688
ksys_ioctl+0xab/0xd0 fs/ioctl.c:705
__do_sys_ioctl fs/ioctl.c:712 [inline]
__se_sys_ioctl fs/ioctl.c:710 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:710
do_syscall_64+0xfd/0x620 arch/x86/entry/common.c:293
entry_SYSCALL_64_after_hwframe+0x49/0xbe

The buggy address belongs to the object at ffff88809fedd440
which belongs to the cache kmalloc-32 of size 32
The buggy address is located 12 bytes inside of
32-byte region [ffff88809fedd440, ffff88809fedd460)
The buggy address belongs to the page:
page:ffffea00027fb740 count:1 mapcount:0 mapping:ffff88812c31c1c0
index:0xffff88809feddfc1
flags: 0xfffe0000000100(slab)
raw: 00fffe0000000100 ffffea0002818d48 ffffea00027f48c8 ffff88812c31c1c0
raw: ffff88809feddfc1 ffff88809fedd000 000000010000003f 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff88809fedd300: fb fb fb fb fc fc fc fc 00 00 01 fc fc fc fc fc
ffff88809fedd380: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
> ffff88809fedd400: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
^
ffff88809fedd480: fb fb fb fb fc fc fc fc fb fb fb fb fc fc fc fc
ffff88809fedd500: fb fb fb fb fc fc fc fc fb fb fb fb 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.
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syzbot can test patches for this bug, for details see:
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[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: a844dc4c Linux 4.14.158
git tree: linux-4.14.y
console output: https://syzkaller.appspot.com/x/log.txt?x=14a82361e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=c02bef505ffc02ff
dashboard link: https://syzkaller.appspot.com/bug?extid=835180215fcdeedc3384

compiler: gcc (GCC) 9.0.0 20181231 (experimental)

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=10a4f946e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:

Reported-by: syzbot+835180...@syzkaller.appspotmail.com

kobject: 'loop1' (ffff8880a41073e0): kobject_uevent_env
kobject: 'loop1' (ffff8880a41073e0): fill_kobj_path: path
= '/devices/virtual/block/loop1'
kobject: 'loop4' (ffff8880a41e2420): kobject_uevent_env
kobject: 'loop4' (ffff8880a41e2420): fill_kobj_path: path
= '/devices/virtual/block/loop4'
==================================================================
BUG: KASAN: use-after-free in fbcon_cursor+0x5c6/0x6f0
drivers/video/fbdev/core/fbcon.c:1325
Read of size 2 at addr ffff8880a14f580c by task syz-executor.3/8047

CPU: 1 PID: 8047 Comm: syz-executor.3 Not tainted 4.14.158-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+0x142/0x197 lib/dump_stack.c:58
print_address_description.cold+0x7c/0x1dc mm/kasan/report.c:252
kasan_report_error mm/kasan/report.c:351 [inline]
kasan_report mm/kasan/report.c:409 [inline]
kasan_report.cold+0xa9/0x2af mm/kasan/report.c:393
__asan_report_load2_noabort+0x14/0x20 mm/kasan/report.c:428
fbcon_cursor+0x5c6/0x6f0 drivers/video/fbdev/core/fbcon.c:1325
fbcon_scrolldelta+0x5a4/0x1100 drivers/video/fbdev/core/fbcon.c:2832
fbcon_set_origin+0x39/0x50 drivers/video/fbdev/core/fbcon.c:2883
set_origin+0x2d8/0x3c0 drivers/tty/vt/vt.c:613
vc_do_resize+0x686/0xec0 drivers/tty/vt/vt.c:934
vc_resize+0x4d/0x60 drivers/tty/vt/vt.c:974
vt_ioctl+0x1282/0x2170 drivers/tty/vt/vt_ioctl.c:840
tty_ioctl+0x841/0x1320 drivers/tty/tty_io.c:2661
vfs_ioctl fs/ioctl.c:46 [inline]
file_ioctl fs/ioctl.c:500 [inline]
do_vfs_ioctl+0x7ae/0x1060 fs/ioctl.c:684
SYSC_ioctl fs/ioctl.c:701 [inline]
SyS_ioctl+0x8f/0xc0 fs/ioctl.c:692
do_syscall_64+0x1e8/0x640 arch/x86/entry/common.c:292
entry_SYSCALL_64_after_hwframe+0x42/0xb7
RIP: 0033:0x45a909
RSP: 002b:00007f0b23d9ac78 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: 00007f0b23d9b6d4

R13: 00000000004c7009 R14: 00000000004dd670 R15: 00000000ffffffff

Allocated by task 7985:
save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:59
save_stack+0x45/0xd0 mm/kasan/kasan.c:447
set_track mm/kasan/kasan.c:459 [inline]
kasan_kmalloc mm/kasan/kasan.c:551 [inline]
kasan_kmalloc+0xce/0xf0 mm/kasan/kasan.c:529
__do_kmalloc mm/slab.c:3720 [inline]
__kmalloc+0x15d/0x7a0 mm/slab.c:3729
kmalloc include/linux/slab.h:493 [inline]
kzalloc include/linux/slab.h:661 [inline]
vc_do_resize+0x1da/0xec0 drivers/tty/vt/vt.c:872
vc_resize+0x4d/0x60 drivers/tty/vt/vt.c:974
vt_ioctl+0x1282/0x2170 drivers/tty/vt/vt_ioctl.c:840
tty_ioctl+0x841/0x1320 drivers/tty/tty_io.c:2661
do

[syzbot]

syzbot suspects this issue was fixed by commit:

commit 770adb5d2b8ebe94a92e4c9510f4f2517f4204eb
Author: Linus Torvalds <torv...@linux-foundation.org>
Date: Mon Sep 7 18:45:27 2020 +0000

fbcon: remove soft scrollback code

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=1633aa5b900000
start commit: 312017a4 Linux 4.19.89
git tree: linux-4.19.y

kernel config: https://syzkaller.appspot.com/x/.config?x=67c148d572dbab48
dashboard link: https://syzkaller.appspot.com/bug?extid=164d60ee54cdb30597a9

syz repro: https://syzkaller.appspot.com/x/repro.syz?x=108b0056e00000

If the result looks correct, please mark the issue as fixed by replying with:

#syz fix: fbcon: remove soft scrollback code

For information about bisection process see: https://goo.gl/tpsmEJ#bisection