KASAN: use-after-free Read in vgacon_invert_region

[Kyungtae Kim]

We report the crash:
"KASAN: use-after-free Read in vgacon_invert_region"

This crash was found in v4.17-rc3. Specifically, memory access (read
operation) is invalid, and it is detected by KASAN.

C repro code:
 https://kiwi.cs.purdue.edu/static/alexkkid-fuzzer/repro-ba6c1.c
kernel config:
 https://kiwi.cs.purdue.edu/static/alexkkid-fuzzer/kernel-config-v4.17-rc3

Crash log:
==============================================================
BUG: KASAN: use-after-free in vgacon_invert_region+0xf9/0x100
drivers/video/console/vgacon.c:663
Read of size 2 at addr ffff880000100000 by task syz-executor3/525

CPU: 0 PID: 525 Comm: syz-executor3 Not tainted 4.17.0-rc3 #3
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0xc7/0x138 lib/dump_stack.c:113
 print_address_description+0x78/0x290 mm/kasan/report.c:256
 kasan_report_error mm/kasan/report.c:354 [inline]
 kasan_report+0x234/0x350 mm/kasan/report.c:412
 __asan_report_load2_noabort+0x19/0x20 mm/kasan/report.c:431
 vgacon_invert_region+0xf9/0x100 drivers/video/console/vgacon.c:663
 invert_screen+0x197/0x630 drivers/tty/vt/vt.c:461
 highlight drivers/tty/vt/selection.c:51 [inline]
 set_selection+0x6b7/0xf60 drivers/tty/vt/selection.c:276
 tioclinux+0x126/0x410 drivers/tty/vt/vt.c:2691
 vt_ioctl+0x1036/0x2580 drivers/tty/vt/vt_ioctl.c:362
 tty_ioctl+0x288/0x14c0 drivers/tty/tty_io.c:2646
 vfs_ioctl fs/ioctl.c:46 [inline]
 do_vfs_ioctl+0x191/0xfe0 fs/ioctl.c:686
 ksys_ioctl+0x99/0xb0 fs/ioctl.c:701
 __do_sys_ioctl fs/ioctl.c:708 [inline]
 __se_sys_ioctl fs/ioctl.c:706 [inline]
 __x64_sys_ioctl+0x78/0xb0 fs/ioctl.c:706
 do_syscall_64+0xb0/0x460 arch/x86/entry/common.c:287
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x4497b9
RSP: 002b:00007f720c4bec68 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f720c4bf6cc RCX: 00000000004497b9
RDX: 0000000020000040 RSI: 000200000000541c RDI: 0000000000000013
RBP: 000000000071bf58 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 0000000000005aa8 R14: 00000000006ecb48 R15: 00007f720c4bf700

The buggy address belongs to the page:
page:ffffea0000004000 count:0 mapcount:-127 mapping:0000000000000000 index:0x0
flags: 0x0()
raw: 0000000000000000 0000000000000000 0000000000000000 00000000ffffff80
raw: ffff88013fff9300 ffff88013fff9300 0000000000000008 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff8800000fff00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffff8800000fff80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff880000100000: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
                   ^
 ffff880000100080: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
 ffff880000100100: ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff
==============================================================


Thanks,
Kyungtae Kim