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SubjectKASAN: wild-memory-access Read in insert_char

syzbot found the following crash on:

HEAD commit: 6794862a Merge tag 'for-5.5-rc1-kconfig-tag' of git://git...
git tree: upstream
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
userspace arch: i386
syz repro:
C reproducer:

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

BUG: KASAN: wild-memory-access in memmove include/linux/string.h:395
BUG: KASAN: wild-memory-access in scr_memmovew include/linux/vt_buffer.h:68
BUG: KASAN: wild-memory-access in insert_char+0x206/0x400
Read of size 212 at addr 00000000ffffff3a by task syz-executor266/8995

CPU: 1 PID: 8995 Comm: syz-executor266 Not tainted 5.5.0-rc1-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
__kasan_report.cold+0x5/0x41 mm/kasan/report.c:510
kasan_report+0x12/0x20 mm/kasan/common.c:639
check_memory_region_inline mm/kasan/generic.c:185 [inline]
check_memory_region+0x134/0x1a0 mm/kasan/generic.c:192
memmove+0x24/0x50 mm/kasan/common.c:116
memmove include/linux/string.h:395 [inline]
scr_memmovew include/linux/vt_buffer.h:68 [inline]
insert_char+0x206/0x400 drivers/tty/vt/vt.c:839
csi_at drivers/tty/vt/vt.c:1964 [inline]
do_con_trol+0x41a6/0x61b0 drivers/tty/vt/vt.c:2431
do_con_write.part.0+0xfd9/0x1ef0 drivers/tty/vt/vt.c:2797
do_con_write drivers/tty/vt/vt.c:2565 [inline]
con_write+0x46/0xd0 drivers/tty/vt/vt.c:3135
process_output_block drivers/tty/n_tty.c:595 [inline]
n_tty_write+0x40e/0x1080 drivers/tty/n_tty.c:2333
do_tty_write drivers/tty/tty_io.c:962 [inline]
tty_write+0x496/0x7f0 drivers/tty/tty_io.c:1046
__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]
__ia32_sys_write+0x71/0xb0 fs/read_write.c:620
do_syscall_32_irqs_on arch/x86/entry/common.c:337 [inline]
do_fast_syscall_32+0x27b/0xe16 arch/x86/entry/common.c:408
entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
RIP: 0023:0xf7fc6a39
Code: 00 00 00 89 d3 5b 5e 5f 5d c3 b8 80 96 98 00 eb c4 8b 04 24 c3 8b 1c
24 c3 8b 34 24 c3 8b 3c 24 c3 51 52 55 89 e5 0f 34 cd 80 <5d> 5a 59 c3 90
90 90 90 eb 0d 90 90 90 90 90 90 90 90 90 90 90 90
RSP: 002b:00000000ffdae18c EFLAGS: 00000246 ORIG_RAX: 0000000000000004
RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 0000000020000000
RDX: 0000000000000078 RSI: 00000000080eb080 RDI: 00000000ffdae1e0
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000

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