Re: KASAN: null-ptr-deref Read in insert_char

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit:    b07f636f Merge tag 'tpmdd-next-20200108' of git://git.infr..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=102e1c85e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=18698c0c240ba616
dashboard link: https://syzkaller.appspot.com/bug?extid=7416b988c249396c5e2c
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=1716da9ee00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=10a9cc85e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+7416b988c249396c5e2c@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: null-ptr-deref in memmove include/linux/string.h:395 [inline]
BUG: KASAN: null-ptr-deref in scr_memmovew include/linux/vt_buffer.h:68  
[inline]
BUG: KASAN: null-ptr-deref in insert_char+0x206/0x400  
drivers/tty/vt/vt.c:839
Read of size 4294967294 at addr 0000000000000010 by task  
syz-executor700/10454

CPU: 0 PID: 10454 Comm: syz-executor700 Not tainted 5.5.0-rc5-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]
 __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: 0033:0x447b99
Code: e8 cc 14 03 00 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 ab 0e fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ff82cd33db8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00000000006ddc28 RCX: 0000000000447b99
RDX: 0000000000000078 RSI: 0000000020000000 RDI: 0000000000000004
RBP: 00000000006ddc20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006ddc2c
R13: 00007ffff0ce099f R14: 00007ff82cd349c0 R15: 0000000000000000
==================================================================