KASAN: user-memory-access Read in do_con_write

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    a45ea48e afs: Fix characters allowed into cell names
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=14a62e01e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=708fe207b84027b3
dashboard link: https://syzkaller.appspot.com/bug?extid=d322a47d68f7864e920a
compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+d322a47d68f7864e920a@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: user-memory-access in scr_memmovew include/linux/vt_buffer.h:68 [inline]
BUG: KASAN: user-memory-access in insert_char drivers/tty/vt/vt.c:839 [inline]
BUG: KASAN: user-memory-access in csi_at drivers/tty/vt/vt.c:1964 [inline]
BUG: KASAN: user-memory-access in do_con_trol drivers/tty/vt/vt.c:2431 [inline]
BUG: KASAN: user-memory-access in do_con_write+0x89dd/0xf360 drivers/tty/vt/vt.c:2797
Read of size 2 at addr 000000010000000c by task syz-executor.5/22173

CPU: 1 PID: 22173 Comm: syz-executor.5 Not tainted 5.5.0-rc7-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+0x1fb/0x318 lib/dump_stack.c:118
 __kasan_report+0x167/0x1c0 mm/kasan/report.c:510
 kasan_report+0x26/0x50 mm/kasan/common.c:639
 check_memory_region_inline mm/kasan/generic.c:182 [inline]
 check_memory_region+0x2b6/0x2f0 mm/kasan/generic.c:192
 memmove+0x28/0x60 mm/kasan/common.c:116
 scr_memmovew include/linux/vt_buffer.h:68 [inline]
 insert_char drivers/tty/vt/vt.c:839 [inline]
 csi_at drivers/tty/vt/vt.c:1964 [inline]
 do_con_trol drivers/tty/vt/vt.c:2431 [inline]
 do_con_write+0x89dd/0xf360 drivers/tty/vt/vt.c:2797
 con_write+0x25/0x40 drivers/tty/vt/vt.c:3135
 process_output_block drivers/tty/n_tty.c:595 [inline]
 n_tty_write+0xd0c/0x1200 drivers/tty/n_tty.c:2333
 do_tty_write drivers/tty/tty_io.c:962 [inline]
 tty_write+0x5a1/0x950 drivers/tty/tty_io.c:1046
 __vfs_write+0xb8/0x740 fs/read_write.c:494
 vfs_write+0x270/0x580 fs/read_write.c:558
 ksys_write+0x117/0x220 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+0x7b/0x90 fs/read_write.c:620
 do_syscall_64+0xf7/0x1c0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x45b349
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:00007f9bb5dedc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 00007f9bb5dee6d4 RCX: 000000000045b349
RDX: 0000000000000007 RSI: 0000000020000040 RDI: 0000000000000003
RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00000000ffffffff
R13: 0000000000000c99 R14: 00000000004cd78a R15: 000000000075bf2c
==================================================================


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