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SubjectBUG: unable to handle kernel NULL pointer dereference in insert_char
From
Hello,

syzbot found the following crash on:

HEAD commit: e033e7d4 Merge branch 'dhowells' (patches from DavidH)
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=129b4bfee00000
kernel config: https://syzkaller.appspot.com/x/.config?x=d9290aeb7e6cf1c4
dashboard link: https://syzkaller.appspot.com/bug?extid=8d8b0a62e3d61b05a75c
compiler: gcc (GCC) 9.0.0 20181231 (experimental)

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+8d8b0a62e3d61b05a75c@syzkaller.appspotmail.com

BUG: kernel NULL pointer dereference, address: 0000000000000012
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
PGD 66c47067 P4D 66c47067 PUD 97d45067 PMD 0
Oops: 0002 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 10771 Comm: syz-executor.1 Not tainted 5.5.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
RIP: 0010:memset16 arch/x86/include/asm/string_64.h:25 [inline]
RIP: 0010:scr_memsetw include/linux/vt_buffer.h:36 [inline]
RIP: 0010:insert_char+0x23d/0x400 drivers/tty/vt/vt.c:840
Code: 00 fc ff df 48 89 fa 48 c1 ea 03 0f b6 04 02 84 c0 74 08 3c 01 0f 8e
1e 01 00 00 d1 eb 41 0f b7 85 c8 03 00 00 4c 89 ff 89 d9 <f3> 66 ab 49 8d
bd 78 04 00 00 48 b8 00 00 00 00 00 fc ff df 48 89
RSP: 0018:ffffc90008227858 EFLAGS: 00010202
RAX: 0000000000000720 RBX: 0000000000000001 RCX: 0000000000000001
RDX: 1ffff1100e70c879 RSI: 0000000000000012 RDI: 0000000000000012
RBP: ffffc900082278a0 R08: ffff888051f5c000 R09: ffff888051f5c890
R10: fffffbfff14f69b0 R11: ffffffff8a7b4d87 R12: 0000000000000000
R13: ffff888073864000 R14: ffff888073864334 R15: 0000000000000012
FS: 00007fbfc62aa700(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000012 CR3: 0000000095551000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
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: 0033:0x45af49
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:00007fbfc62a9c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
RAX: ffffffffffffffda RBX: 0000000000000003 RCX: 000000000045af49
RDX: 0000000000000078 RSI: 0000000020000000 RDI: 0000000000000003
RBP: 000000000075bf20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007fbfc62aa6d4
R13: 00000000004ccf01 R14: 00000000004e8538 R15: 00000000ffffffff
Modules linked in:
CR2: 0000000000000012
---[ end trace 7959dc0106335e09 ]---
RIP: 0010:memset16 arch/x86/include/asm/string_64.h:25 [inline]
RIP: 0010:scr_memsetw include/linux/vt_buffer.h:36 [inline]
RIP: 0010:insert_char+0x23d/0x400 drivers/tty/vt/vt.c:840
Code: 00 fc ff df 48 89 fa 48 c1 ea 03 0f b6 04 02 84 c0 74 08 3c 01 0f 8e
1e 01 00 00 d1 eb 41 0f b7 85 c8 03 00 00 4c 89 ff 89 d9 <f3> 66 ab 49 8d
bd 78 04 00 00 48 b8 00 00 00 00 00 fc ff df 48 89
RSP: 0018:ffffc90008227858 EFLAGS: 00010202
RAX: 0000000000000720 RBX: 0000000000000001 RCX: 0000000000000001
RDX: 1ffff1100e70c879 RSI: 0000000000000012 RDI: 0000000000000012
RBP: ffffc900082278a0 R08: ffff888051f5c000 R09: ffff888051f5c890
R10: fffffbfff14f69b0 R11: ffffffff8a7b4d87 R12: 0000000000000000
R13: ffff888073864000 R14: ffff888073864334 R15: 0000000000000012
FS: 00007fbfc62aa700(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000758080 CR3: 0000000095551000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400


---
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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