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SubjectBUG: unable to handle kernel NULL pointer dereference in mem16_serial_out
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Hello,

syzbot found the following crash on:

HEAD commit: e42617b8 Linux 5.5-rc1
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1157cd41e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=3754e2c78c1adb82
dashboard link: https://syzkaller.appspot.com/bug?extid=92f32d4e21fb246d31a2
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=136f7e41e00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=112b7c82e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+92f32d4e21fb246d31a2@syzkaller.appspotmail.com

BUG: kernel NULL pointer dereference, address: 0000000000000003
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
PGD a9a61067 P4D a9a61067 PUD 8fa24067 PMD 0
Oops: 0002 [#1] PREEMPT SMP KASAN
CPU: 1 PID: 9054 Comm: syz-executor150 Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
RIP: 0010:writew arch/x86/include/asm/io.h:66 [inline]
RIP: 0010:mem16_serial_out+0x6c/0x90 drivers/tty/serial/8250/8250_port.c:414
Code: b6 8d e9 00 00 00 49 8d 7d 40 48 b8 00 00 00 00 00 fc ff df 48 89 fa
48 c1 ea 03 d3 e3 80 3c 02 00 75 19 48 63 db 49 03 5d 40 <66> 44 89 23 5b
41 5c 41 5d 5d c3 e8 d4 44 cf fd eb c2 e8 2d 45 cf
RSP: 0018:ffffc90001cf7908 EFLAGS: 00010206
RAX: dffffc0000000000 RBX: 0000000000000003 RCX: 0000000000000000
RDX: 1ffffffff182080e RSI: ffffffff83e38106 RDI: ffffffff8c104070
RBP: ffffc90001cf7920 R08: ffff88808ffac040 R09: ffffed10431421c6
R10: ffffed10431421c5 R11: ffff888218a10e2b R12: 00000000000000bf
R13: ffffffff8c104030 R14: ffffc90001cf7a40 R15: ffffffff8c104188
FS: 0000000000866880(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000003 CR3: 00000000a64a2000 CR4: 00000000001406e0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
serial_port_out include/linux/serial_core.h:265 [inline]
serial8250_do_startup+0x12b9/0x1cf0
drivers/tty/serial/8250/8250_port.c:2077
serial8250_startup+0x62/0x80 drivers/tty/serial/8250/8250_port.c:2329
uart_port_startup drivers/tty/serial/serial_core.c:219 [inline]
uart_startup drivers/tty/serial/serial_core.c:258 [inline]
uart_startup+0x452/0x980 drivers/tty/serial/serial_core.c:249
uart_set_info drivers/tty/serial/serial_core.c:998 [inline]
uart_set_info_user+0x13b4/0x1cf0 drivers/tty/serial/serial_core.c:1023
tty_tiocsserial drivers/tty/tty_io.c:2506 [inline]
tty_ioctl+0xf60/0x14f0 drivers/tty/tty_io.c:2648
vfs_ioctl fs/ioctl.c:47 [inline]
file_ioctl fs/ioctl.c:545 [inline]
do_vfs_ioctl+0x977/0x14e0 fs/ioctl.c:732
ksys_ioctl+0xab/0xd0 fs/ioctl.c:749
__do_sys_ioctl fs/ioctl.c:756 [inline]
__se_sys_ioctl fs/ioctl.c:754 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:754
do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x440219
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 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 fb 13 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffc99622388 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 0000000000440219
RDX: 0000000020000240 RSI: 000000000000541f RDI: 0000000000000003
RBP: 00000000006ca018 R08: 0000000000000000 R09: 00000000004002c8
R10: 0000000000401b30 R11: 0000000000000246 R12: 0000000000401aa0
R13: 0000000000401b30 R14: 0000000000000000 R15: 0000000000000000
Modules linked in:
CR2: 0000000000000003
---[ end trace 2e0575eb0019173e ]---
RIP: 0010:writew arch/x86/include/asm/io.h:66 [inline]
RIP: 0010:mem16_serial_out+0x6c/0x90 drivers/tty/serial/8250/8250_port.c:414
Code: b6 8d e9 00 00 00 49 8d 7d 40 48 b8 00 00 00 00 00 fc ff df 48 89 fa
48 c1 ea 03 d3 e3 80 3c 02 00 75 19 48 63 db 49 03 5d 40 <66> 44 89 23 5b
41 5c 41 5d 5d c3 e8 d4 44 cf fd eb c2 e8 2d 45 cf
RSP: 0018:ffffc90001cf7908 EFLAGS: 00010206
RAX: dffffc0000000000 RBX: 0000000000000003 RCX: 0000000000000000
RDX: 1ffffffff182080e RSI: ffffffff83e38106 RDI: ffffffff8c104070
RBP: ffffc90001cf7920 R08: ffff88808ffac040 R09: ffffed10431421c6
R10: ffffed10431421c5 R11: ffff888218a10e2b R12: 00000000000000bf
R13: ffffffff8c104030 R14: ffffc90001cf7a40 R15: ffffffff8c104188
FS: 0000000000866880(0000) GS:ffff8880ae900000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000003 CR3: 00000000a64a2000 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.
syzbot engineers can be reached at syzkaller@googlegroups.com.

syzbot will keep track of this bug report. See:
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syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches

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