[Hyper-V] VM console orphan newline input during login

[Chris Valean]

In Bionic azure-edge proposed kernel 4.18.0-1008 we're observing an issue that got carried from upstream.
This is the first version we checked specifically for this issue, we don't have yet a good/bad proposed comparison set of versions.

linux-next-5.0.0-9673b4a-20012019 does not have this issue <- daily upstream linux-next
linux-next-4.20.0-6a1d293-27122018 has the issue <- all 4.1x/4.2x series from upstream have this problem.

This behavior can be seen only on Hyper-V, as it must use the VM console UI:
- boot the above kernels.
- try to login with username and password.
- After you input the username and press Enter in the Hyper-V console, another Enter (newline) is automatically received by the console, making it impossible to input the password.

Afterward, the console behaves like a series of newlines are received and it resets after a while, presenting again the linux user login.

Repro rates:
- 100% on WS2019
- on WS2016 depending on situation it might not be visible 100%.

Problem does not exist for Azure, it's specific to Hyper-V VM console only.

[Joshua R. Poulson (jrp)]

While I do not see the commit in the 4.18.0-1008 changelog ( https://launchpad.net/ubuntu/+source/linux-azure-edge/4.18.0-1008.8~18.04.1 ) I would plant the upstream bisect near "Input: hyper-v - fix wakeup from suspend-to-idle" which was merged upstream in December:

commit adac0753c25217a2365b132c87cb2540b51fa89b
Merge: 0072a0c14d5b 6c3516fed7b6
Author: Linus Torvalds <email address hidden>
Date: Tue Dec 4 08:47:04 2018 -0800

    Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/dtor/input

    Pull input updates from Dmitry Torokhov:
     "Mostly new IDs for Elan/Synaptics touchpads, plus a few small fixups"

    * 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/dtor/input:
      Input: omap-keypad - fix keyboard debounce configuration
      Input: xpad - quirk all PDP Xbox One gamepads
      Input: synaptics - enable SMBus for HP 15-ay000
      Input: synaptics - add PNP ID for ThinkPad P50 to SMBus
      Input: elan_i2c - add ACPI ID for Lenovo IdeaPad 330-15ARR
      Input: elan_i2c - add support for ELAN0621 touchpad
      Input: hyper-v - fix wakeup from suspend-to-idle
      Input: atkbd - clean up indentation issue
      Input: st1232 - convert to SPDX identifiers
      Input: migor_ts - convert to SPDX identifiers
      Input: dt-bindings - fix a typo in file input-reset.txt
      Input: cros_ec_keyb - fix button/switch capability reports
      Input: elan_i2c - add ELAN0620 to the ACPI table
      Input: matrix_keypad - check for errors from of_get_named_gpio()

commit 10f91c73cc41ceead210a905dbd196398e99c7d2
Author: Vitaly Kuznetsov <email address hidden>
Date: Thu Nov 15 11:05:10 2018 -0800

    Input: hyper-v - fix wakeup from suspend-to-idle

    It makes little sense but still possible to put Hyper-V guests into
    suspend-to-idle state. To wake them up two wakeup sources were registered
    in the past: hyperv-keyboard and hid-hyperv. However, since
    commit eed4d47efe95 ("ACPI / sleep: Ignore spurious SCI wakeups from
    suspend-to-idle") pm_wakeup_event() from these devices is ignored. Switch
    to pm_wakeup_hard_event() API as these devices are actually the only
    possible way to wakeup Hyper-V guests.

    Fixes: eed4d47efe95 (ACPI / sleep: Ignore spurious SCI wakeups from suspend-to-idle)
    Reviewed-by: Rafael J. Wysocki <email address hidden>
    Acked-by: K. Y. Srinivasan <email address hidden>
    Acked-by: Jiri Kosina <email address hidden>
    Signed-off-by: Vitaly Kuznetsov <email address hidden>
    Signed-off-by: Dmitry Torokhov <email address hidden>

[Joseph Salisbury (jsalisbury)]

This bug is fixed by the following upstream commit:
d3736d82e816 (“tty: Don't hold ldisc lock in tty_reopen() if ldisc present”)

That commit is in mainline as of v5.0-rc2 and has been cc'd to upstream stable. The commit has already been applied to linux-azure master-next.