[Bug 2077576] Re: SSH client doesn't handle properly non-ASCII chars

[Seth Arnold]

This change makes me uneasy:

- I see no terminal-aware filtering applied in the notify_start() ->
xvasprintf() -> writemsg() -> write() path. The remote server may not be
entirely untrusted but it's also not exactly trusted, either, especially
on the first use. There's a long and glorious history of surprising
outcomes due to terminal escape sequences
https://www.cyberark.com/resources/threat-research-blog/dont-trust-this-
title-abusing-terminal-emulators-with-ansi-escape-characters

- I'm not sure it's even necessary: my phone was easily able to read
pure-ascii QR codes as easily as the (admittedly much better looking)
utf-8 based codes. Try a few:

sudo apt install qrencode
u=`cat /proc/sys/kernel/random/uuid` ; for t in ANSI ANSI256 ASCII ASCIIi UTF8 ANSIUTF8 ; do qrencode -t $t $u  ; done ; echo $u ; unset u

Are ascii-encoded qr codes known to be insufficient?

- As for the actual code changes, they seemed fairly straightforward,
and I found no issues. I'm very wary about undoing a safety mechanism
from the past, put in place by people who thought about this
significantly more than I have.

- Upstream might have been engaging for a while but now appears entirely
silent. This reminds me too much of dpkg+zstd, where a similar train of
engagement lead to Ubuntu forking the dpkg ecosystem and carrying a
patch without a clear way to reunify the ecosystem. Will we do the same
to OpenSSH? (We might have already passed this point if we chose to ship
this in Noble rather than wait for Oracular to test this out.)

Thanks

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https://bugs.launchpad.net/bugs/2077576

Title:
  SSH client doesn't handle properly non-ASCII chars

Status in openssh package in Ubuntu:
  Fix Released
Status in openssh source package in Focal:
  Incomplete
Status in openssh source package in Jammy:
  Incomplete
Status in openssh source package in Noble:
  Fix Released

Bug description:
  [ Impact ]

  Non-ascii visible chars (including back-slashes, new lines and so) are
  not properly rendered by clients, showing their octal visualization.

  Such as:

    Hello SSHD \\ We love \360\237\215\225!

  Instead of:

    Hello SSHD \ We love 🍕!

  This is particularly an issue when a server has configured keyboard
  interactive authentication and a PAM module wants to show non-ASCII
  characters such as a QR code for web authentication:

  When using an ubuntu server running authd for web authentication we
  may end up having the login qrcode rendered such as

  \210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210\342\226\210
                          https://ubuntu.com
                                 1337

  Which is clearly unreadable.

  [ Test case ]

  ## Server preparation

  Enable PAM and keyboard interactive authentication in a ssh server:

  Add a configuration file such as:
   /etc/ssh/sshd_config.d/test-ssh-pam.conf

  Containing:

  UsePAM yes
  KbdInteractiveAuthentication yes
  # This was working already; here to check potential regressions
  ForceCommand bash -c "echo Hello from SSHD \ We also love 🍕!; $SHELL"

  It's also suggested to check for regressions using a `Banner` option
  in sshd, pointing to a file with utf-8 contents.

  Restart the server:

    sudo systemctl restart ssh.service

  Edit the sshd PAM configuration file, adding as first line:

    auth    requisite pam_echo.so Hello SSHD \ We love 🍕!

  Can be done with the command:
    sudo sed '1 iauth    requisite pam_echo.so Hello SSHD! \\ We love 🍕!' \
     -i /etc/pam.d/sshd

  ## Client test

  In the same host:

   ssh -o PubkeyAuthentication=no \
       -o PasswordAuthentication=no \
       -o PreferredAuthentications=keyboard-interactive \
       $USER at localhost

  The client should show:

  Hello SSHD \ We love 🍕!
  ($USER at localhost) Password:
  ...
  Hello from SSHD \ We also love 🍕!

  Retry the same with another host and without keyboard authentication
  enabled in the server side.

  To verify the fix in more complex scenario it's possible to follow the instructions of configuring authd:
   - https://github.com/ubuntu/authd/wiki/05--How%E2%80%90to-log-in-over-SSH

  Once authd is configured, the user should be able to scan a QrCode
  from a ssh session.

  ## Cleanup

  Revert the changes done in the cleanup phase, after test is done

  sudo sed '/pam_echo\.so/d' -i /etc/pam.d/sshd
  sudo rm /etc/ssh/sshd_config.d/test-ssh-pam.conf

  [ Regression potential ]

  SSH info messages are not shown by the client. Even though those
  aren't covered by this change, it's important to check for regressions
  in any output that SSH exposes to the user. So banners and other
  messages should be checked for regressions.

  These kind of messages are normally shown only when PAM *and* keyboard
  interaction are enabled in the server side, so it should not affect
  the default ubuntu servers behavior.

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