Re: Monitor commands related to display server passwords

[Daniel P . Berrangé]

On Wed, Nov 30, 2022 at 09:02:56AM +0100, Markus Armbruster wrote:
> We have a couple of password-related commands, and I'm not sure about
> which ones should be used.  In order of appearance:
> 
> * HMP change vnc
> 
>   Change a VNC server password.  Unlike set_password below, there's no
>   way to select a display other than the first.
> 
>   Note: if change's second argument isn't "vnc", w're changing removable
>   media.  If you call your block device "vnc", you cannot change its
>   media.  Hilarious.

Note, QMP equivalent is blockdev-change-medium, which is an wrapper
around a blockdev-open-tray/remove-medium/insert-medium/close-tray
sequence.  'change <blockdev>' maps to this.

If you call your blockdev 'vnc' you're not getting sympathy
from me ;-P But seriously, I agree with your point.

>   Password prompting (with hidden user input) since commit 7084851534
>   "VNC password authentication, by Daniel P. Berrange." (v0.9.1,
>   2007-08-25).
> 
>   Password argument since commit 2569da0cb6 "Accept password as an
>   argument to 'change vnc password' monitor command (Chris Webb)"
>   (v0.10.0, 2008-12-10).
> 
>   Nowadays, this wraps around QMP change-vnc-password, discussed below.

> * HMP and QMP set_password, expire_password
> 
>   Change a VNC or Spice server password.  For Spice, can optionally fail
>   when connections exist, or disconnect them.
> 
>   HMP commands wrap around the respective QMP command, as they should.
> 
>   HMP set_password does not support password prompting like "change vnc"
>   does.
> 
>   Commands are present even when both CONFIG_VNC and CONFIG_SPICE are
>   off.  Attempts to use them are rejected manually.  Defeats
>   introspection.
> 
>   Since commit 7572150c18 "vnc/spice: add set_passwd monitor command."
>   (v0.14.0, 2010-12-09)
> 
>   Support for VNC displays other than the first since commit 675fd3c96b
>   "qapi/monitor: allow VNC display id in set/expire_password" (v7.0.0,
>   2022-03-02).
> 
> * QMP change-vnc-password
> 
>   Can only target the first VNC display, unlike set_password.
> 
>   Command present only with CONFIG_VNC.
> 
>   Since commit 270b243f91 "qapi: Introduce change-vnc-password" (v1.1,
>   2012-01-18).

IIRC, this was designed as a 1-1 mapping to replace the QMP
'change vnc' command, except it was obviously redundant
since we had already added 'set_passwd' by that point. I
vaguely recall this was all just an oversight on part of
author and reviewers. 

> Do we really need / want both set_password and change-vnc-password in
> QMP?

Nope.

> On the one hand, set_password feels outdated from a QAPI point of view:
> it violates the naming rules, and it defeats introspection.  On the
> other hand, it's more powerful.
> 
> Do we really need / want both set_password and "change vnc" in HMP?
> set_password is more powerful, but only "change vnc" supports password
> prompting.
> 
> Getting rid of "change vnc" would fix the "cannot change media for block
> device named 'vnc'" wart.

> Related: QCryptoSecret objects.

snip

> Currently used by various block backends and the tls-creds-x509 object.
> 
> Would it make sense with display servers, too?

In 6.0 I introduced support for 'password-secret' to SPICE and VNC
command line.

I don't know why, but I only deprecated 'password' in SPICE and
not in VNC.

I didn't wire up any QMP commands todo live password changes. If
the display was already configured with 'password-secret', you
could delete and re-create the existing named secret object
using object-add/object-del, since we fetch the secret value
on every auth check.

There's no way to change from password-off to password-on mode
and vica-verca.

Also no way to change other things like expiry time,

We since gained the 'display-update' command, which could be
extended to allow change expiry time, and turning on/off
use of passwords, and even changing what 'secret' they
point to.

So overall I say

 * Deprecate VNC 'password' option
 * Deprecated QMP and HMP commands for changing VNC/SPICE
   password
 * Extend 'display-update' other other misc live changes

With regards,
Daniel
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