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Re: [Xen-devel] Individual passwords for guest VNC servers ?

Masami Watanabe wrote:
Hi all,

Thanks all point about security, I'll do as follows.
I thought that the point was the following two.

I've always been of the opinion that security is best left to other places in the stack. With that in mind..

1. Storage place of encrypted password
  Should I store it in /etc/xen/passwd ?
    Or, should I wait for DB of Xen that will be released in the future?
  In the latter case, the release time and information, I want you to
  teach it.
  Now, I think we have no choice but to use /etc/xen/passwd.

I would say, forget about password storage entirely. Have qemu-dm take an fd on the command line (this would also be an acceptable patch for upstream qemu too btw). Have qemu-dm use that fd to read the password.

Then, I would just stick the password in the domain's configuration file. Perhaps:

vncpassword = '...';

As an added bonus, if vncpassword is empty, xm could prompt the user for a password. Then, xm passes the password as part of the configuration file. It's debatable whether Xend should filter out the vncpassword parameter on a domain list. I probably would just to be on the same side for now.

2. Method of Xen VNC Server receiving stored password
  By way of xenstore. However, it is necessary to consider xenstore-ls.

Xenstore is readable by too many things IMHO. Doesn't seem like a good choice for something like this.

and I think that the following is a problem. - The key that encrypt challenge data is fixed. It is necessary to encrypt the challenge data by the same logic as the
  standard VNC client.
However, there is no necessity for even managing the key as well as standard VNC Server.
  Only the domain manager should know the key used for the DES decryption.
There is no necessity that is stored, and maintained on the Xent side.

Okay, I'm a bit confused by how you state things here. The VNC auth session looks something like this:

Server generates a random, one-time 16 byte piece of data for the challenge.
Server sends challenge to client
Client encrypts challenge with password (null-padded to 8 bytes in length)
Client sends password to server

The key lives entirely within vnc.c within qemu-dm. I'd just read 16 bytes from /dev/[u]random to generate the key.

BTW, make sure you use the des.c from an existing VNC server. There are a few incompatible changes between it and the standard des.c.


Anthony Liguori

When the domain is generated, the domain manager only has input the key.
Xen preserves only the data encrypted with the key that only the manager
When the domain is generated, Xen inputs the key that only the manager
knows. And, the key is passed to xend and qemu-dm.

As soon as the above-mentioned decision is made, I will think about


On Fri, 22 Sep 2006 14:54:24 +0100, Ian Pratt wrote:
Passing around passwords either on the command line, or environment is
big red flag from a security POV. Also the Xen guest & xend config
all default to world readable. I think we should follow the Apache
and store the passwords out-of-band from the main config. eg

   (vncpasswordfile '/etc/xen/vncpassword')

At this point it would make sense to have one password file for all
and store them in format:  'vm-name:  pw-hash'
The new life cycle management stuff in post 3.0.3 xend changes this
quite a bit as a config file is only used when initially creating a VM,
and then information about it gets stored in xend's database. The
current password associated with a VM would be one of the parameters
stored in the database, and should be updated using 'xm vnc-password' or
shuch like.
As Ian just suggested we could have command 'xm password'  for
these passwords (cf apache's  htpasswd command)

Now when launching qemu-dm, we can either pass the path to the
file on its command line,   eg  -passwordfile /etc/xen/password, or
passs the actual password to qemu-dm down a pipe (eg qemu-dm would
the password from filehandle 3 upon startup). The latter would be my
preference, since then we could isolate the password handling stuff in
Xend, and not duplicate it in qemu-dm, and the paravirt  equivalent.
I wouldn't rely on qemu-dm staying in dom0. I think the information
should be passed transiently via xenstore.

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