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Re: [Qemu-devel] [PATCH V8 08/14] Introduce file lock for the block laye

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [PATCH V8 08/14] Introduce file lock for the block layer
Date: Wed, 7 Sep 2011 17:10:51 +0300
User-agent: Mutt/1.5.21 (2010-09-15)

On Wed, Sep 07, 2011 at 09:56:52AM -0400, Stefan Berger wrote:
> On 09/07/2011 09:16 AM, Michael S. Tsirkin wrote:
> >On Wed, Sep 07, 2011 at 09:06:05AM -0400, Stefan Berger wrote:
> >>>>First: There are two ways to encrypt the data.
> >>>>
> >>>>One comes with the QCoW2 type of image and it comes for free. Set
> >>>>the encryption flag when creating the QCoW2 file and one has to
> >>>>provide a key to access the QCoW2. I found this mode problematic for
> >>>>users since it required me to go through the monitor every time I
> >>>>started the VM. Besides that the key is provided so late that all
> >>>>devices are already initialized and if the wrong key was provided
> >>>>the only thing the TPM can do is to go into shutdown mode since
> >>>>there is state on the QCoW2 but it cannot be decrypted. This also
> >>>>became problematic when doing migrations with libvirt for example
> >>>>and one was to have a wrong key/password installed on the target
> >>>>side -- graceful termination of the migration is impossible.
> >OK let's go back to this for a moment. Add a load
> >callback, access file there. On failure, return
> >an error. migration fails gracefully, and
> >management can retry, or migrate to another node,
> >or whatever.
> >
> >What's the problem exactly?
> >
> >
> The switch-over from source to destination already happened when the
> key is finally passed and you just won't be able to access the QCoW2
> in case the key was wrong.

This is exactly what happens with any kind of othe rmigration errror.
So fail migration, and source can get restarted if necessary.

> Similar problems occur when you start a
> VM with an encrypted QCoW2 image. The monitor will prompt you for
> the password and then you start the VM and if the password was wrong
> the OS just won't be able to access the image.
>    Stefan

Why can't you verify the password?


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