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Re: [Qemu-devel] [RFC 0/5] block: File descriptor passing using -open-ho

From: Kevin Wolf
Subject: Re: [Qemu-devel] [RFC 0/5] block: File descriptor passing using -open-hook-fd
Date: Wed, 02 May 2012 10:20:17 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:11.0) Gecko/20120329 Thunderbird/11.0.1

Am 01.05.2012 22:25, schrieb Anthony Liguori:
> Thanks for sending this out Stefan.
> On 05/01/2012 10:31 AM, Stefan Hajnoczi wrote:
>> Libvirt can take advantage of SELinux to restrict the QEMU process and 
>> prevent
>> it from opening files that it should not have access to.  This improves
>> security because it prevents the attacker from escaping the QEMU process if
>> they manage to gain control.
>> NFS has been a pain point for SELinux because it does not support labels 
>> (which
>> I believe are stored in extended attributes).  In other words, it's not
>> possible to use SELinux goodness on QEMU when image files are located on NFS.
>> Today we have to allow QEMU access to any file on the NFS export rather than
>> restricting specifically to the image files that the guest requires.
>> File descriptor passing is a solution to this problem and might also come in
>> handy elsewhere.  Libvirt or another external process chooses files which 
>> is allowed to access and provides just those file descriptors - QEMU cannot
>> open the files itself.
>> This series adds the -open-hook-fd command-line option.  Whenever QEMU needs 
>> to
>> open an image file it sends a request over the given UNIX domain socket.  The
>> response includes the file descriptor or an errno on failure.  Please see the
>> patches for details on the protocol.
>> The -open-hook-fd approach allows QEMU to support file descriptor passing
>> without changing -drive.  It also supports snapshot_blkdev and other commands
>> that re-open image files.
>> Anthony Liguori<address@hidden>  wrote most of these patches.  I added a
>> demo -open-hook-fd server and added some small fixes.  Since Anthony is
>> traveling right now I'm sending the RFC for discussion.
> What I like about this approach is that it's useful outside the block layer 
> and 
> is conceptionally simple from a QEMU PoV.  We simply delegate open() to 
> libvirt 
> and let libvirt enforce whatever rules it wants.
> This is not meant to be an alternative to blockdev, but even with blockdev, I 
> think we still want to use a mechanism like this even with blockdev.

What does it provide on top?

This doesn't look like something that I'd like a lot. qemu should be
able to continue to run no matter what the management tool does, whether
it responds to RPCs properly or whether it has crashed. You need a
really good use case for the RPC that cannot be covered otherwise in
order to justify this.


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