[libvirt] [Qemu-devel] [RFC 0/5] block: File descriptor passing using -open-hook-fd

[Corey Bryant]

On 05/01/2012 06:15 PM, Eric Blake wrote:
> On 05/01/2012 03:53 PM, Anthony Liguori wrote:
>
>>> I think (correct me if I'm wrong) libvirt should be aware of any file
>>> that qemu
>>> asks it to open. So from a security point of view, libvirt can prevent
>>> opening a
>>> file if it isn't affiliated with the guest.
>>
>> Right, libvirt can maintain a whitelist of files QEMU is allowed to open
>> (which is already has because it needs to label these files).
>
> Indeed.
>
>>   The only
>> complexity is that it's not a straight strcmp().  The path needs to be
>> (carefully) broken into components with '.' and '..' handled
>> appropriately.  But this shouldn't be that difficult to do.
>
> Libvirt would probably canonicalize path names, both when sticking them
> in the whitelist, and in validating the requests from qemu - agreed that
> it's not difficult.
>
> More importantly, libvirt needs to start tracking the backing chain of
> any qcow2 or qed file as part of the domain XML; and operations like
> 'block-stream' would update not only the chain, but also the whitelist.
>   In the drive-reopen case, this means that libvirt would have to be
> careful when to change labeling - provide access to the new files before
> drive-reopen, then revoke access to files after drive-reopen completes.
>   In other words, having the -open-hook-fd client pass a command to
> libvirt at the time it is closing an fd would help libvirt know when
> qemu has quit using a file, which might make it easier to revoke SELinux
> labels at that time.
>

If we were to go with this approach, I think the following updates would 
be required for libvirt.  Could you let me know if I'm missing anything?

libvirt tasks:
- Introduce a data structure to store file whitelist per guest
- Add -open-hook-fd option to QEMU command line and pass Unix
   domain socket fd to QEMU
- Create open() handler that handles requests from QEMU to open
   files and passes back fd
- Potentially also handle close requests from QEMU?  Would allow
   libvirt to update XML and whitelist (as well as SELinux labels).
- Canonicalize path names when putting them in whitelist and
   when validating requests from QEMU
- XML updates to track backing chain of qcow2 and qed files
- Update whitelist and XML chain when QEMU monitor commands are
   used to open new files: block-stream, drive-reopen, drive_add,
   savevm, snapshot_blkdev, change

Updates would also be required for SELinux and AppArmor policy to allow 
libvirt open of NFS files, and allow QEMU read/write (no open allowed) 
of NFS Files.

-- 
Regards,
Corey