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Re: [libvirt] [Qemu-devel] [PATCH v4 0/5] Per-guest configurable user/group for QEMU processes





On 09/14/2012 09:51 AM, Daniel P. Berrange wrote:
On Fri, Sep 14, 2012 at 09:31:26AM -0400, Corey Bryant wrote:


On 09/14/2012 04:40 AM, Daniel P. Berrange wrote:
On Tue, Sep 11, 2012 at 02:13:38PM -0400, Corey Bryant wrote:
Are there any other requirements that need to be taken care of to
enable execution of QEMU guests under separate unprivileged user IDs
(ie. DAC isolation)?

At this point, this patch series (Per-guest configurable user/group
for QEMU processes) is upstream, allowing libvirt to execute guests
under separate unprivileged user IDs.  Additionally, the QEMU bridge
helper series is upstream, allowing QEMU to allocate a tap device
and attach it to a bridge when run under an unprivileged user ID (http://www.redhat.com/archives/libvir-list/2012-August/msg00277.html).

Is there any other feature in QEMU that requires QEMU to be run as root?

Well those features you mention are for two separate issues. When
running libvirt privileged (qemu:///system), QEMU was already run
as non-root (qemu:qemu). The per-guest user/group was just making
sure that QEMU VMs were  isolated from each other using user IDs.
Since libvirtd is running privileged, it can either set permissions
or open things on QEMU's behalf. All this side of things really
works already.

Ok good. This is really what I was getting at and you answered my
question.  So we now have DAC isolation of QEMU guests when running
with the qemu:///system URI and there shouldn't be any issues
running unprivileged guests from a privileged libvirt.


The TAP device bridge helper is something that's needed when running
libvirtd itself unprivileged (eg the per user qemu:///session libvirtd).
In this case libvirtd can't access privileged resources at all, hence
the setuid TAP helper was required.


Ah, that's right, the bridge helper is really only benefiting
libvirt when running with the qemu:///session URI.

Is there a desire to get to a point where libvirt can do everything
under a session URI that it can do today under a system URI?  Then
libvirt and guests could all run unprivileged.  I'm sure it's a lot
of work.. I'm just asking. :)

Well if you want to give a VM a raw block device someone/thing needs to
be running privileged to set an ACL on the device to le the unprivileged
VM use it. Similarly for PCI device passthrough. Traditionally in the
qemu:///system case, libvirt can deal with this. In a qemu:///session
case the sysadmin would have had to setup ACLs/permissions on the
devices / files ahead of time.

Perhaps these are things that could eventually be taken care of by a setuid root helper with reduced capabilities, allowing libvirt to run unprivileged.

--
Regards,
Corey Bryant


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