[libvirt] running Libvirt from source code, IPC_LOCK and VFIO

[Yuval Shaia]

+ Kamal and Marcel

On Mon, Feb 04, 2019 at 09:58:47AM +0100, Michal Privoznik wrote:
> On 2/1/19 7:04 PM, Daniel Henrique Barboza wrote:
> > Hi,
> > 
> > I'm facing a strange behavior when running Libvirt from source code,
> > latest upstream, on an Ubuntu 18.04.1 LTS Power 9 server. My QEMU
> > guest - which is using VFIO and GPU passthrough - breaks on boot when
> > trying to allocate a DMA window inside KVM.
> > 
> > Debugging the code, I've found out that the problem is related to the
> > process
> > not having CAP_IPC_LOCK - at least from the host kernel perspective.
> > 
> > This is strange because:
> > 
> > - the same VM running directly from QEMU command line works
> > - the same VM running in the system Libvirt (v4.0.0, Ubuntu version)
> > also works
> > 
> > What am I missing? My understanding on Linux process is that a process
> > running as root should inherit the same capabilities of the user, which
> > includes
> > CAP_IPC_LOCK. Running Libvirt from source code should grant ipc_lock
> > to it ... right?
> 
> No. Ideally, you trust libvirt and want it to manage devices on your system
> thus it needs all the capabilities. But qemu spawn by libvirt should have no
> capabilities as libvirt set up everything that's needed for qemu to run. But
> this is hard to get right - qemu changes and so does the capabilities it may
> require (these depend on domain configuration anyway). Therefore, it is
> possible to set libvirt so it does not drop capabilities for qemu process -
> see clear_emulator_capabilities in qemu.conf - but then libvirt can't
> guarantee that a compromised qemu does no harm.

In my case it is not a matter of risk of a malicious guest, it is that the
device cannot utilize the host device *unless* it has the 'lock'
capability.

> 
> This corresponds with your finding about ./configure - if there is no
> libncap-ng found there's no way for libvirt to drop capabilities and thus it
> doesn't do that.
> 
> Michal
> 
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