Re: [libvirt] running Libvirt from source code, IPC_LOCK and VFIO
On 2/4/19 7:48 AM, Christian Ehrhardt wrote:
On Mon, Feb 4, 2019 at 9:59 AM Michal Privoznik
<mprivozn(a)redhat.com> 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
FYI Ubuntu's version is build with capng for the cap dropping feature
as discussed in this thread.
libcap-ng-dev is installed as build dependencies and "--with-capng"
is set on all Linux builds.
Good to know. "jus disable ligcap-ng support" wasn't a solution that I
was going
to propose, but with this info I can explain to other people why this isn't
viable at all.
>> 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.
Thanks for the explanation.
What I need to do then is to see exactly the capabilities Iam missing
from QEMU
(aside from IPC_LOCK) and see if we can get Libvirt to set them.
Thanks,
DHB
>
> 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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