+ 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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