Re: [libvirt] AMD SEV's /dev/sev permissions and probing QEMU for capabilities

On Fri, Jan 18, 2019 at 12:51:50PM +0000, Singh, Brijesh wrote: > > On 1/18/19 3:39 AM, Erik Skultety wrote: > > Hi, > > this is a summary of a private discussion I've had with guys CC'd on this email > > about finding a solution to [1] - basically, the default permissions on > > /dev/sev (below) make it impossible to query for SEV platform capabilities, > > since by default we run QEMU as qemu:qemu when probing for capabilities. It's > > worth noting is that this is only relevant to probing, since for a proper QEMU > > VM we create a mount namespace for the process and chown all the nodes (needs a > > SEV fix though). > > > > # ll /dev/sev > > crw-------. 1 root root > > > > I suggested either force running QEMU as root for probing (despite the obvious > > security implications) or using namespaces for probing too. Dan argued that > > this would have a significant perf impact and suggested we ask systemd to add a > > global udev rule. > > > > I proceeded with cloning [1] to systemd and creating an udev rule that I planned > > on submitting to systemd upstream - the initial idea was to mimic /dev/kvm and > > make it world accessible to which Brijesh from AMD expressed a concern that > > regular users might deplete the resources (limit on the number of guests > > allowed by the platform). > > > During private discussion I didn't realized that we are discussing a > probe issue hence things I have said earlier may not be applicable > during the probe. The /dev/sev is managed by the CCP (aka PSP) driver. > The /dev/sev is used for communicating with the SEV FW running inside > the PSP. The SEV FW offers platform and guest specific services. The > guest specific services are used during the guest launch, these services > are available through KVM driver only. Whereas the platform services can > be invoked at anytime. A typical platform specific services are: > > - importing certificates > > - exporting certificates > > - querying the SEV FW version etc etc > > In case of the probe we are not launch SEV guest hence we should not be > worried about depleting the SEV ASID resources. > > IIRC, libvirt uses QEMP query-sev-capabilities to probe the SEV support. > QEMU executes the below sequence to complete the request: > > 1. Exports the platform certificates  (this is when /dev/sev is accessed). > > 2. Read the host MSR to determine the C-bit and reduced phys-bit position > > I don't see any reason why we can't give world a 'read' permission to > /dev/sev. Anyone should be able to export the certificates and query Okay, makes sense to me. The problem I see is the sev_platform_ioctl function in QEMU which makes an _IOWR request, therefore the file descriptor being opened in sev_get_capabilities is O_RDWR. Now, I only understand ioctl from what I've read in the man page, so I don't quite understand the need for IOWR here - but my honest guess would be that it's because the commands like SEV_PDH_CERT_EXPORT or SEV_PLATFORM_STATUS need to be copied from userspace to kernel to instruct kernel which services we want, ergo _IOWR, is that right?

In any case, a fix of some sort needs to land in QEMU first, because no udev rule would fix the current situation. Afterwards, I expect that having a rule like this: KERNEL=="sev", GROUP="kvm", MODE="0644" and a selinux policy rule adding the kvm_device_t label, we should be fine, do we agree on that?