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

On Wed, Jan 23, 2019 at 01:55:06PM +0100, Erik Skultety wrote: > 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? I'm not seeing any permissions checks in the sev_ioctl() function in the kernel, so IIUC, that means any permissions are entirely based on whether you can open the /dev/sev, once open you can run any ioctl. What, if anything, enforces which ioctls you can run when the device is only O_RDONLY vs O_RDWR ?

> 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? Based on what I think I see above, this looks like a bad idea. It still looks like we can solve this entirely in libvirt by just giving the libvirt capabilities probing code CAP_DAC_OVERRIDE. This would make libvirt work for all currently released SEV support in kernel/qemu.