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

On 1/23/19 7:36 AM, Daniel P. Berrangé wrote: > On Wed, Jan 23, 2019 at 02:33:01PM +0100, Erik Skultety wrote: >> On Wed, Jan 23, 2019 at 01:24:13PM +0000, Daniel P. Berrangé wrote: >>> On Wed, Jan 23, 2019 at 02:22:12PM +0100, Erik Skultety wrote: >>>> On Wed, Jan 23, 2019 at 01:10:42PM +0000, Daniel P. Berrangé wrote: >>>>> 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 ? >>>> >>>> I don't know, that's why I'm asking, because the manual didn't make it any >>>> clear for me whether there's a connection between the device permissions and >>>> ioctls that you're allowed to run. >>>> >>>>> >>>>>> 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. >>>> >>>> Sure we can, but that would make libvirt the only legitimate user of /dev/sev >>>> and everything else would require the admin to change the permissions >>>> explicitly so that other apps could at least retrieve the platform info, if >>>> it was intended to be for public use? >>>> Additionally, we'll still get shot down by SELinux because svirt_t wouldn't be >>>> able to access /dev/sev by default. >>> >>> That's separate form probing and just needs SELinux policy to define >>> a new sev_device_t type and grant svirt_t access to it. >> >> I know, I misread "we can solve this entirely in libvirt" then, I thought you >> the SELinux part was included in the statement, my bad then. Still, back to the >> original issue, we could technically do both, libvirt would have run qemu with >> CAP_DAC_OVERRIDE and we keep working with everything's been released for >> SEV in kernel/qemu and for everyone else, systemd might add 0644 for /dev/sev, >> that way, everyone's happy, not that I'd be a fan of libvirt often having >> to work around something because projects underneath wouldn't backport fixes to >> all the distros we support, thus leaving the dirty work to us. > > Setting 0644 for /dev/sev looks unsafe to me unless someone can show > where the permissions checks take place for the many ioctls that > /dev/sev allows, such that only SEV_PDH_CERT_EXPORT or SEV_PLATFORM_STATUS > is allowed when /dev/sev is opened by a user who doesn't have write > permissions. > Agree its not safe to do 0644. Currently, anyone who has access to /dev/sev (read or write) will be able to execute SEV platform command. In other words there is no permission check per command basis. I must admit that while developing the driver I was under assumption that only root will ever access the /dev/sev device hence overlooked it. But now knowing that others may also need to access the /dev/sev, I can submit patch in kernel to do per command access control. Until then, can we follow Daniel's recommendation to elevate privilege of the probing code?