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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?

-Brijesh


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