Re: [PATCH v13 0/6] Support query and use SGX

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On Thu, Jul 21, 2022 at 03:12:05PM +0200, Michal Prívozník wrote:
> On 7/21/22 10:06, Daniel P. Berrangé wrote:
> > On Wed, Jul 20, 2022 at 11:12:56PM +0000, Yang, Lin A wrote:
> >>> This version is a bit better than the previous one. But we're at version
> >>> 13 and I am still unable to even start a guest. Please, make sure that this
> >>> basic functionality works in v14, because this is plain waste of precious
> >>> review bandwidth.
> >>>
> >>> Anyway, as usual, I've uploaded my suggested fixes here:
> >>>
> >>> https://gitlab.com/MichalPrivoznik/libvirt/-/commits/sgx/
> >>
> >> Sorry to hear it didn't work in your environment. We definitely tested it
> >> several times and it works well for both QEMU 6.2.0 and 7.0.0.
> 
> Alright, I finally made it work. The problem was with memfd backend.
> I'll post patch for that soon.
> 
> >>
> >> Let me try to reproduce it with the domain xml you shared before.
> >>
> >> By my best guess, if you see "qemu-system-x86_64:***: 
> >> invalid object type: memory-backend-epc" error, it means QEMU didn't
> >> get enough permission to launch SGX VM.
> >>
> >> Pls add /dev/sgx_vepc, /dev/sgx_enclave and /dev/sgx_provision to the 
> >> "cgroup_device_acl" list in /etc/libvirt/qemu.conf. QEMU requires those access
> >> to assign EPC, but it was denied  by libvirt’s cgroup controllers by default.
> >>
> >> cgroup_device_acl = [  
> >>     "/dev/null", "/dev/full", "/dev/zero",  
> >>     ...
> >>     "/dev/sgx_vepc", 
> >>     "/dev/sgx_enclave”, 
> >>     "/dev/sgx_provision” 
> >> ] 
> >>
> >> Also in /etc/libvirt/qemu.conf, set the runtime user to uid 0,  since QEMU needs to
> >> read and write to those sgx devices, like /dev/sgx_vepc. Unfortunately, it is owned
> >> by root with file mode 600, so QEMU has to launch as root. 
> >>
> >> user = "+0"
> >>
> >> It would be really helpful if you can use these steps to see whether it resolve 
> >> the issue. I will add a doc somewhere to include all steps are required for use to
> >> use sgx in libvirt.
> > 
> > The need to customize qemu.conf to change cgroups ACLs and set uid==0 makes
> > this patch series unusal in the real world deployments. It cannot be merged
> > with such problems existing.
> > 
> 
> Agreed. While libvirt can allow /dev/sgx* in CGroups (we do that for
> other devices, including NVDIMM and virtio-pmem types of <memory/>),
> it's more tricky with relabelling.
> 
> By default, when available, libvirt creates a separate mount namespace
> for each QEMU process and creates a very small /dev there, with only
> those nodes that QEMU needs. Now, if libvirt is fixed (I have follow up
> patches on top of this series) the /dev/sgx* nodes are created there AND
> I have another patch that sets DAC/SELinux label on them so that uid=0
> is no longer needed. What I worry about though, is the case when this
> namespace feature is disabled. Then libvirt should not touch /dev/sgx*
> because that might compromise security in the system.

That might in turn require the ability to pass in pre-opened FDs for
the devices to QEMU.

> 
> > Are the /dev/sgx* fundamentally required to be restricted to root access
> > only, or is it safe to make them accessible to non-root ? ie If a malicious
> > user has access to those files, what is the impact they have ? Bear in mind
> > that QEMU itself can be malicious if the guest compromises it.
> 
> If we get an agreement here, I can cleanup this v13 and post v14 that
> include all patches mentioned.
> 
> Michal
> 

With regards,
Daniel
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