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

[Daniel P . Berrangé]

On Wed, Jan 30, 2019 at 02:39:54PM +0100, Erik Skultety wrote:
> > > > though, we need a #ifdef check for existance of PR_CAP_AMBIENT
> > > >
> > > > > An alternative question I've been playing ever since we exchanged the 
> > > > > last few
> > > > > emails is that can't we wait until the ioctls are compared against 
> > > > > permissions
> > > > > in kernel so that upstream libvirt (and downstream too for that 
> > > > > matter) doesn't
> > > > > have to work around it and stick with that workaround for eternity?
> > > >
> > > > IIUC, the SEV feature has already shipped with distros, so we'd 
> > > > effectively
> > > > be saying that what we already shipped is unusable to libvirt. This 
> > > > doesn't
> > > > feel like a desirable story to me.
> > >
> > > It was, but it never worked, it always has been broken in this way. When 
> > > we
> > > were merging this upstream, we had a terrible shortage of machines and we 
> > > had
> > > to share, so the first person to provision the machine had already taken 
> > > care
> > > of the permissions in order to test so that led to this issue having been
> > > overlooked until now. If it ever worked as expected and then we broke it, 
> > > then
> > > any fix from our side would make sense but otherwise I believe we should 
> > > fix
> > > this bottom up.
> >
> > Well technically it would work if libvirt was configured to run as
> > root:root, but yes, that is not a normal or recommended configuration.
> >
> > Personally I have a preference for userspace solutions, as those are
> > pretty straightforward to roll out to people as patches in existing
> > releases. Deploying kernel updates is a higher bar to cross for an
> > existing release.
> 
> So, can you compile the prctl stuff in kernel conditionally? If so, then 
> that's
> a problem because you may end up with a platform where SEV is supported within
> kernel, but you don't have the ambient stuff we have to conditionally compile
> in libvirt, so you end up with broken SEV support anyway, I wanted to argue
> with centos 7, but the ambient set support was backported to 3.10, so the only
> distro where we'd have a problem from userspace POV would be debian 8, but 
> then
> again the kernel there is so old that neither SEV is supported there.
> 
> I understand your point, but it also sounds very agile and I don't think that
> compensating with "something that is fast" for "something that is right" is 
> the
> way to go in the long term. Especially since we almost never deprecate stuff
> and we can't break compatibility. Trying to work around every issue coming
> from your dependencies in your project is highly unsustainable.

With the launching of VMs we've got to a place where libvirt is pretty
robust about being able to grant access regardless of what the host OS
has done for permissions in /dev.  I think its desirable that this same
flexibility extends to capabilities probing, which is somethign the
dac_override approach allows. IOW, even if the kernel changes /dev/sev
as previously discussed, I would keep the dac_override stuff for probing
capabilities forever. This makes sure we'll work even if the distro in
question has strictly locked down permissions on /dev/kvm or /dev/sev,
diverging from the default udev settup


Regards,
Daniel
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