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

[Erik Skultety]

On Wed, Jan 30, 2019 at 06:18:22PM +0000, Daniel P. Berrangé wrote:
> 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

Fair enough, I resent the series where I only exposed /dev/sev to machines that
need it and applied the DAC_OVERRIDE_PATCH on top of that.
https://www.redhat.com/archives/libvir-list/2019-January/msg01343.html

Erik