On Thu, May 09, 2024 at 01:58:21PM GMT, Peter Krempa wrote: > On Thu, May 02, 2024 at 19:39:39 +0200, Andrea Bolognani wrote: > > +# libvirt will normally prevent migration if the storage backing the VM is > > not > > +# on a shared filesystems. Sometimes, however, the storage *is* shared > > despite > > +# not being detected as such: for example, this is the case when one of the > > +# hosts involved in the migration is exporting its local storage to the > > other > > +# one via NFS. > > I wanted to suggest using VIR_MIGRATE_UNSAFE flag to bypass this check, > but doing so without disabling dynamic ownership on the image itself > would still make the security driver cut off access to this file.
This was suggested in the past, but the argument against it was that it's too vague: we want to skip the check on whether the storage is shared, and that one only, while "unsafe migration" could potentially cover a lot of unrelated scenarios. The other argument against using a migration flag was that it's an all-or-nothing proposition: either you perform the check for all paths, or you skip it for all paths. A configuration option allows us more granular control, where we can designate exactly which paths should not be subject to the check, retaining the default behavior for all the other ones. > > +# Any directory listed here will be assumed to live on a shared filesystem, > > +# making migration possible in scenarios such as the one described above. > > +# > > +# Due to how label remembering is implemented, it will generally be > > necessary > > +# to set remember_owner=0 *on both sides of the migration* as well. > > So IIUC the problem here is that on the box where the storage is local > this will create the XATTR entries used for remembering the lablels > (which also includes a refcount), which given that older NFS doesn't > support security labelling would then be leaked? Yeah, the expectation when remembering is enabled is that both sides of the migration will keep the record updated. This works when XATTRs/SELinux labels are supported by the shared filesystem or are *not* supported and both sides access it through it, in which case it all ends up being no-op and works by omission :) In our scenario, migrating from local to NFS will work but then migrating back won't, because the existing information will lead libvirt to believe that the image is already in use - which it is, but not in the way that it expects :) In general, all the bookkeeping and relabeling is pretty much broken by design when migration is involved, and things only appear to work because requests are silently ignored. > Looking at the code we seem to be calling 'qemuSecurityRestoreAllLabel' > with the 'migrated' flag set when migrating at all times. This should > make it theoretically possible to simply clear out the XATTRs for any > file which resides on the paths in question if we're migrating away > rather than ignoring them, which would solve the issue. This would involve touching the file's attributes on the source side after the VM is already running on the destination side, which is something that the existing code understandably steers very clear of. I've tried fairly hard to make things work "just so" and by now I'm convinced that there is no solution which gets it right without adding a lot of additional complexity/fragility and introducing its own trade-offs. Asking users to disable owner remembering is not too unreasonable IMO. Especially when we take into account the fact that I'm targeting KubeVirt's needs specifically with this feature, and they *already* do that anyway. > This would go together with the 'syntax-sugar'-ish flavor to this > feature. Additionally it would help in case, when the user configures > these paths after the VM was started and thus the XATTRs are already > applied. Those would not then be cleared on migration. That sounds like a corner case of a corner case, and a direct result of the admin failing to configure the host correctly in the first place. I wouldn't be too concerned about it. > I also wanted to suggest to modify this paragraph to suggest a lesser > hammer to disable ownersip remembering, as that will disable it also for > the non-shared paths, but IIRC we don't have flags to disable it > individually (in contrast with the 'dynamic_ownership' flag which can be > disabled per-image via the 'relabel' attribute). I'm not aware of anything of the sort either. -- Andrea Bolognani / Red Hat / Virtualization _______________________________________________ Devel mailing list -- devel@lists.libvirt.org To unsubscribe send an email to devel-le...@lists.libvirt.org
