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[libvirt] Restore original security labels

Dear list,

it's been a while since I've tried to get the patches in [1].
However, it turned out that we need completely different approach. Now
I'd like to revisit that decision.

The problem is: libvirt sets various security labels (DAC, selinux) in
order for a file to be readable by a qemu process. However, it doesn't
record the original labels, so in process of tearing the domain down, we
restore "defaults" (in case of DAC we set root:root instead of
john:doe). Moreover, if a file is to be shared among multiple domains we
can't restore the label as it would make it inaccessible for other qemu

My implementation dealt with this problem using XATTRs: one to store the
original label, the other one as a reference counter. For each labeling
the counter is increased. For each attempt to restore the label the
counter is decreased. The original label is restored iff the counter is
zero. However, this approach doesn't work well with two libvirtd
instances fighting over a file. But one can argue that this is something
for cluster. The question is - do we want to reimplement cluster in libvirt?

I think my approach seems like reasonable trade-off. So what's your
opinion on this?


1: http://www.redhat.com/archives/libvir-list/2013-March/msg01289.html

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