Re: [libvirt] [PATCH 00/17] Avoid races with udev
Michal Privoznik <mprivozn(a)redhat.com> [2016-10-26, 02:52PM +0200]:
I've came across interesting bug recently. The problem was that
user tried to start a domain, but qemu was denied access to some
device. Even though we relabelled it initially. By debugging I
found the root cause: while we were starting qemu, udev came and
restored original security labels. Sigh. We have two options
here:
a) write out series of udev rules so that whenever it tries to
relabel something our rule will stop it from doing so
b) write a small helper binary that will udev call in order to:
1) detect whether device is in use by libvirt
2) get seclabel that was set by libvirt
These patches implement the latter approach. While these patches
make life easier for us, there is still a race when udev might
restore the device's seclabel before we had chance to flush
internal database of seclabels for the helper binary. This is
something I'm currently focusing on. But before I get there, here
are patches that makes the problem much more bearable.
In case you want to try these patches, here are some scratch builds:
https://mprivozn.fedorapeople.org/udev/
Also, you can find them on my branch:
https://github.com/zippy2/libvirt/commits/udev_labels2
This beast is turned off by default, to turn it on you'll need to add:
write_udev=1
to qemu.conf.
Hello Michal,
will this work (or can be made working) for file-based disk images as
well? Currently, the user can provoke a race condition when defining two
domains using the same file-based disk image and not (wrongly)
specifying the <shareable/> element. What happens is that when the while
the first domain is running, the second domain start will relabel disk
image for its own usage, essentially cutting of the first running
domain. This caused a crash of the first QEMU process until
https://patchwork.ozlabs.org/patch/658163/, now the first domain still
reports I/O errors.
I have investigated this problem for a while now and have seen various
bug reports (https://bugzilla.redhat.com/show_bug.cgi?id=1191802,
https://bugzilla.redhat.com/show_bug.cgi?id=547546) and your attempt to
fix this
(https://www.redhat.com/archives/libvir-list/2015-October/msg00331.html)
but unfortunately this disruptive behaviour still exists.
Best regards,
Bjoern
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