On Mon, Jun 7 2021 at 09:01:08 AM -0400, Daniel Walsh <dwalsh@redhat.com> wrote:
On 6/4/21 09:59, Daniel P. Berrangé wrote:
On Thu, Jun 03, 2021 at 10:14:24PM -0400, Link Dupont wrote:
reproducible scenarios Alright. I reran my tests with a CentOS 8 guest. On CentOS 8 (with a virtiofs filesystem and with xattr on), the type of files in the mounted hierarchy are unlabeled_t. I can work around that by switching SELinux in
On Thu, Jun 3 2021 at 08:56:46 PM -0400, Link Dupont <link@sub-pop.net> wrote: the guest to permissive or disabled. cc Dan Walsh. I was discussing this with Dan Walsh yesterday in general.
In general, if we want to enable SELinux both on host and guest, then both host and guest should have same SELinux policy. Otherwise there will be lot of different kind of conflicts because both host and guest will try to work with same selinux label. I guess that in practice this will be very hard to achieve as people will run different host and guest flavors and these might have different policies. Yeah, I think there's little to no chance of people keeping the same SELinux policy in host/guest, except in very tightly controlled narrow use cases where the host admin exerts direct control over
On Fri, Jun 04, 2021 at 09:44:39AM -0400, Vivek Goyal wrote: the precise guest config.
So another option is to rename selinux xattr in virtiofs so that any selinux xattr coming from guest is saved as user.virtiofs.security.selinux xattr on host. That way host and guest can have their separate labels without interfering with each other. David Gilbert already has added support for this. I can't remember the exact syntax but you can figure it out from documentation here in xattr remappig section. For general purpose virt usage, I think remapping in some way is likely to be needed as the default strategy.
https://github.com/qemu/qemu/blob/master/docs/tools/virtiofsd.rst
But I have question with selinux xattr remapping. What will happen to initial labels when fs is exported. I mean until and unless some process in guest labels all the exported files, they all with either be unlabeled or pick some generic label for all the files. I'd say you need some mechanism to force a re-label inside the guest. Normally a relabel will be done in /.autorelabel file is present, or in certain other scenarios like selinux policy RPM updates.
We wouldn't want to force a relabel neccesarily for the entire FS if we're just hotplugging a new virtiofs export though. So perhaps there's scope for supporting usage of a per-mount point relabel trigger. eg Host creates $VIRTIOFS-ROOT/.autorelabel and whenever the guest sees a new virtiofs export arriving, it can look for $VIRTIOFS-MOUNT-POINT/.autorelabel
Another option is, can we use a single label for whole of the virtiofs (using context=<label>) option in guest. That way nothing is saved in files as such. But this means that processes in guest can't have different selinux labels on different virtiofs dir/files. Forcing a single label for the entire export is passable as a fallback plan. This is what people have done for years with NFS v3 mounts. It has annoying usage limitations though, so if at all possible remapping is a preferrable approach.
Dan, what do you think?
Thanks Vivek
With a CentOS 7 guest, things get less usable. I digested this to a reproducible scenario.
Build a disk image with `virt-builder`, configuring the CentOS Plus kernel to get 9p support.
virt-builder centos-7.8 \ --root-password password:centos \ --output centos-7.8.qcow2 \ --install yum-utils \ --run-command 'yum-config-manager --enable centosplus' \ --run-command 'sed -ie "s/DEFAULTKERNEL=kernel/DEFAULTKERNEL=kernel-plus/" /etc/sysconfig/kernel' \ --append-line '/etc/dracut.conf.d/virtio.conf:add_drivers+="virtio_scsi virtio_pci virtio_console"' \ --append-line '/etc/modules-load.d/9pnet_virtio.conf:9pnet_virtio' \ --install kernel-plus \ --append-line '/etc/fstab:home /home 9p trans=virtio,version=9p2000.L 0 0'
Install the volume into the `default` pool.
sudo install -m644 centos-7.8.qcow2 /var/lib/libvirt/images
Next, define a domain using the disk image (using `virt-install` here for "easy mode").
virt-install \ --import \ --os-variant centos7.0 \ --name centos \ --ram 2048 \ --disk path=/var/lib/libvirt/images/centos-7.8.qcow2 \ --memorybacking access.mode=shared \ --filesystem source=/home,target=home,accessmode=passthrough \ --autoconsole none
Now with SELinux enforcing, I cannot list the contents of the directories in the mounted hierarchy.
[root@localhost ~]# ls -lZ /home/link ls: cannot open directory /home/link: Permission denied
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Regards, Daniel
The separate XAttr support would also allow virtiofsd to work with labels inside of the container when the virtiofsd is confined on the host.
I would guess/hope virtiofsd on the host will be confined and only able to write certain labels like container_file_t or svirt_image_t. If you want to allow virt file systems within the "VM" to be able to set labels, these labels will have to be something other then SELinux labels on the host, or SELinux will prevent them from being set.
I don't think the guest needs to be able to set labels on the virtiofs filesystems; as long as it can read labels that grant read and write access as the guest user, I'd call it a success. The way I use this feature, I'm treating my host filesystem as the "source", and really only mounting it into the guest as a zero-delay synchronization. (I previously did this using sync solutions like rsync or lsyncd, but those have a slight delay before files sync over.) If the labels as seen by the guest allow for reading and writing files from the guest to the host filesystem, I'd be happy. Doing any relabeling from the host is totally fine.