On 07/19/11 15:58, Eric Blake wrote: > On 07/19/2011 07:27 AM, Jes Sorensen wrote: >> Eric, what happens if libvirt in an selinux environment tells QEMU to >> launch using an image file that is backed by backing file(s)? > > Before starting qemu, libvirt first parses all the image files, to see > if any of them have backing images. For every qcow2 or qed image with a > backing file, libvirt sets the SELinux context of both the qcow2 image > and its backing file so that qemu will be able to successfully open() > them. But if any of those files reside on NFS, then it is not possible > to label individual files, so it requires setting the SELinux bool > virt_use_nfs, which thus gives qemu the power to open() arbitrary files > on NFS, and you've lost security.
Urgh, libvirt parsing image files is really unfortunate, it really doesn't give me warm fuzzy feelings :( libvirt really should not know about internals of image formats. > It would be nice if libvirt had a way to pass fds for every disk and > backing file up front; then, SELinux can work around the lack of NFS > per-file labelling by blocking open() in qemu. In fact, this has > already been proposed: A cleaner solution seems to have libvirt provide a call-back allowing QEMU to call out and have libvirt open a file descriptor instead. This way libvirt can validate it and open it for QEMU and pass it back. If we cannot do something like this, I would prefer to have backing files on NFS should simply not be supported when running in an selinux setup. Cheers, Jes
