Re: [libvirt] [Qemu-devel] [RFC 0/5] block: File descriptor passing using -open-hook-fd

Am 01.05.2012 22:25, schrieb Anthony Liguori: > Thanks for sending this out Stefan. > > On 05/01/2012 10:31 AM, Stefan Hajnoczi wrote: >> Libvirt can take advantage of SELinux to restrict the QEMU process and prevent >> it from opening files that it should not have access to. This improves >> security because it prevents the attacker from escaping the QEMU process if >> they manage to gain control. >> >> NFS has been a pain point for SELinux because it does not support labels (which >> I believe are stored in extended attributes). In other words, it's not >> possible to use SELinux goodness on QEMU when image files are located on NFS. >> Today we have to allow QEMU access to any file on the NFS export rather than >> restricting specifically to the image files that the guest requires. >> >> File descriptor passing is a solution to this problem and might also come in >> handy elsewhere. Libvirt or another external process chooses files which QEMU >> is allowed to access and provides just those file descriptors - QEMU cannot >> open the files itself. >> >> This series adds the -open-hook-fd command-line option. Whenever QEMU needs to >> open an image file it sends a request over the given UNIX domain socket. The >> response includes the file descriptor or an errno on failure. Please see the >> patches for details on the protocol. >> >> The -open-hook-fd approach allows QEMU to support file descriptor passing >> without changing -drive. It also supports snapshot_blkdev and other commands >> that re-open image files. >> >> Anthony Liguori<aliguori(a)us.ibm.com> wrote most of these patches. I added a >> demo -open-hook-fd server and added some small fixes. Since Anthony is >> traveling right now I'm sending the RFC for discussion. > > What I like about this approach is that it's useful outside the block layer and > is conceptionally simple from a QEMU PoV. We simply delegate open() to libvirt > and let libvirt enforce whatever rules it wants. > > This is not meant to be an alternative to blockdev, but even with blockdev, I > think we still want to use a mechanism like this even with blockdev. What does it provide on top? This doesn't look like something that I'd like a lot. qemu should be able to continue to run no matter what the management tool does, whether it responds to RPCs properly or whether it has crashed. You need a really good use case for the RPC that cannot be covered otherwise in order to justify this.