Description of problem:

In qemu it's possible to set arbitrary "URIs" as backing sources.
Where by "URI" I mean some qemu URI such as nbd://...  eg:

$ getenforce 
Enforcing

$ nbdkit -r /usr/lib64/nbdkit/plugins/nbdkit-xz-plugin.so \
      file=/tmp/f17x64.img.xz -f -U /tmp/socket &

$ qemu-img create -f qcow2 -b 'nbd:unix:/tmp/socket' overlay.qcow2
Formatting 'overlay.qcow2', fmt=qcow2 size=34359738368 backing_file='nbd:unix:/tmp/socket' encryption=off cluster_size=65536 lazy_refcounts=off 
$ qemu-img info overlay.qcow2 
image: overlay.qcow2
file format: qcow2
virtual size: 32G (34359738368 bytes)
disk size: 196K
cluster_size: 65536
backing file: nbd:unix:/tmp/socket    <--- note

$ guestfish -a overlay.qcow2 -i -v
[...]
libguestfs: [06549ms] launch libvirt guest
libguestfs: error: could not create appliance through libvirt: internal error process exited while connecting to monitor: qemu-system-x86_64: -drive file=/tmp/overlay.qcow2,if=none,id=drive-scsi0-0-0-0,format=qcow2,cache=writeback: Failed to connect to socket: Permission denied
qemu-system-x86_64: -drive file=/tmp/overlay.qcow2,if=none,id=drive-scsi0-0-0-0,format=qcow2,cache=writeback: could not open disk image /tmp/overlay.qcow2: Permission denied
 [code=1 domain=10]

$ killall nbdkit ; rm /tmp/socket

The SELinux error is:

time->Mon Sep 23 15:44:55 2013
type=SYSCALL msg=audit(1379947495.028:30674): arch=c000003e syscall=42 success=no exit=-13 a0=d a1=7fff50eb1e80 a2=6e a3=b items=0 ppid=1 pid=13478 auid=1000 uid=1000 gid=1000 euid=1000 suid=1000 fsuid=1000 egid=1000 sgid=1000 fsgid=1000 ses=1 tty=(none) comm="qemu-system-x86" exe="/usr/bin/qemu-system-x86_64" subj=unconfined_u:system_r:svirt_t:s0:c244,c608 key=(null)
type=AVC msg=audit(1379947495.028:30674): avc:  denied  { write } for  pid=13478 comm="qemu-system-x86" name="socket" dev="dm-2" ino=1968389 scontext=unconfined_u:system_r:svirt_t:s0:c244,c608 tcontext=unconfined_u:object_r:user_tmp_t:s0 tclass=sock_file

which indicates that the socket has not been relabelled.

Version-Release number of selected component (if applicable):

libvirt-1.0.5.5-1.fc19.x86_64

How reproducible:

100%

Steps to Reproduce:
1. See above.

Additional information:

This works fine if SELinux is permissive.