Description of problem:
I have a libvirt guest with a CDROM device. All attempts to use the 'change' command to alter the CDROM media result in QEMU attempting to open the new media for read-write and failing as selinux blocks this illegal write access attempt.

Furthermore, QMP does not correctly report this EACCESS error instead returning an 'undefined error' message

This is my full guest XML. In particular notice a readonly CDROM and readonly Floppy device

# virsh dumpxml f12i686
<domain type='kvm' id='4'>
  <name>f12i686</name>
  <uuid>0301f2c1-05fc-dddf-089b-60e874f11e64</uuid>
  <memory>1048576</memory>
  <currentMemory>1048576</currentMemory>
  <vcpu cpuset='1'>4</vcpu>
  <os>
    <type arch='x86_64' machine='pc-0.12'>hvm</type>
    <boot dev='hd'/>
  </os>
  <features>
    <acpi/>
    <apic/>
    <pae/>
  </features>
  <cpu match='exact'>
    <model>core2duo</model>
    <topology sockets='1' cores='2' threads='1'/>
    <feature policy='require' name='lahf_lm'/>
    <feature policy='require' name='sse4.1'/>
    <feature policy='require' name='xtpr'/>
    <feature policy='require' name='cx16'/>
    <feature policy='require' name='tm2'/>
    <feature policy='require' name='est'/>
    <feature policy='require' name='vmx'/>
    <feature policy='require' name='ds_cpl'/>
    <feature policy='require' name='pbe'/>
    <feature policy='require' name='tm'/>
    <feature policy='require' name='ht'/>
    <feature policy='require' name='ss'/>
    <feature policy='require' name='acpi'/>
    <feature policy='require' name='ds'/>
  </cpu>
  <clock offset='utc'/>
  <on_poweroff>destroy</on_poweroff>
  <on_reboot>restart</on_reboot>
  <on_crash>restart</on_crash>
  <devices>
    <emulator>/usr/libexec/qemu-kvm</emulator>
    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/var/lib/libvirt/images/f12i686.img'/>
      <target dev='vda' bus='virtio'/>
      <alias name='virtio-disk0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x04' function='0x0'/>
    </disk>
    <disk type='file' device='cdrom'>
      <driver name='qemu'/>
      <source file='/var/lib/libvirt/images/boot.iso'/>
      <target dev='hdc' bus='ide'/>
      <readonly/>
      <alias name='ide0-1-0'/>
      <address type='drive' controller='0' bus='1' unit='0'/>
    </disk>
    <disk type='file' device='floppy'>
      <driver name='qemu'/>
      <target dev='fda' bus='fdc'/>
      <readonly/>
      <alias name='fdc0-0-0'/>
      <address type='drive' controller='0' bus='0' unit='0'/>
    </disk>
    <controller type='ide' index='0'>
      <alias name='ide0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x01' function='0x1'/>
    </controller>
    <controller type='fdc' index='0'>
      <alias name='fdc0'/>
    </controller>
    <interface type='network'>
      <mac address='52:54:00:4c:7e:aa'/>
      <source network='default'/>
      <target dev='vnet0'/>
      <model type='virtio'/>
      <alias name='net0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x06' function='0x0'/>
    </interface>
    <serial type='pty'>
      <source path='/dev/pts/11'/>
      <target port='0'/>
      <alias name='serial0'/>
    </serial>
    <console type='pty' tty='/dev/pts/11'>
      <source path='/dev/pts/11'/>
      <target port='0'/>
      <alias name='serial0'/>
    </console>
    <input type='tablet' bus='usb'>
      <alias name='input0'/>
    </input>
    <input type='mouse' bus='ps2'/>
    <graphics type='vnc' port='5900' autoport='yes' keymap='en-us'/>
    <sound model='es1370'>
      <alias name='sound0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x05' function='0x0'/>
    </sound>
    <video>
      <model type='cirrus' vram='9216' heads='1'/>
      <alias name='video0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x02' function='0x0'/>
    </video>
  </devices>
  <seclabel type='dynamic' model='selinux'>
    <label>system_u:system_r:svirt_t:s0:c112,c115</label>
    <imagelabel>system_u:object_r:svirt_image_t:s0:c112,c115</imagelabel>
  </seclabel>
</domain>


The corresponding full QEMU command line run is:

/usr/libexec/qemu-kvm -S -M pc-0.12 -cpu core2duo,+lahf_lm,+sse4.1,+xtpr,+cx16,+tm2,+est,+vmx,+ds_cpl,+pbe,+tm,+ht,+ss,+acpi,+ds -enable-kvm -m 1024 -smp 4,sockets=1,cores=2,threads=1 -name f12i686 -uuid 0301f2c1-05fc-dddf-089b-60e874f11e64 -nodefaults -chardev socket,id=monitor,path=/var/lib/libvirt/qemu/f12i686.monitor,server,nowait -mon chardev=monitor,mode=control -rtc base=utc -boot c -drive file=/var/lib/libvirt/images/f12i686.img,if=none,id=drive-virtio-disk0,boot=on,format=raw -device virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0 -drive file=/var/lib/libvirt/images/boot.iso,if=none,media=cdrom,id=drive-ide0-1-0,readonly=on -device ide-drive,bus=ide.1,unit=0,drive=drive-ide0-1-0,id=ide0-1-0 -drive if=none,id=drive-fdc0-0-0,readonly=on -global isa-fdc.driveA=drive-fdc0-0-0 -netdev tap,fd=19,id=hostnet0 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:4c:7e:aa,bus=pci.0,addr=0x6 -chardev pty,id=serial0 -device isa-serial,chardev=serial0 -usb -device usb-tablet,id=input0 -vnc 127.0.0.1:0,password -k en-us -vga cirrus -device ES1370,id=sound0,bus=pci.0,addr=0x5 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x3

Notice in particular the 'readonly=on' flag against the CDROM and floppy drives.

QEMU is running as 'qemu:qemu' user/group and SELinux is enforcing


Now attempting to change the CDROM media

  # virsh attach-disk f12i686 /var/lib/libvirt/images/boot.iso hdc --driver file --type cdrom --mode readonly

This results in the following QMP command:

  {"execute":"change","arguments":{"device":"drive-ide0-1-0","target":"/var/lib/libvirt/images/boot.iso"}}

Which returns

  {"error": {"class": "UndefinedError", "desc": "An undefined error has ocurred", "data": {}}}

The strace of the QEMU binary shows

19215 stat("/var/lib/libvirt/images/boot.iso", {st_mode=S_IFREG|0644,
st_size=203423744, ...}) = 0
19215 stat("/var/lib/libvirt/images/boot.iso", {st_mode=S_IFREG|0644,
st_size=203423744, ...}) = 0
19215 open("/var/lib/libvirt/images/boot.iso", O_RDONLY|O_SYNC|O_CLOEXEC) = 21
19215 close(21)                         = 0
19215 stat("/var/lib/libvirt/images/boot.iso", {st_mode=S_IFREG|0644,
st_size=203423744, ...}) = 0
19215 open("/var/lib/libvirt/images/boot.iso", O_RDWR|O_SYNC|O_CLOEXEC) = -1
EACCES (Permission denied)
19215 write(19, "{\"error\": {\"class\": \"UndefinedError\", \"desc\": \"An
undefined error has ocurred\", \"data\": {}}}\r\n", 94) = 94

So it initially opens it readonly, but then closes it and re-opens read-write and fails since SELinux blocks this illegal access attempt.

So there are two problems:

 - The EACCESS error condition is not being reported back via QMP properly

 - Either 
    * QEMU needs to honour the original 'readonly=on' flag for this device when changing media
   Or
    * The 'change' command needs to allow specification of the readonly flag for the new media

I imagine probably the latter is the one that needs fixing, since it also doesn't have any way to specify other things like file format (qcow2, raw, etc)


Version-Release number of selected component (if applicable):
qemu-kvm-0.12.1.2-2.69.el6.x86_64

How reproducible:
Always

Steps to Reproduce:
1. Define the guest XML above
2. Run virsh attach-disk f12i686 /var/lib/libvirt/images/boot.iso hdc --driver file --type cdrom --mode readonly
3. See the undefined error returned

NB, this test scenario relies on SELinux being Enforcing to prevent QEMU's attempt to open it read-write.

  
Actual results:
QEMU opens the boot.iso  readwrite and fails with poor error message

Expected results:
QEMU opens the boot.iso  readonly and suceeds


Additional info: