Libguestfs - May 2013 - Re: How to use libguestfs access LVM as non-root user?

On Fri, May 31, 2013 at 4:17 PM, Richard W.M. Jones
<rjones@redhat.com>wrote:
> On Fri, May 31, 2013 at 04:03:32PM +0800, Qiu Yu wrote:
> > Actually I'm looking into an issue with OpenStack / Libvirt
manipulating
> > LVM as an image backend. When the logical volume is created, udev
rules
> > will set the ownership to root:disk. After libvirt actually starting
an
> > instance, device node ownership will change to qemu:qemu by libvirt.
> Then,
> > if you stop an instance, the device node ownership will change back to
> > root:root. Seems there's some inconsistency here.
>
> This is actually a well-known bug in libvirt.
>
> https://bugzilla.redhat.com/show_bug.cgi?id=796072
> (marked as a duplicate of a private bug 547546 for some reason --
> I will try to get this made non-private)
>
> You can turn off libvirt's ownership management (set dynamic_ownership
> = 0 in /etc/libvirt/qemu.conf) however this will cause other problems.
> I don't think there is a proper solution to this yet.
>
> Rich.
>
>
Richard,

Really thanks for the advice. But seems I'm hitting another permission
issue regarding libguestfs and LVM image.

Following code snippet is stripped from OpenStack data injection logic.
Running as non-root user (stack) will report "guestfs_launch failed"
error,
and the debug message shows it failed to open the LVM device node.

I've already set up the group permission ('disk' and 'fuse')
for the
'stack' user. Could you shed some lights of what else should I look
into?
Thanks!

$ cat test.py
import guestfs
g = guestfs.GuestFS()
g.add_drive_opts("/dev/xenvg/123", format="qcow2")
g.launch()

$ id
uid=501(stack) gid=504(stack) groups=504(stack),6(disk),498(fuse)

$ ll /dev/mapper/xenvg-123
lrwxrwxrwx 1 root root 7 May 31 17:16 /dev/mapper/xenvg-123 -> ../dm-2
$ ll /dev/dm-2
brw-rw---- 1 root disk 253, 2 May 31 18:06 /dev/dm-2

$ python test.py
libguestfs: trace: add_drive_opts "/dev/xenvg/123"
"format:qcow2"
libguestfs: trace: add_drive_opts = 0
libguestfs: trace: launch
libguestfs: trace: launch = -1 (error)
Traceback (most recent call last):
  File "test.py", line 4, in <module>
    g.launch()
  File "/usr/lib/python2.6/site-packages/guestfs.py", line 244, in
launch
    return libguestfsmod.launch (self._o)
RuntimeError: guestfs_launch failed, see earlier error messages
libguestfs: trace: close

$ export LIBGUESTFS_DEBUG=1
$ python test.py
libguestfs: new guestfs handle 0x26a8250
libguestfs: trace: add_drive_opts "/dev/xenvg/123"
"format:qcow2"
libguestfs: trace: add_drive_opts = 0
libguestfs: trace: launch
libguestfs: [00000ms] febootstrap-supermin-helper --verbose -f checksum
'/usr/lib64/guestfs/supermin.d' x86_64
supermin helper [00000ms] whitelist = (not specified), host_cpu = x86_64,
kernel = (null), initrd = (null), appliance = (null)
supermin helper [00000ms] inputs[0] = /usr/lib64/guestfs/supermin.d
checking modpath /lib/modules/2.6.32-358.6.2.ns8.el6.x86_64 is a directory
picked vmlinuz-2.6.32-358.6.2.ns8.el6.x86_64 because modpath
/lib/modules/2.6.32-358.6.2.ns8.el6.x86_64 exists
supermin helper [00000ms] finished creating kernel
supermin helper [00000ms] visiting /usr/lib64/guestfs/supermin.d
supermin helper [00000ms] visiting /usr/lib64/guestfs/supermin.d/base.img
supermin helper [00000ms] visiting /usr/lib64/guestfs/supermin.d/daemon.img
supermin helper [00000ms] visiting /usr/lib64/guestfs/supermin.d/hostfiles
supermin helper [00015ms] visiting /usr/lib64/guestfs/supermin.d/init.img
supermin helper [00015ms] adding kernel modules
supermin helper [00080ms] finished creating appliance
libguestfs: [00083ms] begin testing qemu features
libguestfs: [00097ms] finished testing qemu features
libguestfs: accept_from_daemon: 0x26a8250 g->state = 1
[00098ms] /usr/libexec/qemu-kvm \
    -global virtio-blk-pci.scsi=off \
    -drive file=/dev/xenvg/123,cache=off,format=qcow2,if=virtio \
    -nodefconfig \
    -enable-kvm \
    -nodefaults \
    -nographic \
    -m 500 \
    -no-reboot \
    -device virtio-serial \
    -serial stdio \
    -chardev socket,path=/tmp/libguestfssaaw6T/guestfsd.sock,id=channel0 \
    -device virtserialport,chardev=channel0,name=org.libguestfs.channel.0 \
    -kernel /var/tmp/.guestfs-501/kernel.30285 \
    -initrd /var/tmp/.guestfs-501/initrd.30285 \
    -append 'panic=1 console=ttyS0 udevtimeout=300 no_timer_check acpi=off
printk.time=1 cgroup_disable=memory selinux=0 guestfs_verbose=1
TERM=screen-bce ' \
    -drive
file=/var/tmp/.guestfs-501/root.30285,snapshot=on,if=virtio,cache=unsafeqemu-kvm:
-drive file=/dev/xenvg/123,cache=off,format=qcow2,if=virtio: could not open
disk image /dev/xenvg/123: Invalid argument
libguestfs: child_cleanup: 0x26a8250: child process died
libguestfs: trace: launch = -1 (error)
Traceback (most recent call last):
  File "test.py", line 4, in <module>
    g.launch()
  File "/usr/lib/python2.6/site-packages/guestfs.py", line 244, in
launch
    return libguestfsmod.launch (self._o)
RuntimeError: guestfs_launch failed, see earlier error messages
libguestfs: trace: close
libguestfs: closing guestfs handle 0x26a8250 (state 0)

--
Qiu Yu

On Fri, May 31, 2013 at 06:39:53PM +0800, Qiu Yu wrote:
> [00098ms] /usr/libexec/qemu-kvm \
>     -global virtio-blk-pci.scsi=off \
>     -drive file=/dev/xenvg/123,cache=off,format=qcow2,if=virtio \
>     -nodefconfig \
>     -enable-kvm \
>     -nodefaults \
>     -nographic \
>     -m 500 \
>     -no-reboot \
>     -device virtio-serial \
>     -serial stdio \
>     -chardev socket,path=/tmp/libguestfssaaw6T/guestfsd.sock,id=channel0 \
>     -device virtserialport,chardev=channel0,name=org.libguestfs.channel.0 \
>     -kernel /var/tmp/.guestfs-501/kernel.30285 \
>     -initrd /var/tmp/.guestfs-501/initrd.30285 \
>     -append 'panic=1 console=ttyS0 udevtimeout=300 no_timer_check
acpi=off
> printk.time=1 cgroup_disable=memory selinux=0 guestfs_verbose=1
> TERM=screen-bce ' \
>     -drive
>
file=/var/tmp/.guestfs-501/root.30285,snapshot=on,if=virtio,cache=unsafeqemu-kvm:
> -drive file=/dev/xenvg/123,cache=off,format=qcow2,if=virtio: could not open
> disk image /dev/xenvg/123: Invalid argument
I'm assuming it's because the format is wrong (ie. not qcow2 but raw).

The error message is a little bit obscure and could be better, but we
do rely on qemu printing something sensible instead of just "Invalid
argument".

What happens if you do:

  file -bsL /dev/xenvg/123

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v

On Fri, May 31, 2013 at 06:39:53PM +0800, Qiu Yu wrote:
> $ cat test.py
> import guestfs
> g = guestfs.GuestFS()
> g.add_drive_opts("/dev/xenvg/123", format="qcow2")
Another option is to completely omit the 'format = ...' option.

Then libguestfs will autodetect the format for you.

However this is not as secure as specifying the format:
  http://libguestfs.org/guestfs.3.html#cve-2010-3851
so in production code you should specify the format if you know it,
especially if the guest is not trusted.

In recent libguestfs >= 1.20 you can also query the format using:
`g.disk_format (filename)' but you should still be careful that
you don't accidentally hit CVE-2010-3851.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into KVM guests.
http://libguestfs.org/virt-v2v

On Fri, May 31, 2013 at 7:25 PM, Richard W.M. Jones
<rjones@redhat.com>wrote:
> On Fri, May 31, 2013 at 06:39:53PM +0800, Qiu Yu wrote:
> > [00098ms] /usr/libexec/qemu-kvm \
> >     -global virtio-blk-pci.scsi=off \
> >     -drive file=/dev/xenvg/123,cache=off,format=qcow2,if=virtio \
> >     -nodefconfig \
> >     -enable-kvm \
> >     -nodefaults \
> >     -nographic \
> >     -m 500 \
> >     -no-reboot \
> >     -device virtio-serial \
> >     -serial stdio \
> >     -chardev
socket,path=/tmp/libguestfssaaw6T/guestfsd.sock,id=channel0
> \
> >     -device
> virtserialport,chardev=channel0,name=org.libguestfs.channel.0 \
> >     -kernel /var/tmp/.guestfs-501/kernel.30285 \
> >     -initrd /var/tmp/.guestfs-501/initrd.30285 \
> >     -append 'panic=1 console=ttyS0 udevtimeout=300 no_timer_check
> acpi=off
> > printk.time=1 cgroup_disable=memory selinux=0 guestfs_verbose=1
> > TERM=screen-bce ' \
> >     -drive
> >
>
file=/var/tmp/.guestfs-501/root.30285,snapshot=on,if=virtio,cache=unsafeqemu-kvm:
> > -drive file=/dev/xenvg/123,cache=off,format=qcow2,if=virtio: could not
> open
> > disk image /dev/xenvg/123: Invalid argument
>
> I'm assuming it's because the format is wrong (ie. not qcow2 but
raw).
>
> The error message is a little bit obscure and could be better, but we
> do rely on qemu printing something sensible instead of just "Invalid
> argument".
>
>
Oh, yes. It's indeed the wrong 'format' argument causing the issue.
Sorry
I'm not familiar with libguestfs, 'invalid argument' should be an
obvious
hint for me to follow on.

For others who might interests, if you use qcow2 image with OpenStack and
choose LVM as image backend, nova libvirt driver will convert it raw when
creating a new instance. However, data injection using libguestfs will
still assuming the it is qcow2 format, hence hit the above issue. Not sure
it is a bug or not, probably one should specify "use_cow_images =
False" to
avoid the problem in this case.

--
Qiu Yu