[libvirt] [PATCH] Updated to deal with specifying user IDs to that do not map to usernames

[Daniel P. Berrange]

On Tue, Jun 07, 2016 at 11:31:02AM -0400, Laine Stump wrote:
> On 06/07/2016 11:15 AM, Peter Krempa wrote:
> > On Tue, Jun 07, 2016 at 15:50:38 +0100, Daniel Berrange wrote:
> > > On Tue, Jun 07, 2016 at 08:24:14AM +0200, Peter Krempa wrote:
> > > > On Mon, Jun 06, 2016 at 14:25:23 -0500, Roy Keene wrote:
> > > > > Patch to libvirt master to avoid failing when a user ID is specified,
> > > > > e.g. for <seclabel type='dac'>, that does not map to a user name.
> > > > > 
> > > > > This is useful if you want to run each VM as a separate user and not
> > > > > bother creating an /etc/passwd entry for each UID.
> > > > For this use case you shall prefix the name with a +. Please refer to
> > > > the documentation on seclabels.
> > > Empirically that does not currently work:
> > > 
> > > # virsh dumpxml serial | grep --after 2 '<seclabel'
> > >    <seclabel type='static' model='dac' relabel='no'>
> > >      <label>+21421:+12421421</label>
> > >    </seclabel>
> > > 
> > > # virsh start serial
> > > error: Failed to start domain serial
> > > error: Cannot access storage file '/var/lib/libvirt/images/demo.qcow2' (as uid:21421, gid:12421421): Success
> > > 
> > > # ls -al /var/lib/libvirt/images/demo.qcow2
> > > -rw-r--r--. 1 21421 12421421 197120 Apr 30  2015 /var/lib/libvirt/images/demo.qcow2
> > > 
> > > 
> > > Looking at the libvirtd logs we see
> > > 
> > > 2016-06-07 14:49:13.724+0000: 13490: debug : qemuDomainCheckDiskPresence:3954 : Checking for disk presence
> > > 2016-06-07 14:49:16.551+0000: 13490: error : virGetUserEnt:801 : Failed to find user record for uid '21421'
> > > 2016-06-07 14:49:16.551+0000: 13490: error : virStorageFileGetMetadataRecurse:3114 : Cannot access storage file '/var/lib/libvirt/images/demo.qcow2' (as uid:21421, gid:12421421): Success
> > > 
> > > 
> > > So even though the QEMU driver has honoured the '+' syntax, some of the
> > > things QEMU is calling appears to be trying to resolve the UID back into
> > > a user password record and failing.
> > Indeed.  This is caused by calling virFileAccessibleAs which calls
> > virGetGroupList so that it can add all groups for the given UID which is
> > very strange. The group list is then set after forking at attempting to
> > check file presence. I hate root squashed NFS.
> > 
> > I guess we could just skip reporting the error if we can't get the
> > group list in that case and just set the provided numerical UID/GID and
> > try it that way.
> 
> Without taking the time to wind through the code - are you saying that we're
> attempting to get the grouplist for a different uid while running as root,
> then doing a fork/seteuid, and then adding the groups that we learned
> pre-fork?
> 
> If that's the case, then I think the same results can be achieved by getting
> the grouplist *after* seteuid(). It looks like getgroups() gets the groups
> for the current uid, while mgetgroups() gets the groups for a specified
> username, so getgroups() would avoid the need for a username.

I don't think that's going to work as you have a chicken nand egg
scenario. ie getgroups() is just reporting the list of supplementary
groups set by a previous call to initgroups(). It is the inigroups
call that we need to make and why we're lookiing up the grouplist
beforehand.


Regards,
Daniel
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