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Re: [libvirt] [PATCH v2 7/7] qemu_security: Lock metadata while relabelling



On Thu, Aug 23, 2018 at 05:45:42PM +0200, Michal Privoznik wrote:
> On 08/23/2018 05:01 PM, Daniel P. Berrangé wrote:
> > On Thu, Aug 23, 2018 at 04:54:01PM +0200, Michal Privoznik wrote:
> >> On 08/20/2018 07:17 PM, Michal Prívozník wrote:
> >>> On 08/20/2018 05:16 PM, Daniel P. Berrangé wrote:
> >>>> On Tue, Aug 14, 2018 at 01:19:43PM +0200, Michal Privoznik wrote:
> >>>>> Fortunately, we have qemu wrappers so it's sufficient to put
> >>>>> lock/unlock call only there.
> >>>>>
> >>>>> Signed-off-by: Michal Privoznik <mprivozn redhat com>
> >>>>> ---
> >>>>>  src/qemu/qemu_security.c | 107 +++++++++++++++++++++++++++++++++++++++++++++++
> >>>>>  1 file changed, 107 insertions(+)
> >>>>>
> >>
> >>
> >>>> I'm wondering if this is really the right level to plug in the metadata
> >>>> locking ?  What about if we just pass a virLockManagerPtr to the security
> >>>> drivers and let them lock each resource at the time they need to modify
> >>>> its metadata. This will trivially guarantee that we always lock the exact
> >>>> set of files that we'll be changing metadata on.
> >>>>
> >>>> eg in SELinux driver the virSecuritySELinuxSetFileconHelper method
> >>>> would directly call virLockManagerAcquire & virLockManagerRelease,
> >>>> avoiding the entire virDomainLock  abstraction which was really
> >>>> focused around managing the content locks around lifecycle state
> >>>> changes.
> >>>>
> >>>
> >>> Yeah, I vaguely recall writing code like this (when I was trying to
> >>> solve this some time ago). Okay, let me see if that's feasible.
> >>>
> >>> But boy, this is getting hairy.
> >>
> >> So as I'm writing these patches I came to realize couple of things:
> >>
> >> a) instead of domain PID we should pass libvirtd PID because we want to
> >> release the locks if libvirtd dies not domain.
> >>
> >> b) that, however, leads to a problem because
> >> virLockManagerLockDaemonAcquire() closes the socket to virtlockd causing
> >> it to kill the owner of the lock, i.e. it kills libvirtd immediately.
> > 
> > This is fine ;-P
> > 
> >> c) even if I hack around b) so that we connect only once for each
> >> lock+unlock pair call, we would still connect dozens of times when
> >> starting a domain (all the paths we label times all active secdrivers).
> >> So we should share connection here too.
> > 
> > Yeah, makes sense.
> > 
> >> Now question is how do do this effectively and cleanly (code-wise). For
> >> solving b) we can have a new flag, say VIR_LOCK_MANAGER_KEEP_CONNECT
> >> that would cause  virLockManagerLockDaemonAcquire() to save the
> >> (connection, program, counter) tuple somewhere into lock driver private
> >> data so that virLockManagerLockDaemonRelease() called with the same flag
> >> can re-use the data.
> >>
> >> For c) I guess we will need to open the connection in
> >> virLockManagerLockDaemonNew(), put the socket FD into event loop so that
> >> EOF is caught and initiate reopen in that case. However, I'm not sure if
> >> this is desirable - to be constantly connected to virtlockd.
> > 
> > Can we use a model similar to what I did for the shared secondary
> > driver connections.
> > 
> > By default a call to virGetConnectNetwork() will open a new connection.
> > 
> > To avoid opening & closing 100's of connections though, some places
> > will call virSetConnectNetwork() to store a pre-opened connection in
> > a thread local. That stays open until virSetConnectNetwork is called
> > again passing in a NULL.
> > 
> > We would put such a cache around any bits of code that triggers
> > many connection calls to virlockd.
> 
> Actually, would sharing connection for case c) work?
> 
> Consider the following scenario: two threads A and B starting two
> different domains, both of them which want to relabel /dev/blah.
> 
> Now, say thread A gets to lock the path first. It does so, and proceed
> to chown().
> 
> Then thread B wants to lock the same path. It tries to do so, but has to
> wait until A unlocks it. However, at this point virtlockd is stuck in
> virFileLock() call (it waits for the lock to be released).
> 
> Because virtlockd runs single threaded it doesn't matter anymore that A
> will try to unlock the path eventually. virtlockd has deadlocked.
> 
> 
> I don't see any way around this :(

Oh right, yes, that kills the idea of using a WAIT flag for lockspaces,
unless we want to introduce threads ingto virtlockd, but we can't do
that because Linux has totally fubard  locking across execve() :-(

Regards,
Daniel
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