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

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 :(


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