Jussi and I discussed this quite a bit this morning. He explained that this is 
the process:
1. mediascanner2 scans files on the filesystem (eg, ~/Music and ~/Videos) and 
stores paths, file metadata (artist, album, etc) in ~/.cache/mediascanner2
2. apps use the QML plugin (which uses libmediascanner) to query the data in 
~/.cache/mediascanner2
3. apps then can use the thumbnailer api to request the album via last.fm (this 
is cached on disk for later requests)

In '1', above, mediascanner2 runs unconfined and is a session service.
It does not need any special consideration (though it should have an
apparmor profile). In '2', the use of libmediascanner is in process to
the app and therefore the app currently requires the reserved
'music_files_read/video_files_read' policy group otherwise an app can
steal the mediascanner2 database for data mining, etc. For '3', there is
no concern because the thumbnailer just fetches whatever it is told from
last.fm (which the app could do itself).

What we need to protect is the scanned information in
~/.cache/mediascanner2 and access to ~/.cache/mediascanner2 should be
considered privileged. DBus service should be created as a proxy to the
data in ~/.cache/mediascanner2. This DBus service has direct access to
~/.cache/mediascanner2 and apps only have access to this DBus service
(presumably via the QML plugin/libmediascanner). This service then
becomes the trusted helper since it can perform the apparmor integration
in this bug and the trust store integration in bug #1315381. Once this
happens, this DBus api can be added to unprivileged policy groups (eg,
audio/video).

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https://bugs.launchpad.net/bugs/1303962

Title:
  please integrate mediascanner2 and media-hub with apparmor

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