Reply to post: Solution

Solution

Even if you completely eliminate the side channel info leaks*, these kind of issues will remain as long as an app can hijack the running activity.

Can it be made so that apps require a special permission to start an activity while another app is running? (Unless initiated by activating a notification or an intent by the currently running app.)

If that's not possible, perhaps the change in app can be made more obvious in the UI. For example, if an app tries to start an activity while another is running, the old app could be shown to fade/zoom out to display the home screen for a fraction of a second, then fade/zoom in to display the second app? This could be accompanied by a hovering alert saying "Background process MyCoolGame is replacing MyBankApp". Sure, some users won't notice, but those that do will hopefully get the app reported. With increasing device resolutions, it may also be possible to show the current running app in the status bar (kind of like how a browser's URL bar mitigates against phishing attacks), which doesn't rely on the user noticing an animation.

*Some methods to mitigate these side channels are to disallow access to other processes /proc directories, and delay and/or reduce the resolution or of other readable info like network stats (e.g. network counters could be delayed by a couple of minutes, and only be accurate to 1MB increments unless the app is privileged).