@sw-pschmied originally post this in #38285

While looking through the Moby source code was found /proc/asound to be
shared with containers as read-only (as defined in
https://github.com/moby/moby/blob/master/oci/defaults.go#L128).

This can lead to two information leaks.

---

**Leak of media playback status of the host**

Steps to reproduce the issue:

- Listen to music/Play a YouTube video/Do anything else that involves
sound output
- Execute docker run --rm ubuntu:latest bash -c "sleep 7; cat
/proc/asound/card*/pcm*p/sub*/status | grep state | cut -d ' ' -f2 |
grep RUNNING || echo 'not running'"
- See that the containerized process is able to check whether someone
on the host is playing music as it prints RUNNING
- Stop the music output
- Execute the command again (The sleep is delaying the output because
information regarding playback status isn't propagated instantly)
- See that it outputs not running

**Describe the results you received:**

A containerized process is able to gather information on the playback
status of an audio device governed by the host. Therefore a process of a
container is able to check whether and what kind of user activity is
present on the host system. Also, this may indicate whether a container
runs on a desktop system or a server as media playback rarely happens on
server systems.

The description above is in regard to media playback - when examining
`/proc/asound/card*/pcm*c/sub*/status` (`pcm*c` instead of `pcm*p`) this
can also leak information regarding capturing sound, as in recording
audio or making calls on the host system.

Signed-off-by: Jonathan A. Schweder <jonathanschweder@gmail.com>