top zeigt nur aktuelle Benutzerprozesse

Eine Möglichkeit ist /proc montiert entweder mit hidepid=1 oder hidepid=2 . Diese Mount-Option wurde in späteren Linux-Kerneln hinzugefügt und irgendwann um CentOS 5.9 und 6.3 herum zurückportiert.

Mount options
   The proc filesystem supports the following mount options:

   hidepid=n (since Linux 3.3)
          This option controls who can access the information in
          /proc/[pid] directories.  The argument, n, is one of the
          following values:

          0   Everybody may access all /proc/[pid] directories.  This is
              the traditional behavior, and the default if this mount
              option is not specified.

          1   Users may not access files and subdirectories inside any
              /proc/[pid] directories but their own (the /proc/[pid]
              directories themselves remain visible).  Sensitive files
              such as /proc/[pid]/cmdline and /proc/[pid]/status are now
              protected against other users.  This makes it impossible
              to learn whether any user is running a specific program
              (so long as the program doesn't otherwise reveal itself by
              its behavior).

          2   As for mode 1, but in addition the /proc/[pid] directories
              belonging to other users become invisible.  This means
              that /proc/[pid] entries can no longer be used to discover
              the PIDs on the system.  This doesn't hide the fact that a
              process with a specific PID value exists (it can be
              learned by other means, for example, by "kill -0 $PID"),
              but it hides a process's UID and GID, which could
              otherwise be learned by employing stat(2) on a /proc/[pid]
              directory.  This greatly complicates an attacker's task of
              gathering information about running processes (e.g.,
              discovering whether some daemon is running with elevated
              privileges, whether another user is running some sensitive
              program, whether other users are running any program at
              all, and so on).

Eine andere Möglichkeit (vom Poster gefunden und als Referenzinformationen zu dieser Antwort hinzugefügt) ist grsecurity, das eine Funktion zum Ausblenden der Prozesse anderer Benutzer vor nicht privilegierten Benutzern als Teil der Dateisystemhärtung bietet.

$ uname -r
3.14.32-xxxx-grs-ipv6-64