mm: restrict access to /proc/slabinfo

Historically /proc/slabinfo has 0444 permissions and is accessible to
the world. slabinfo contains rather private information related both to
the kernel and userspace tasks. Depending on the situation, it might
reveal either private information per se or information useful to make
another targeted attack. Some examples of what can be learned by
reading/watching for /proc/slabinfo entries:

1) dentry (and different *inode*) number might reveal other processes fs
activity. The number of dentry 'active objects' doesn't strictly show
file count opened/touched by a process, however, there is a good
correlation between them. The patch 'proc: force dcache drop on
unauthorized access' relies on the privacy of dentry count.

2) different inode entries might reveal the same information as (1), but
these are more fine granted counters. If a filesystem is mounted in a
private mount point (or even a private namespace) and fs type differs from
other mounted fs types, fs activity in this mount point/namespace is
revealed. If there is a single ecryptfs mount point, the whole fs
activity of a single user is revealed. Number of files in ecryptfs
mount point is a private information per se.

3) fuse_* reveals number of files / fs activity of a user in a user
private mount point. It is approx. the same severity as ecryptfs
infoleak in (2).

4) sysfs_dir_cache similar to (2) reveals devices' addition/removal,
which can be otherwise hidden by 'chmod 0700 /sys/'. With 0444 slabinfo
the precise number of sysfs files is known to the world.

5) buffer_head might reveal some kernel activity. With other
information leaks an attacker might identify what specific kernel
routines generate buffer_head activity.

6) *kmalloc* infoleaks are very situational. Attacker should watch for
the specific kmalloc size entry and filter the noise related to the unrelated
kernel activity. If an attacker has relatively silent victim system, he
might get rather precise counters.

Additional information sources might significantly increase the slabinfo
infoleak benefits. E.g. if an attacker knows that the processes
activity on the system is very low (only core daemons like syslog and
cron), he may run setxid binaries / trigger local daemon activity /
trigger network services activity / await sporadic cron jobs activity
/ etc. and get rather precise counters for fs and network activity of
these privileged tasks, which is unknown otherwise.

Also hiding slabinfo is a one step to complicate exploitation of kernel
heap overflows (and possibly, other bugs). The related discussion:

http://thread.gmane.org/gmane.linux.kernel/1108378

World readable slabinfo simplifies kernel developers' job of debugging
kernel bugs (e.g. memleaks), but I believe it does more harm than
benefits. For most users 0444 slabinfo is an unreasonable attack vector.

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>