kmsg: honor dmesg_restrict sysctl on /dev/kmsg

kmsg: honor dmesg_restrict sysctl on /dev/kmsg

Originally, the addition of dmesg_restrict covered both the syslog
method of accessing dmesg, as well as /dev/kmsg itself.  This was done
indirectly by security_syslog calling cap_syslog before doing any LSM
checks.

However, commit 12b3052c3ee ("capabilities/syslog: open code cap_syslog
logic to fix build failure") moved the code around and pushed the checks
into the caller itself.  That seems to have inadvertently dropped the
checks for dmesg_restrict on /dev/kmsg.  Most people haven't noticed
because util-linux dmesg(1) defaults to using the syslog method for access
in older versions.  With util-linux 2.22 and a kernel newer than 3.5,
dmesg(1) defaults to reading directly from /dev/kmsg.

Fix this by making an explicit check in the devkmsg_open function.

This fixes https://bugzilla.redhat.com/show_bug.cgi?id=903192

Signed-off-by: Josh Boyer <jwboyer@redhat.com>
Reported-by: Christian Kujau <lists@nerdbynature.de>
Cc: Eric Paris <eparis@redhat.com>
Cc: James Morris <jmorris@namei.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>