Create a new entry "display" in the procfs attr directory for
controlling which LSM security information is displayed for a
process. A process can only read or write its own display value.

The name of an active LSM that supplies hooks for
human readable data may be written to "display" to set the
value. The name of the LSM currently in use can be read from
"display". At this point there can only be one LSM capable
of display active. A helper function lsm_task_display() is
provided to get the display slot for a task_struct.

Setting the "display" requires that all security modules using
setprocattr hooks allow the action. Each security module is
responsible for defining its policy.

AppArmor hook provided by John Johansen [off-list ref]
SELinux hook provided by Stephen Smalley [off-list ref]

Reviewed-by: Kees Cook <redacted>
Acked-by: Stephen Smalley <redacted>
Acked-by: Paul Moore <paul@paul-moore.com>
Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Cc: linux-api@vger.kernel.org
---
 fs/proc/base.c                       |   1 +
 include/linux/lsm_hooks.h            |  17 +++
 security/apparmor/include/apparmor.h |   3 +-
 security/apparmor/lsm.c              |  32 +++++
 security/security.c                  | 169 ++++++++++++++++++++++++---
 security/selinux/hooks.c             |  11 ++
 security/selinux/include/classmap.h  |   2 +-
 security/smack/smack_lsm.c           |   7 ++
 8 files changed, 223 insertions(+), 19 deletions(-)

diff --git a/fs/proc/base.c b/fs/proc/base.c
index 0f707003dda5..7432f24f0132 100644
--- a/fs/proc/base.c
+++ b/fs/proc/base.c

@@ -2806,6 +2806,7 @@ static const struct pid_entry attr_dir_stuff[] = {
 	ATTR(NULL, "fscreate",		0666),
 	ATTR(NULL, "keycreate",		0666),
 	ATTR(NULL, "sockcreate",	0666),
+	ATTR(NULL, "display",		0666),