smack: limit privilege by label

There have been a number of requests to make the Smack LSM
enforce MAC even in the face of privilege, either capability
based or superuser based. This is not universally desired,
however, so it seems desirable to make it optional. Further,
at least one legacy OS implemented a scheme whereby only
processes running with one particular label could be exempt
from MAC. This patch supports these three cases.

If /smack/onlycap is empty (unset or null-string) privilege
is enforced in the normal way.

If /smack/onlycap contains a label only processes running with
that label may be MAC exempt.

If the label in /smack/onlycap is the star label ("*") the
semantics of the star label combine with the privilege
restrictions to prevent any violations of MAC, even in the
presence of privilege.

Again, this will be independent of the privilege scheme.

Signed-off-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: James Morris <jmorris@namei.org>

1 files changed, 9 insertions, 1 deletions

diff --git a/security/smack/smack_access.c b/security/smack/smack_access.c
index f6b5f6e..79ff21e 100644
--- a/security/smack/smack_access.c
+++ b/security/smack/smack_access.c
@@ -157,7 +157,7 @@ int smk_access(char *subject_label, char *object_label, int request)
  *
  * This function checks the current subject label/object label pair
  * in the access rule list and returns 0 if the access is permitted,
- * non zero otherwise. It allows that current my have the capability
+ * non zero otherwise. It allows that current may have the capability
  * to override the rules.
  */
 int smk_curacc(char *obj_label, u32 mode)
@@ -168,6 +168,14 @@ int smk_curacc(char *obj_label, u32 mode)
 	if (rc == 0)
 		return 0;
 
+	/*
+	 * Return if a specific label has been designated as the
+	 * only one that gets privilege and current does not
+	 * have that label.
+	 */
+	if (smack_onlycap != NULL && smack_onlycap != current->security)
+		return rc;
+
 	if (capable(CAP_MAC_OVERRIDE))
 		return 0;