audit: audit feature to only allow unsetting the loginuid

This is a new audit feature which only grants processes with
CAP_AUDIT_CONTROL the ability to unset their loginuid.  They cannot
directly set it from a valid uid to another valid uid.  The ability to
unset the loginuid is nice because a priviledged task, like that of
container creation, can unset the loginuid and then priv is not needed
inside the container when a login daemon needs to set the loginuid.

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Eric Paris <eparis@redhat.com>

3 files changed, 7 insertions, 2 deletions

diff --git a/include/uapi/linux/audit.h b/include/uapi/linux/audit.h
index 9eddf2ca614f..05e5e8fc2ac4 100644
--- a/include/uapi/linux/audit.h
+++ b/include/uapi/linux/audit.h
@@ -386,7 +386,8 @@ struct audit_features {
         __u32   lock;           /* which features to lock */
 };
 
-#define AUDIT_LAST_FEATURE      -1
+#define AUDIT_FEATURE_ONLY_UNSET_LOGINUID       0
+#define AUDIT_LAST_FEATURE                      AUDIT_FEATURE_ONLY_UNSET_LOGINUID
 
 #define audit_feature_valid(x)          ((x) >= 0 && (x) <= AUDIT_LAST_FEATURE)
 #define AUDIT_FEATURE_TO_MASK(x)        (1 << ((x) & 31)) /* mask for __u32 */
diff --git a/kernel/audit.c b/kernel/audit.c
index 29ee6a421c6c..fbfa3a74decb 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -144,7 +144,8 @@ static struct audit_features af = {.vers = AUDIT_FEATURE_VERSION,
                                    .features = 0,
                                    .lock = 0,};
 
-static char *audit_feature_names[0] = {
+static char *audit_feature_names[1] = {
+        "only_unset_loginuid",
 };
 
 
diff --git a/kernel/auditsc.c b/kernel/auditsc.c
index c75d7813aef2..924c0bf048d2 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -1974,6 +1974,9 @@ static int audit_set_loginuid_perm(kuid_t loginuid)
         /* it is set, you need permission */
         if (!capable(CAP_AUDIT_CONTROL))
                 return -EPERM;
+        /* reject if this is not an unset and we don't allow that */
+        if (is_audit_feature_set(AUDIT_FEATURE_ONLY_UNSET_LOGINUID) && uid_valid(loginuid))
+                return -EPERM;
         return 0;
 }