[PATCH 6/7] audit: audit feature to only allow unsetting the loginuid

[Eric Paris]

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 at redhat.com>
---
 include/uapi/linux/audit.h | 3 ++-
 kernel/audit.c             | 3 ++-
 kernel/auditsc.c           | 3 +++
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/audit.h b/include/uapi/linux/audit.h
index a053243..2963b5a 100644
--- a/include/uapi/linux/audit.h
+++ b/include/uapi/linux/audit.h
@@ -380,7 +380,8 @@ 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 3acbbc8..a5c470b 100644
--- a/kernel/audit.c
+++ b/kernel/audit.c
@@ -145,7 +145,8 @@ 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 eea28c1..e5dbbc6 100644
--- a/kernel/auditsc.c
+++ b/kernel/auditsc.c
@@ -1972,6 +1972,9 @@ 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;
 }
 
-- 
1.8.2.1