apparmor: fix lsm_get_self_attr()

In apparmor_getselfattr() when an invalid AppArmor attribute is
requested, or a value hasn't been explicitly set for the requested
attribute, the label passed to aa_put_label() is not properly
initialized which can cause problems when the pointer value is non-NULL
and AppArmor attempts to drop a reference on the bogus label object.

Cc: Casey Schaufler <casey@schaufler-ca.com>
Cc: John Johansen <john.johansen@canonical.com>
Fixes: 223981db ("AppArmor: Add selfattr hooks")
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Reviewed-by: Paul Moore <paul@paul-moore.com>
[PM: description changes as discussed with MS]
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/apparmor/lsm.c

@@ -782,7 +782,7 @@ static int apparmor_getselfattr(unsigned int attr, struct lsm_ctx __user *lx,
 	int error = -ENOENT;
 	struct aa_task_ctx *ctx = task_ctx(current);
 	struct aa_label *label = NULL;
-	char *value;
+	char *value = NULL;
 
 	switch (attr) {
 	case LSM_ATTR_CURRENT: