proc,security: move restriction on writing /proc/pid/attr nodes to proc

Processes can only alter their own security attributes via
/proc/pid/attr nodes.  This is presently enforced by each individual
security module and is also imposed by the Linux credentials
implementation, which only allows a task to alter its own credentials.
Move the check enforcing this restriction from the individual
security modules to proc_pid_attr_write() before calling the security hook,
and drop the unnecessary task argument to the security hook since it can
only ever be the current task.

Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Acked-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/smack/smack_lsm.c

@@ -3620,7 +3620,6 @@ static int smack_getprocattr(struct task_struct *p, char *name, char **value)
 
 /**
  * smack_setprocattr - Smack process attribute setting
- * @p: the object task
  * @name: the name of the attribute in /proc/.../attr
  * @value: the value to set
  * @size: the size of the value
@@ -3630,8 +3629,7 @@ static int smack_getprocattr(struct task_struct *p, char *name, char **value)
  *
  * Returns the length of the smack label or an error code
  */
-static int smack_setprocattr(struct task_struct *p, char *name,
-			     void *value, size_t size)
+static int smack_setprocattr(const char *name, void *value, size_t size)
 {
 	struct task_smack *tsp = current_security();
 	struct cred *new;
@@ -3639,13 +3637,6 @@ static int smack_setprocattr(struct task_struct *p, char *name,
 	struct smack_known_list_elem *sklep;
 	int rc;
 
-	/*
-	 * Changing another process' Smack value is too dangerous
-	 * and supports no sane use case.
-	 */
-	if (p != current)
-		return -EPERM;
-
 	if (!smack_privileged(CAP_MAC_ADMIN) && list_empty(&tsp->smk_relabel))
 		return -EPERM;