capabilities: Use RCU to protect task lookup in sys_capget

cap_get_target_pid() protects the task lookup with tasklist_lock.
security_capget() is called under tasklist_lock as well but
tasklist_lock does not protect anything there. The capabilities are
protected by RCU already.

So tasklist_lock only protects the lookup and prevents the task going
away, which can be done with rcu_read_lock() as well.
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Signed-off-by: NJames Morris <jmorris@namei.org>

kernel/capability.c

@@ -135,7 +135,7 @@ static inline int cap_get_target_pid(pid_t pid, kernel_cap_t *pEp,
 	if (pid && (pid != task_pid_vnr(current))) {
 		struct task_struct *target;
 
-		read_lock(&tasklist_lock);
+		rcu_read_lock();
 
 		target = find_task_by_vpid(pid);
 		if (!target)
@@ -143,7 +143,7 @@ static inline int cap_get_target_pid(pid_t pid, kernel_cap_t *pEp,
 		else
 			ret = security_capget(target, pEp, pIp, pPp);
 
-		read_unlock(&tasklist_lock);
+		rcu_read_unlock();
 	} else
 		ret = security_capget(current, pEp, pIp, pPp);