sys: Fix missing rcu protection for __task_cred() access

commit c69e8d9c (CRED: Use RCU to access another task's creds and to
release a task's own creds) added non rcu_read_lock() protected access
to task creds of the target task in set_prio_one().

The comment above the function says:
 * - the caller must hold the RCU read lock

The calling code in sys_setpriority does read_lock(&tasklist_lock) but
not rcu_read_lock(). This works only when CONFIG_TREE_PREEMPT_RCU=n.
With CONFIG_TREE_PREEMPT_RCU=y the rcu_callbacks can run in the tick
interrupt when they see no read side critical section.

There is another instance of __task_cred() in sys_setpriority() itself
which is equally unprotected.

Wrap the whole code section into a rcu read side critical section to
fix this quick and dirty.

Will be revisited in course of the read_lock(&tasklist_lock) -> rcu
crusade.

Oleg noted further:

This also fixes another bug here. find_task_by_vpid() is not safe
without rcu_read_lock(). I do not mean it is not safe to use the
result, just find_pid_ns() by itself is not safe.

Usually tasklist gives enough protection, but if copy_process() fails
it calls free_pid() lockless and does call_rcu(delayed_put_pid().
This means, without rcu lock find_pid_ns() can't scan the hash table
safely.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
LKML-Reference: <20091210004703.029784964@linutronix.de>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/sys.c

@@ -163,6 +163,7 @@ SYSCALL_DEFINE3(setpriority, int, which, int, who, int, niceval)
 	if (niceval > 19)
 		niceval = 19;
 
+	rcu_read_lock();
 	read_lock(&tasklist_lock);
 	switch (which) {
 		case PRIO_PROCESS:
@@ -200,6 +201,7 @@ SYSCALL_DEFINE3(setpriority, int, which, int, who, int, niceval)
 	}
 out_unlock:
 	read_unlock(&tasklist_lock);
+	rcu_read_unlock();
 out:
 	return error;
 }